diff --git a/toolchain/check/action.cpp b/toolchain/check/action.cpp
index c588f5f2c2aa..2426de19f6ae 100644
--- a/toolchain/check/action.cpp
+++ b/toolchain/check/action.cpp
@@ -102,7 +102,7 @@ static auto AddDependentActionSpliceImpl(Context& context,
   if (!result_type_inst_id.has_value()) {
     result_type_inst_id = AddDependentActionTypeInst(
         context, action.loc_id,
-        SemIR::TypeOfInst{.type_id = SemIR::TypeType::SingletonTypeId,
+        SemIR::TypeOfInst{.type_id = SemIR::TypeType::TypeId,
                           .inst_id = inst_id});
   }
   return AddInst(
@@ -132,11 +132,11 @@ static auto RefineOperand(Context& context, SemIR::LocId loc_id,
       auto type_inst_id = context.types().GetInstId(inst.type_id());
       inst_id = AddDependentActionSpliceImpl(
           context,
-          SemIR::LocIdAndInst(loc_id,
-                              SemIR::RefineTypeAction{
-                                  .type_id = SemIR::InstType::SingletonTypeId,
-                                  .inst_id = *inst_id,
-                                  .inst_type_inst_id = type_inst_id}),
+          SemIR::LocIdAndInst(
+              loc_id,
+              SemIR::RefineTypeAction{.type_id = SemIR::InstType::TypeId,
+                                      .inst_id = *inst_id,
+                                      .inst_type_inst_id = type_inst_id}),
           type_inst_id);
     }
 
diff --git a/toolchain/check/call.cpp b/toolchain/check/call.cpp
index d51820749188..6306f7359a39 100644
--- a/toolchain/check/call.cpp
+++ b/toolchain/check/call.cpp
@@ -101,13 +101,12 @@ static auto PerformCallToGenericClass(Context& context, SemIR::LocId loc_id,
                           /*self_type_id=*/SemIR::InstId::None,
                           /*self_id=*/SemIR::InstId::None, arg_ids);
   if (!callee_specific_id) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
-  return GetOrAddInst<SemIR::ClassType>(
-      context, loc_id,
-      {.type_id = SemIR::TypeType::SingletonTypeId,
-       .class_id = class_id,
-       .specific_id = *callee_specific_id});
+  return GetOrAddInst<SemIR::ClassType>(context, loc_id,
+                                        {.type_id = SemIR::TypeType::TypeId,
+                                         .class_id = class_id,
+                                         .specific_id = *callee_specific_id});
 }
 
 // Performs a call where the callee is the name of a generic interface, such as
@@ -123,7 +122,7 @@ static auto PerformCallToGenericInterface(
                           /*self_type_id=*/SemIR::InstId::None,
                           /*self_id=*/SemIR::InstId::None, arg_ids);
   if (!callee_specific_id) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   return GetOrAddInst(
       context, loc_id,
@@ -149,23 +148,23 @@ static auto BuildCalleeSpecificFunction(
     // This is an associated function in an interface; the callee is the
     // specific function in the impl that corresponds to the specific function
     // we deduced.
-    callee_id = GetOrAddInst(
-        context, context.insts().GetLocId(generic_callee_id),
-        SemIR::SpecificImplFunction{
-            .type_id = GetSingletonType(
-                context, SemIR::SpecificFunctionType::SingletonInstId),
-            .callee_id = generic_callee_id,
-            .specific_id = callee_specific_id});
+    callee_id =
+        GetOrAddInst(context, context.insts().GetLocId(generic_callee_id),
+                     SemIR::SpecificImplFunction{
+                         .type_id = GetSingletonType(
+                             context, SemIR::SpecificFunctionType::InstId),
+                         .callee_id = generic_callee_id,
+                         .specific_id = callee_specific_id});
   } else {
     // This is a regular generic function. The callee is the specific function
     // we deduced.
-    callee_id = GetOrAddInst(
-        context, context.insts().GetLocId(generic_callee_id),
-        SemIR::SpecificFunction{
-            .type_id = GetSingletonType(
-                context, SemIR::SpecificFunctionType::SingletonInstId),
-            .callee_id = generic_callee_id,
-            .specific_id = callee_specific_id});
+    callee_id =
+        GetOrAddInst(context, context.insts().GetLocId(generic_callee_id),
+                     SemIR::SpecificFunction{
+                         .type_id = GetSingletonType(
+                             context, SemIR::SpecificFunctionType::InstId),
+                         .callee_id = generic_callee_id,
+                         .specific_id = callee_specific_id});
   }
 
   // Add the `self` argument back if there was one.
@@ -208,7 +207,7 @@ static auto PerformCallToFunction(Context& context, SemIR::LocId loc_id,
       EntityKind::Function, callee_function.enclosing_specific_id,
       callee_function.self_type_id, callee_function.self_id, arg_ids);
   if (!callee_specific_id) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   if (callee_specific_id->has_value()) {
@@ -240,7 +239,7 @@ static auto PerformCallToFunction(Context& context, SemIR::LocId loc_id,
     case SemIR::InitRepr::Incomplete:
       // Don't form an initializing expression with an incomplete type.
       // CheckFunctionReturnType will have diagnosed this for us if needed.
-      return_info.type_id = SemIR::ErrorInst::SingletonTypeId;
+      return_info.type_id = SemIR::ErrorInst::TypeId;
       break;
   }
 
@@ -262,7 +261,7 @@ auto PerformCall(Context& context, SemIR::LocId loc_id, SemIR::InstId callee_id,
   // Try treating the callee as a function first.
   auto callee_function = GetCalleeFunction(context.sem_ir(), callee_id);
   if (callee_function.is_error) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   if (callee_function.function_id.has_value()) {
     return PerformCallToFunction(context, loc_id, callee_id, callee_function,
@@ -287,7 +286,7 @@ auto PerformCall(Context& context, SemIR::LocId loc_id, SemIR::InstId callee_id,
       CARBON_DIAGNOSTIC(CallToNonCallable, Error,
                         "value of type {0} is not callable", TypeOfInstId);
       context.emitter().Emit(loc_id, CallToNonCallable, callee_id);
-      return SemIR::ErrorInst::SingletonInstId;
+      return SemIR::ErrorInst::InstId;
     }
   }
 }
diff --git a/toolchain/check/check_unit.cpp b/toolchain/check/check_unit.cpp
index ea5d4080f948..c3d68d40ab71 100644
--- a/toolchain/check/check_unit.cpp
+++ b/toolchain/check/check_unit.cpp
@@ -88,7 +88,7 @@ auto CheckUnit::Run() -> void {
 auto CheckUnit::InitPackageScopeAndImports() -> void {
   // Importing makes many namespaces, so only canonicalize the type once.
   auto namespace_type_id =
-      GetSingletonType(context_, SemIR::NamespaceType::SingletonInstId);
+      GetSingletonType(context_, SemIR::NamespaceType::InstId);
 
   // Define the package scope, with an instruction for `package` expressions to
   // reference.
diff --git a/toolchain/check/class.cpp b/toolchain/check/class.cpp
index a8f869c05163..566eba98c4cc 100644
--- a/toolchain/check/class.cpp
+++ b/toolchain/check/class.cpp
@@ -56,7 +56,7 @@ static auto CheckCompleteAdapterClassType(
         .Build(class_info.adapt_id, AdaptWithBase)
         .Note(class_info.base_id, AdaptWithBaseHere)
         .Emit();
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   if (!field_decls.empty()) {
@@ -67,7 +67,7 @@ static auto CheckCompleteAdapterClassType(
         .Build(class_info.adapt_id, AdaptWithFields)
         .Note(field_decls.front(), AdaptWithFieldHere)
         .Emit();
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   for (auto inst_id : body) {
@@ -84,7 +84,7 @@ static auto CheckCompleteAdapterClassType(
             .Build(class_info.adapt_id, AdaptWithVirtual)
             .Note(inst_id, AdaptWithVirtualHere)
             .Emit();
-        return SemIR::ErrorInst::SingletonInstId;
+        return SemIR::ErrorInst::InstId;
       }
     }
   }
@@ -97,8 +97,7 @@ static auto CheckCompleteAdapterClassType(
 
   return AddInst<SemIR::CompleteTypeWitness>(
       context, node_id,
-      {.type_id =
-           GetSingletonType(context, SemIR::WitnessType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::WitnessType::InstId),
        // TODO: Use InstId from the adapt declaration.
        .object_repr_type_inst_id = context.types().GetInstId(object_repr_id)});
 }
@@ -112,10 +111,10 @@ static auto AddStructTypeFields(
     field_decl.index =
         SemIR::ElementIndex{static_cast<int>(struct_type_fields.size())};
     ReplaceInstPreservingConstantValue(context, field_decl_id, field_decl);
-    if (field_decl.type_id == SemIR::ErrorInst::SingletonTypeId) {
+    if (field_decl.type_id == SemIR::ErrorInst::TypeId) {
       struct_type_fields.push_back(
           {.name_id = field_decl.name_id,
-           .type_inst_id = SemIR::ErrorInst::SingletonTypeInstId});
+           .type_inst_id = SemIR::ErrorInst::TypeInstId});
       continue;
     }
     auto unbound_element_type =
@@ -141,8 +140,8 @@ static auto BuildVtable(Context& context, Parse::NodeId node_id,
     LoadImportRef(context, base_vtable_id);
     auto canonical_base_vtable_id =
         context.constant_values().GetConstantInstId(base_vtable_id);
-    if (canonical_base_vtable_id == SemIR::ErrorInst::SingletonInstId) {
-      return SemIR::ErrorInst::SingletonInstId;
+    if (canonical_base_vtable_id == SemIR::ErrorInst::InstId) {
+      return SemIR::ErrorInst::InstId;
     }
     auto base_vtable_inst_block = context.inst_blocks().Get(
         context.insts()
@@ -183,7 +182,7 @@ static auto BuildVtable(Context& context, Parse::NodeId node_id,
   }
   return AddInst<SemIR::Vtable>(
       context, node_id,
-      {.type_id = GetSingletonType(context, SemIR::VtableType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::VtableType::InstId),
        .virtual_functions_id = context.inst_blocks().Add(vtable)});
 }
 
@@ -222,7 +221,7 @@ static auto CheckCompleteClassType(
     struct_type_fields.push_back(
         {.name_id = SemIR::NameId::Vptr,
          .type_inst_id = context.types().GetInstId(
-             GetPointerType(context, SemIR::VtableType::SingletonInstId))});
+             GetPointerType(context, SemIR::VtableType::InstId))});
   }
   if (base_type_id.has_value()) {
     auto base_decl = context.insts().GetAs<SemIR::BaseDecl>(class_info.base_id);
@@ -242,14 +241,13 @@ static auto CheckCompleteClassType(
 
   auto struct_type_inst_id = AddTypeInst<SemIR::StructType>(
       context, node_id,
-      {.type_id = SemIR::TypeType::SingletonTypeId,
+      {.type_id = SemIR::TypeType::TypeId,
        .fields_id =
            AddStructTypeFields(context, struct_type_fields, field_decls)});
 
   return AddInst<SemIR::CompleteTypeWitness>(
       context, node_id,
-      {.type_id =
-           GetSingletonType(context, SemIR::WitnessType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::WitnessType::InstId),
        .object_repr_type_inst_id = struct_type_inst_id});
 }
 
diff --git a/toolchain/check/control_flow.cpp b/toolchain/check/control_flow.cpp
index 258d930ff20d..2a38b56cd187 100644
--- a/toolchain/check/control_flow.cpp
+++ b/toolchain/check/control_flow.cpp
@@ -106,9 +106,9 @@ auto SetBlockArgResultBeforeConstantUse(Context& context,
     const_id = context.constant_values().Get(
         literal.value().value.ToBool() ? if_true : if_false);
   } else {
-    CARBON_CHECK(cond_const_id == SemIR::ErrorInst::SingletonConstantId,
+    CARBON_CHECK(cond_const_id == SemIR::ErrorInst::ConstantId,
                  "Unexpected constant branch condition.");
-    const_id = SemIR::ErrorInst::SingletonConstantId;
+    const_id = SemIR::ErrorInst::ConstantId;
   }
 
   if (const_id.is_constant()) {
diff --git a/toolchain/check/convert.cpp b/toolchain/check/convert.cpp
index fda030be195a..6c943925e938 100644
--- a/toolchain/check/convert.cpp
+++ b/toolchain/check/convert.cpp
@@ -125,9 +125,9 @@ static auto MakeElementAccessInst(Context& context, SemIR::LocId loc_id,
     // index so that we don't need an integer literal instruction here, and
     // remove this special case.
     auto index_id = block.template AddInst<SemIR::IntValue>(
-        loc_id, {.type_id = GetSingletonType(
-                     context, SemIR::IntLiteralType::SingletonInstId),
-                 .int_id = context.ints().Add(static_cast<int64_t>(i))});
+        loc_id,
+        {.type_id = GetSingletonType(context, SemIR::IntLiteralType::InstId),
+         .int_id = context.ints().Add(static_cast<int64_t>(i))});
     return AddInst<AccessInstT>(block, loc_id,
                                 {elem_type_id, aggregate_id, index_id});
   } else {
@@ -222,7 +222,7 @@ static auto ConvertTupleToArray(Context& context, SemIR::TupleType tuple_type,
                         "list of initializers");
       context.emitter().Emit(value_loc_id, ArrayInitDependentBound);
     }
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   if (tuple_elem_types.size() != array_bound) {
     if (target.diagnose) {
@@ -241,7 +241,7 @@ static auto ConvertTupleToArray(Context& context, SemIR::TupleType tuple_type,
                                  : ArrayInitFromLiteralArgCountMismatch,
                              *array_bound, tuple_elem_types.size());
     }
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   PendingBlock target_block_storage(&context);
@@ -272,8 +272,8 @@ static auto ConvertTupleToArray(Context& context, SemIR::TupleType tuple_type,
             context, value_loc_id, value_id, src_type_inst_id, literal_elems,
             ConversionTarget::FullInitializer, return_slot_arg_id,
             array_type.element_type_inst_id, target_block, i, i);
-    if (init_id == SemIR::ErrorInst::SingletonInstId) {
-      return SemIR::ErrorInst::SingletonInstId;
+    if (init_id == SemIR::ErrorInst::InstId) {
+      return SemIR::ErrorInst::InstId;
     }
     inits.push_back(init_id);
   }
@@ -324,7 +324,7 @@ static auto ConvertTupleToTuple(Context& context, SemIR::TupleType src_type,
       context.emitter().Emit(value_loc_id, TupleInitElementCountMismatch,
                              dest_elem_types.size(), src_elem_types.size());
     }
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // If we're forming an initializer, then we want an initializer for each
@@ -358,8 +358,8 @@ static auto ConvertTupleToTuple(Context& context, SemIR::TupleType src_type,
             context, value_loc_id, value_id, src_type_inst_id, literal_elems,
             inner_kind, target.init_id, dest_type_inst_id, target.init_block, i,
             i);
-    if (init_id == SemIR::ErrorInst::SingletonInstId) {
-      return SemIR::ErrorInst::SingletonInstId;
+    if (init_id == SemIR::ErrorInst::InstId) {
+      return SemIR::ErrorInst::InstId;
     }
     new_block.Set(i, init_id);
   }
@@ -426,7 +426,7 @@ static auto ConvertStructToStructOrClass(
                              ToClass, dest_elem_fields_size,
                              src_elem_fields.size());
     }
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // Prepare to look up fields in the source by index.
@@ -508,7 +508,7 @@ static auto ConvertStructToStructOrClass(
                                    target.type_id, dest_field.name_id);
           }
         }
-        return SemIR::ErrorInst::SingletonInstId;
+        return SemIR::ErrorInst::InstId;
       }
     }
     auto src_field = src_elem_fields[src_field_index];
@@ -521,8 +521,8 @@ static auto ConvertStructToStructOrClass(
             literal_elems, inner_kind, target.init_id, dest_field.type_inst_id,
             target.init_block, src_field_index,
             src_field_index + dest_vptr_offset, dest_vtable_id);
-    if (init_id == SemIR::ErrorInst::SingletonInstId) {
-      return SemIR::ErrorInst::SingletonInstId;
+    if (init_id == SemIR::ErrorInst::InstId) {
+      return SemIR::ErrorInst::InstId;
     }
     new_block.Set(i, init_id);
   }
@@ -571,8 +571,8 @@ static auto ConvertStructToClass(
   CARBON_CHECK(dest_class_info.inheritance_kind != SemIR::Class::Abstract);
   auto object_repr_id =
       dest_class_info.GetObjectRepr(context.sem_ir(), dest_type.specific_id);
-  if (object_repr_id == SemIR::ErrorInst::SingletonTypeId) {
-    return SemIR::ErrorInst::SingletonInstId;
+  if (object_repr_id == SemIR::ErrorInst::TypeId) {
+    return SemIR::ErrorInst::InstId;
   }
   auto dest_struct_type =
       context.types().GetAs<SemIR::StructType>(object_repr_id);
@@ -755,7 +755,7 @@ static auto GetTransitiveAdaptedType(Context& context, SemIR::TypeId type_id)
 static auto DiagnoseConversionFailureToConstraintValue(
     Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
     SemIR::TypeId target_type_id) -> void {
-  CARBON_DCHECK(target_type_id == SemIR::TypeType::SingletonTypeId ||
+  CARBON_DCHECK(target_type_id == SemIR::TypeType::TypeId ||
                 context.types().Is<SemIR::FacetType>(target_type_id));
 
   auto type_of_expr_id = context.insts().Get(expr_id).type_id();
@@ -888,8 +888,8 @@ static auto PerformBuiltinConversion(
                                       .init_id = foundation_init_id,
                                       .init_block = target.init_block,
                                       .diagnose = target.diagnose});
-        if (foundation_value_id == SemIR::ErrorInst::SingletonInstId) {
-          return SemIR::ErrorInst::SingletonInstId;
+        if (foundation_value_id == SemIR::ErrorInst::InstId) {
+          return SemIR::ErrorInst::InstId;
         }
       }
 
@@ -993,7 +993,7 @@ static auto PerformBuiltinConversion(
     }
   }
 
-  if (target.type_id == SemIR::TypeType::SingletonTypeId) {
+  if (target.type_id == SemIR::TypeType::TypeId) {
     // A tuple of types converts to type `type`.
     // TODO: This should apply even for non-literal tuples.
     if (auto tuple_literal = value.TryAs<SemIR::TupleLiteral>()) {
@@ -1047,7 +1047,7 @@ static auto PerformBuiltinConversion(
     // FacetTypes, but we assume constant values for impl lookup at the moment.
     if (!const_value_id.has_value()) {
       context.TODO(loc_id, "conversion of runtime facet value");
-      const_value_id = SemIR::ErrorInst::SingletonInstId;
+      const_value_id = SemIR::ErrorInst::InstId;
     }
 
     if (auto facet_access_type_inst =
@@ -1072,7 +1072,7 @@ static auto PerformBuiltinConversion(
         sem_ir.types().GetConstantId(target.type_id));
     if (lookup_result.has_value()) {
       if (lookup_result.has_error_value()) {
-        return SemIR::ErrorInst::SingletonInstId;
+        return SemIR::ErrorInst::InstId;
       } else {
         // We bind the input value to the target `FacetType` with a
         // `FacetValue`, which requires an instruction of type `TypeType`. So if
@@ -1080,12 +1080,11 @@ static auto PerformBuiltinConversion(
         // `FacetAccessType` instruction.
         auto type_inst_id = SemIR::TypeInstId::None;
         if (sem_ir.types().Is<SemIR::FacetType>(value_type_id)) {
-          type_inst_id =
-              AddTypeInst(context, loc_id,
-                          SemIR::FacetAccessType{
-                              .type_id = SemIR::TypeType::SingletonTypeId,
-                              .facet_value_inst_id = const_value_id,
-                          });
+          type_inst_id = AddTypeInst(context, loc_id,
+                                     SemIR::FacetAccessType{
+                                         .type_id = SemIR::TypeType::TypeId,
+                                         .facet_value_inst_id = const_value_id,
+                                     });
         } else {
           type_inst_id = context.types().GetAsTypeInstId(const_value_id);
         }
@@ -1104,7 +1103,7 @@ static auto PerformBuiltinConversion(
         DiagnoseConversionFailureToConstraintValue(context, loc_id, value_id,
                                                    target.type_id);
       }
-      return SemIR::ErrorInst::SingletonInstId;
+      return SemIR::ErrorInst::InstId;
     }
   }
 
@@ -1118,8 +1117,8 @@ static auto PerformCopy(Context& context, SemIR::InstId expr_id, bool diagnose)
     -> SemIR::InstId {
   auto expr = context.insts().Get(expr_id);
   auto type_id = expr.type_id();
-  if (type_id == SemIR::ErrorInst::SingletonTypeId) {
-    return SemIR::ErrorInst::SingletonInstId;
+  if (type_id == SemIR::ErrorInst::TypeId) {
+    return SemIR::ErrorInst::InstId;
   }
 
   if (InitReprIsCopyOfValueRepr(context.sem_ir(), type_id)) {
@@ -1135,7 +1134,7 @@ static auto PerformCopy(Context& context, SemIR::InstId expr_id, bool diagnose)
                       "cannot copy value of type {0}", TypeOfInstId);
     context.emitter().Emit(expr_id, CopyOfUncopyableType, expr_id);
   }
-  return SemIR::ErrorInst::SingletonInstId;
+  return SemIR::ErrorInst::InstId;
 }
 
 auto PerformAction(Context& context, SemIR::LocId loc_id,
@@ -1154,10 +1153,9 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
 
   // Start by making sure both sides are non-errors. If any part is an error,
   // the result is an error and we shouldn't diagnose.
-  if (sem_ir.insts().Get(expr_id).type_id() ==
-          SemIR::ErrorInst::SingletonTypeId ||
-      target.type_id == SemIR::ErrorInst::SingletonTypeId) {
-    return SemIR::ErrorInst::SingletonInstId;
+  if (sem_ir.insts().Get(expr_id).type_id() == SemIR::ErrorInst::TypeId ||
+      target.type_id == SemIR::ErrorInst::TypeId) {
+    return SemIR::ErrorInst::InstId;
   }
 
   if (SemIR::GetExprCategory(sem_ir, expr_id) == SemIR::ExprCategory::NotExpr) {
@@ -1169,7 +1167,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
                         "expression cannot be used as a value");
       context.emitter().Emit(expr_id, UseOfNonExprAsValue);
     }
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // We can only perform initialization for complete, non-abstract types. Note
@@ -1208,13 +1206,13 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
             return context.emitter().Build(loc_id, AbstractTypeInInit,
                                            target.type_id);
           })) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // Check whether any builtin conversion applies.
   expr_id =
       PerformBuiltinConversion(context, loc_id, expr_id, target, vtable_id);
-  if (expr_id == SemIR::ErrorInst::SingletonInstId) {
+  if (expr_id == SemIR::ErrorInst::InstId) {
     return expr_id;
   }
 
@@ -1233,7 +1231,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
     auto target_type_inst_id = context.types().GetInstId(target.type_id);
     return AddDependentActionSplice(
         context, loc_id,
-        SemIR::ConvertToValueAction{.type_id = SemIR::InstType::SingletonTypeId,
+        SemIR::ConvertToValueAction{.type_id = SemIR::InstType::TypeId,
                                     .inst_id = expr_id,
                                     .target_type_inst_id = target_type_inst_id},
         target_type_inst_id);
@@ -1254,7 +1252,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
       if (!target.diagnose) {
         return context.emitter().BuildSuppressed();
       }
-      if (target.type_id == SemIR::TypeType::SingletonTypeId ||
+      if (target.type_id == SemIR::TypeType::TypeId ||
           sem_ir.types().Is<SemIR::FacetType>(target.type_id)) {
         CARBON_DIAGNOSTIC(
             ConversionFailureNonTypeToFacet, Error,
@@ -1265,7 +1263,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
         return context.emitter().Build(
             loc_id, ConversionFailureNonTypeToFacet,
             target.kind == ConversionTarget::ExplicitAs, expr_id,
-            target.type_id == SemIR::TypeType::SingletonTypeId, target.type_id);
+            target.type_id == SemIR::TypeType::TypeId, target.type_id);
       } else {
         CARBON_DIAGNOSTIC(ConversionFailure, Error,
                           "cannot{0:| implicitly} convert expression of type "
@@ -1280,7 +1278,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
     });
 
     // Pull a value directly out of the initializer if possible and wanted.
-    if (expr_id != SemIR::ErrorInst::SingletonInstId &&
+    if (expr_id != SemIR::ErrorInst::InstId &&
         CanUseValueOfInitializer(sem_ir, target.type_id, target.kind)) {
       expr_id = AddInst<SemIR::ValueOfInitializer>(
           context, loc_id, {.type_id = target.type_id, .init_id = expr_id});
@@ -1309,7 +1307,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
                    sem_ir.insts().Get(expr_id));
 
     case SemIR::ExprCategory::Error:
-      return SemIR::ErrorInst::SingletonInstId;
+      return SemIR::ErrorInst::InstId;
 
     case SemIR::ExprCategory::Initializing:
       if (target.is_initializer()) {
@@ -1423,7 +1421,7 @@ auto ConvertToBoolValue(Context& context, SemIR::LocId loc_id,
                         SemIR::InstId value_id) -> SemIR::InstId {
   return ConvertToValueOfType(
       context, loc_id, value_id,
-      GetSingletonType(context, SemIR::BoolType::SingletonInstId));
+      GetSingletonType(context, SemIR::BoolType::InstId));
 }
 
 auto ConvertForExplicitAs(Context& context, Parse::NodeId as_node,
@@ -1459,7 +1457,7 @@ auto ConvertCallArgs(Context& context, SemIR::LocId call_loc_id,
         .Build(call_loc_id, MissingObjectInMethodCall)
         .Note(callee_decl_id, InCallToFunction)
         .Emit();
-    self_id = SemIR::ErrorInst::SingletonInstId;
+    self_id = SemIR::ErrorInst::InstId;
   }
 
   return CallerPatternMatch(context, callee_specific_id, callee.self_param_id,
@@ -1469,11 +1467,11 @@ auto ConvertCallArgs(Context& context, SemIR::LocId call_loc_id,
 
 auto ExprAsType(Context& context, SemIR::LocId loc_id, SemIR::InstId value_id,
                 bool diagnose) -> TypeExpr {
-  auto type_inst_id = ConvertToValueOfType(context, loc_id, value_id,
-                                           SemIR::TypeType::SingletonTypeId);
-  if (type_inst_id == SemIR::ErrorInst::SingletonInstId) {
-    return {.inst_id = SemIR::ErrorInst::SingletonTypeInstId,
-            .type_id = SemIR::ErrorInst::SingletonTypeId};
+  auto type_inst_id =
+      ConvertToValueOfType(context, loc_id, value_id, SemIR::TypeType::TypeId);
+  if (type_inst_id == SemIR::ErrorInst::InstId) {
+    return {.inst_id = SemIR::ErrorInst::TypeInstId,
+            .type_id = SemIR::ErrorInst::TypeId};
   }
 
   auto type_const_id = context.constant_values().Get(type_inst_id);
@@ -1483,8 +1481,8 @@ auto ExprAsType(Context& context, SemIR::LocId loc_id, SemIR::InstId value_id,
                         "cannot evaluate type expression");
       context.emitter().Emit(loc_id, TypeExprEvaluationFailure);
     }
-    return {.inst_id = SemIR::ErrorInst::SingletonTypeInstId,
-            .type_id = SemIR::ErrorInst::SingletonTypeId};
+    return {.inst_id = SemIR::ErrorInst::TypeInstId,
+            .type_id = SemIR::ErrorInst::TypeId};
   }
 
   return {.inst_id = context.types().GetAsTypeInstId(type_inst_id),
diff --git a/toolchain/check/convert.h b/toolchain/check/convert.h
index a3f425daa386..ac32105c2fc9 100644
--- a/toolchain/check/convert.h
+++ b/toolchain/check/convert.h
@@ -119,9 +119,9 @@ auto ConvertCallArgs(Context& context, SemIR::LocId call_loc_id,
 
 // A type that has been converted for use as a type expression.
 struct TypeExpr {
-  // The converted expression of type `type`, or `ErrorInst::SingletonInstId`.
+  // The converted expression of type `type`, or `ErrorInst::InstId`.
   SemIR::TypeInstId inst_id;
-  // The corresponding type, or `ErrorInst::SingletonTypeId`.
+  // The corresponding type, or `ErrorInst::TypeId`.
   SemIR::TypeId type_id;
 };
 
diff --git a/toolchain/check/deduce.cpp b/toolchain/check/deduce.cpp
index 299620b22164..880da32774df 100644
--- a/toolchain/check/deduce.cpp
+++ b/toolchain/check/deduce.cpp
@@ -321,7 +321,7 @@ auto DeductionContext::Deduce() -> bool {
                                                 param_type_id)
                          : TryConvertToValueOfType(context(), loc_id_, arg_id,
                                                    param_type_id);
-      if (arg_id == SemIR::ErrorInst::SingletonInstId) {
+      if (arg_id == SemIR::ErrorInst::InstId) {
         return false;
       }
     }
@@ -578,7 +578,7 @@ auto DeductionContext::CheckDeductionIsComplete() -> bool {
           NoteGenericHere(context(), generic_id_, diag);
           diag.Emit();
         }
-        deduced_arg_id = SemIR::ErrorInst::SingletonInstId;
+        deduced_arg_id = SemIR::ErrorInst::InstId;
       }
     }
 
diff --git a/toolchain/check/eval.cpp b/toolchain/check/eval.cpp
index 4281412d00ff..4765cb0f0b29 100644
--- a/toolchain/check/eval.cpp
+++ b/toolchain/check/eval.cpp
@@ -247,7 +247,7 @@ static auto GetPhase(const SemIR::ConstantValueStore& constant_values,
                      SemIR::ConstantId constant_id) -> Phase {
   if (!constant_id.is_constant()) {
     return Phase::Runtime;
-  } else if (constant_id == SemIR::ErrorInst::SingletonConstantId) {
+  } else if (constant_id == SemIR::ErrorInst::ConstantId) {
     return Phase::UnknownDueToError;
   }
   switch (constant_values.GetDependence(constant_id)) {
@@ -285,7 +285,7 @@ static auto MakeConstantResult(Context& context, SemIR::Inst inst, Phase phase)
       return context.constants().GetOrAdd(inst,
                                           SemIR::ConstantDependence::Template);
     case Phase::UnknownDueToError:
-      return SemIR::ErrorInst::SingletonConstantId;
+      return SemIR::ErrorInst::ConstantId;
     case Phase::Runtime:
       return SemIR::ConstantId::NotConstant;
   }
@@ -293,9 +293,8 @@ static auto MakeConstantResult(Context& context, SemIR::Inst inst, Phase phase)
 
 // Forms a `constant_id` describing why an evaluation was not constant.
 static auto MakeNonConstantResult(Phase phase) -> SemIR::ConstantId {
-  return phase == Phase::UnknownDueToError
-             ? SemIR::ErrorInst::SingletonConstantId
-             : SemIR::ConstantId::NotConstant;
+  return phase == Phase::UnknownDueToError ? SemIR::ErrorInst::ConstantId
+                                           : SemIR::ConstantId::NotConstant;
 }
 
 // Converts a bool value into a ConstantId.
@@ -334,11 +333,10 @@ static auto MakeFacetTypeResult(Context& context,
                                 const SemIR::FacetTypeInfo& info, Phase phase)
     -> SemIR::ConstantId {
   SemIR::FacetTypeId facet_type_id = context.facet_types().Add(info);
-  return MakeConstantResult(
-      context,
-      SemIR::FacetType{.type_id = SemIR::TypeType::SingletonTypeId,
-                       .facet_type_id = facet_type_id},
-      phase);
+  return MakeConstantResult(context,
+                            SemIR::FacetType{.type_id = SemIR::TypeType::TypeId,
+                                             .facet_type_id = facet_type_id},
+                            phase);
 }
 
 // `GetConstantValue` checks to see whether the provided ID describes a value
@@ -797,7 +795,7 @@ static auto PerformArrayIndex(EvalContext& eval_context, SemIR::ArrayIndex inst)
         eval_context.emitter().Emit(
             eval_context.GetDiagnosticLoc(inst.index_id), ArrayIndexOutOfBounds,
             {.type = index->type_id, .value = index_val}, aggregate_type_id);
-        return SemIR::ErrorInst::SingletonConstantId;
+        return SemIR::ErrorInst::ConstantId;
       }
     }
   }
@@ -824,11 +822,11 @@ static auto MakeIntTypeResult(Context& context, SemIR::LocId loc_id,
                               SemIR::IntKind int_kind, SemIR::InstId width_id,
                               Phase phase) -> SemIR::ConstantId {
   auto result = SemIR::IntType{
-      .type_id = GetSingletonType(context, SemIR::TypeType::SingletonInstId),
+      .type_id = GetSingletonType(context, SemIR::TypeType::InstId),
       .int_kind = int_kind,
       .bit_width_id = width_id};
   if (!ValidateIntType(context, loc_id, result)) {
-    return SemIR::ErrorInst::SingletonConstantId;
+    return SemIR::ErrorInst::ConstantId;
   }
   return MakeConstantResult(context, result, phase);
 }
@@ -1101,7 +1099,7 @@ static auto PerformBuiltinIntShiftOp(Context& context, SemIR::LocId loc_id,
         builtin_kind == SemIR::BuiltinFunctionKind::IntLeftShift,
         {.type = rhs.type_id, .value = rhs_orig_val});
     // TODO: Is it useful to recover by returning 0 or -1?
-    return SemIR::ErrorInst::SingletonConstantId;
+    return SemIR::ErrorInst::ConstantId;
   }
 
   if (rhs_orig_val.isNegative() &&
@@ -1115,7 +1113,7 @@ static auto PerformBuiltinIntShiftOp(Context& context, SemIR::LocId loc_id,
         builtin_kind == SemIR::BuiltinFunctionKind::IntLeftShift,
         {.type = rhs.type_id, .value = rhs_orig_val});
     // TODO: Is it useful to recover by returning 0 or -1?
-    return SemIR::ErrorInst::SingletonConstantId;
+    return SemIR::ErrorInst::ConstantId;
   }
 
   llvm::APInt result_val;
@@ -1134,7 +1132,7 @@ static auto PerformBuiltinIntShiftOp(Context& context, SemIR::LocId loc_id,
         context.emitter().Emit(loc_id, CompileTimeUnsizedShiftOutOfRange,
                                {.type = rhs.type_id, .value = rhs_orig_val},
                                IntStore::MaxIntWidth);
-        return SemIR::ErrorInst::SingletonConstantId;
+        return SemIR::ErrorInst::ConstantId;
       }
       lhs_val = lhs_val.sext(
           IntStore::CanonicalBitWidth(lhs_val.getSignificantBits() + *width));
@@ -1178,7 +1176,7 @@ static auto PerformBuiltinBinaryIntOp(Context& context, SemIR::LocId loc_id,
     case SemIR::BuiltinFunctionKind::IntUMod:
       if (rhs_val.isZero()) {
         DiagnoseDivisionByZero(context, loc_id);
-        return SemIR::ErrorInst::SingletonConstantId;
+        return SemIR::ErrorInst::ConstantId;
       }
       break;
     default:
@@ -1423,7 +1421,7 @@ static auto MakeConstantForBuiltinCall(EvalContext& eval_context,
       // Allow errors to be diagnosed for both sides of the operator before
       // returning here if any error occurred on either side.
       if (!lhs_facet_type_id.has_value() || !rhs_facet_type_id.has_value()) {
-        return SemIR::ErrorInst::SingletonConstantId;
+        return SemIR::ErrorInst::ConstantId;
       }
       // Reuse one of the argument instructions if nothing has changed.
       if (lhs_facet_type_id == rhs_facet_type_id) {
@@ -1438,8 +1436,7 @@ static auto MakeConstantForBuiltinCall(EvalContext& eval_context,
     }
 
     case SemIR::BuiltinFunctionKind::IntLiteralMakeType: {
-      return context.constant_values().Get(
-          SemIR::IntLiteralType::SingletonInstId);
+      return context.constant_values().Get(SemIR::IntLiteralType::InstId);
     }
 
     case SemIR::BuiltinFunctionKind::IntMakeTypeSigned: {
@@ -1458,14 +1455,13 @@ static auto MakeConstantForBuiltinCall(EvalContext& eval_context,
         break;
       }
       if (!ValidateFloatBitWidth(context, loc_id, arg_ids[0])) {
-        return SemIR::ErrorInst::SingletonConstantId;
+        return SemIR::ErrorInst::ConstantId;
       }
-      return context.constant_values().Get(
-          SemIR::LegacyFloatType::SingletonInstId);
+      return context.constant_values().Get(SemIR::LegacyFloatType::InstId);
     }
 
     case SemIR::BuiltinFunctionKind::BoolMakeType: {
-      return context.constant_values().Get(SemIR::BoolType::SingletonInstId);
+      return context.constant_values().Get(SemIR::BoolType::InstId);
     }
 
     // Integer conversions.
@@ -1598,7 +1594,7 @@ static auto MakeConstantForCall(EvalContext& eval_context, SemIR::LocId loc_id,
   //
   // TODO: Use a better representation for this.
   if (call.args_id == SemIR::InstBlockId::None) {
-    return SemIR::ErrorInst::SingletonConstantId;
+    return SemIR::ErrorInst::ConstantId;
   }
 
   // Find the constant value of the callee.
@@ -1631,7 +1627,7 @@ static auto MakeConstantForCall(EvalContext& eval_context, SemIR::LocId loc_id,
       ReplaceFieldWithConstantValue(eval_context, &call, &SemIR::Call::args_id,
                                     &phase);
   if (phase == Phase::UnknownDueToError) {
-    return SemIR::ErrorInst::SingletonConstantId;
+    return SemIR::ErrorInst::ConstantId;
   }
 
   // If any operand of the call is non-constant, the call is non-constant.
@@ -1743,7 +1739,7 @@ static auto TryEvalTypedInst(EvalContext& eval_context, SemIR::InstId inst_id,
         // The result is an instruction.
         return MakeConstantResult(
             eval_context.context(),
-            SemIR::InstValue{.type_id = SemIR::InstType::SingletonTypeId,
+            SemIR::InstValue{.type_id = SemIR::InstType::TypeId,
                              .inst_id = result_inst_id},
             Phase::Concrete);
       }
@@ -1880,10 +1876,10 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
   if (auto facet_type = base_facet_inst.TryAs<SemIR::FacetType>()) {
     info = GetConstantFacetTypeInfo(eval_context, facet_type->facet_type_id,
                                     &phase);
-  } else if (base_facet_type_id == SemIR::ErrorInst::SingletonTypeId) {
-    return SemIR::ErrorInst::SingletonConstantId;
+  } else if (base_facet_type_id == SemIR::ErrorInst::TypeId) {
+    return SemIR::ErrorInst::ConstantId;
   } else {
-    CARBON_CHECK(base_facet_type_id == SemIR::TypeType::SingletonTypeId,
+    CARBON_CHECK(base_facet_type_id == SemIR::TypeType::TypeId,
                  "Unexpected type_id: {0}, inst: {1}", base_facet_type_id,
                  base_facet_inst);
   }
@@ -1906,10 +1902,10 @@ auto TryEvalTypedInst<SemIR::WhereExpr>(EvalContext& eval_context,
                          inst_id)) {
         SemIR::ConstantId lhs = eval_context.GetConstantValue(impls->lhs_id);
         SemIR::ConstantId rhs = eval_context.GetConstantValue(impls->rhs_id);
-        if (rhs != SemIR::ErrorInst::SingletonConstantId &&
+        if (rhs != SemIR::ErrorInst::ConstantId &&
             IsPeriodSelf(eval_context, lhs)) {
           auto rhs_inst_id = eval_context.constant_values().GetInstId(rhs);
-          if (rhs_inst_id == SemIR::TypeType::SingletonInstId) {
+          if (rhs_inst_id == SemIR::TypeType::InstId) {
             // `.Self impls type` -> nothing to do.
           } else {
             auto facet_type =
diff --git a/toolchain/check/eval_inst.cpp b/toolchain/check/eval_inst.cpp
index 1eb2919cc98d..b7c88fbb6490 100644
--- a/toolchain/check/eval_inst.cpp
+++ b/toolchain/check/eval_inst.cpp
@@ -124,7 +124,7 @@ auto EvalConstantInst(Context& context, SemIR::ClassDecl inst)
 
   // A non-generic class declaration evaluates to the class type.
   return ConstantEvalResult::NewSamePhase(
-      SemIR::ClassType{.type_id = SemIR::TypeType::SingletonTypeId,
+      SemIR::ClassType{.type_id = SemIR::TypeType::TypeId,
                        .class_id = inst.class_id,
                        .specific_id = SemIR::SpecificId::None});
 }
@@ -311,8 +311,7 @@ auto EvalConstantInst(Context& context, SemIR::NameRef inst)
 
 auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                       SemIR::RequireCompleteType inst) -> ConstantEvalResult {
-  auto witness_type_id =
-      GetSingletonType(context, SemIR::WitnessType::SingletonInstId);
+  auto witness_type_id = GetSingletonType(context, SemIR::WitnessType::InstId);
 
   // If the type is a concrete constant, require it to be complete now.
   auto complete_type_id =
diff --git a/toolchain/check/eval_inst.h b/toolchain/check/eval_inst.h
index 7648bdf68f36..3718c88ea5cb 100644
--- a/toolchain/check/eval_inst.h
+++ b/toolchain/check/eval_inst.h
@@ -87,7 +87,7 @@ class ConstantEvalResult {
 };
 
 constexpr ConstantEvalResult ConstantEvalResult::Error =
-    Existing(SemIR::ErrorInst::SingletonConstantId);
+    Existing(SemIR::ErrorInst::ConstantId);
 
 constexpr ConstantEvalResult ConstantEvalResult::NotConstant =
     ConstantEvalResult(SemIR::ConstantId::NotConstant);
diff --git a/toolchain/check/facet_type.cpp b/toolchain/check/facet_type.cpp
index 1f31b8d4ef71..96fc2560ec42 100644
--- a/toolchain/check/facet_type.cpp
+++ b/toolchain/check/facet_type.cpp
@@ -19,8 +19,7 @@ auto FacetTypeFromInterface(Context& context, SemIR::InterfaceId interface_id,
   SemIR::FacetTypeId facet_type_id = context.facet_types().Add(
       SemIR::FacetTypeInfo{.extend_constraints = {{interface_id, specific_id}},
                            .other_requirements = false});
-  return {.type_id = SemIR::TypeType::SingletonTypeId,
-          .facet_type_id = facet_type_id};
+  return {.type_id = SemIR::TypeType::TypeId, .facet_type_id = facet_type_id};
 }
 
 // Returns `true` if the `FacetAccessWitness` of `witness_id` matches
@@ -69,7 +68,7 @@ auto InitialFacetTypeImplWitness(
 
   auto facet_type_id =
       context.types().GetTypeIdForTypeInstId(facet_type_inst_id);
-  CARBON_CHECK(facet_type_id != SemIR::ErrorInst::SingletonTypeId);
+  CARBON_CHECK(facet_type_id != SemIR::ErrorInst::TypeId);
   auto facet_type = context.types().GetAs<SemIR::FacetType>(facet_type_id);
   // TODO: This is currently a copy because I'm not sure whether anything could
   // cause the facet type store to resize before we are done with it.
@@ -82,8 +81,7 @@ auto InitialFacetTypeImplWitness(
          .impl_id = SemIR::ImplId::None});
     return AddInst<SemIR::ImplWitness>(
         context, witness_loc_id,
-        {.type_id =
-             GetSingletonType(context, SemIR::WitnessType::SingletonInstId),
+        {.type_id = GetSingletonType(context, SemIR::WitnessType::InstId),
          .witness_table_id = witness_table_inst_id,
          .specific_id = self_specific_id});
   }
@@ -94,7 +92,7 @@ auto InitialFacetTypeImplWitness(
                                  context, witness_loc_id, facet_type_inst_id,
                                  is_definition);
                            })) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   const auto& interface =
@@ -115,7 +113,7 @@ auto InitialFacetTypeImplWitness(
         context.inst_blocks().AddUninitialized(assoc_entities.size());
     table = context.inst_blocks().GetMutable(elements_id);
     for (auto& uninit : table) {
-      uninit = SemIR::ImplWitnessTablePlaceholder::SingletonInstId;
+      uninit = SemIR::ImplWitnessTablePlaceholder::InstId;
     }
 
     auto witness_table_inst_id = AddInst<SemIR::ImplWitnessTable>(
@@ -124,8 +122,7 @@ auto InitialFacetTypeImplWitness(
 
     witness_inst_id = AddInst<SemIR::ImplWitness>(
         context, witness_loc_id,
-        {.type_id =
-             GetSingletonType(context, SemIR::WitnessType::SingletonInstId),
+        {.type_id = GetSingletonType(context, SemIR::WitnessType::InstId),
          .witness_table_id = witness_table_inst_id,
          .specific_id = self_specific_id});
   }
@@ -138,23 +135,23 @@ auto InitialFacetTypeImplWitness(
       continue;
     }
     auto& table_entry = table[access.index.index];
-    if (table_entry == SemIR::ErrorInst::SingletonInstId) {
+    if (table_entry == SemIR::ErrorInst::InstId) {
       // Don't overwrite an error value. This prioritizes not generating
       // multiple errors for one associated constant over picking a value
       // for it to use to attempt recovery.
       continue;
     }
     auto rewrite_inst_id = rewrite.rhs_id;
-    if (rewrite_inst_id == SemIR::ErrorInst::SingletonInstId) {
-      table_entry = SemIR::ErrorInst::SingletonInstId;
+    if (rewrite_inst_id == SemIR::ErrorInst::InstId) {
+      table_entry = SemIR::ErrorInst::InstId;
       continue;
     }
 
     auto decl_id = context.constant_values().GetConstantInstId(
         assoc_entities[access.index.index]);
     CARBON_CHECK(decl_id.has_value(), "Non-constant associated entity");
-    if (decl_id == SemIR::ErrorInst::SingletonInstId) {
-      table_entry = SemIR::ErrorInst::SingletonInstId;
+    if (decl_id == SemIR::ErrorInst::InstId) {
+      table_entry = SemIR::ErrorInst::InstId;
       continue;
     }
 
@@ -170,11 +167,11 @@ auto InitialFacetTypeImplWitness(
                         SemIR::NameId);
       context.emitter().Emit(facet_type_inst_id, RewriteForAssociatedFunction,
                              fn.name_id);
-      table_entry = SemIR::ErrorInst::SingletonInstId;
+      table_entry = SemIR::ErrorInst::InstId;
       continue;
     }
 
-    if (table_entry != SemIR::ImplWitnessTablePlaceholder::SingletonInstId) {
+    if (table_entry != SemIR::ImplWitnessTablePlaceholder::InstId) {
       if (table_entry != rewrite_inst_id) {
         // TODO: Figure out how to print the two different values
         // `const_id` & `rewrite_inst_id` in the diagnostic
@@ -189,7 +186,7 @@ auto InitialFacetTypeImplWitness(
             facet_type_inst_id, AssociatedConstantWithDifferentValues,
             assoc_const.name_id, table_entry, rewrite_inst_id);
       }
-      table_entry = SemIR::ErrorInst::SingletonInstId;
+      table_entry = SemIR::ErrorInst::InstId;
       continue;
     }
 
@@ -227,7 +224,7 @@ auto InitialFacetTypeImplWitness(
         context.emitter().Emit(
             facet_type_inst_id, AssociatedConstantNotConstantAfterConversion,
             assoc_const.name_id, rewrite_inst_id, assoc_const_type_id);
-        rewrite_inst_id = SemIR::ErrorInst::SingletonInstId;
+        rewrite_inst_id = SemIR::ErrorInst::InstId;
       }
     }
 
@@ -268,8 +265,7 @@ auto AllocateFacetTypeImplWitness(Context& context,
   }
 
   llvm::SmallVector<SemIR::InstId> empty_table(
-      assoc_entities.size(),
-      SemIR::ImplWitnessTablePlaceholder::SingletonInstId);
+      assoc_entities.size(), SemIR::ImplWitnessTablePlaceholder::InstId);
   context.inst_blocks().ReplacePlaceholder(witness_id, empty_table);
 }
 
diff --git a/toolchain/check/function.cpp b/toolchain/check/function.cpp
index 8af6e7ba36c1..d9b7421c1a7c 100644
--- a/toolchain/check/function.cpp
+++ b/toolchain/check/function.cpp
@@ -33,8 +33,8 @@ auto CheckFunctionTypeMatches(Context& context,
       new_function.GetDeclaredReturnType(context.sem_ir());
   auto prev_return_type_id =
       prev_function.GetDeclaredReturnType(context.sem_ir(), prev_specific_id);
-  if (new_return_type_id == SemIR::ErrorInst::SingletonTypeId ||
-      prev_return_type_id == SemIR::ErrorInst::SingletonTypeId) {
+  if (new_return_type_id == SemIR::ErrorInst::TypeId ||
+      prev_return_type_id == SemIR::ErrorInst::TypeId) {
     return false;
   }
   if (!context.types().AreEqualAcrossDeclarations(new_return_type_id,
diff --git a/toolchain/check/generic.cpp b/toolchain/check/generic.cpp
index 6300e3e86296..af09b794c148 100644
--- a/toolchain/check/generic.cpp
+++ b/toolchain/check/generic.cpp
@@ -344,7 +344,7 @@ static auto MakeGenericEvalBlock(Context& context, SemIR::GenericId generic_id,
           context, generic_id, region, context.insts().GetLocId(inst_id),
           constants_in_generic, inside_redeclaration, inst.type_id());
       // If the generic declaration is invalid, it can result in an error.
-      if (type_id == SemIR::ErrorInst::SingletonTypeId) {
+      if (type_id == SemIR::ErrorInst::TypeId) {
         break;
       }
       // TODO: Eventually, completeness requirements should be modeled as
diff --git a/toolchain/check/handle_alias.cpp b/toolchain/check/handle_alias.cpp
index 20d2cca90a65..994fea53f3cf 100644
--- a/toolchain/check/handle_alias.cpp
+++ b/toolchain/check/handle_alias.cpp
@@ -56,8 +56,8 @@ auto HandleParseNode(Context& context, Parse::AliasId /*node_id*/) -> bool {
     CARBON_DIAGNOSTIC(AliasRequiresNameRef, Error,
                       "alias initializer must be a name reference");
     context.emitter().Emit(expr_node, AliasRequiresNameRef);
-    alias_type_id = SemIR::ErrorInst::SingletonTypeId;
-    alias_value_id = SemIR::ErrorInst::SingletonInstId;
+    alias_type_id = SemIR::ErrorInst::TypeId;
+    alias_value_id = SemIR::ErrorInst::InstId;
   }
   auto alias_id = AddInst<SemIR::BindAlias>(context, name_context.loc_id,
                                             {.type_id = alias_type_id,
diff --git a/toolchain/check/handle_array.cpp b/toolchain/check/handle_array.cpp
index 444f1e3ea592..30ec24fe00cb 100644
--- a/toolchain/check/handle_array.cpp
+++ b/toolchain/check/handle_array.cpp
@@ -43,16 +43,16 @@ auto HandleParseNode(Context& context, Parse::ArrayExprId node_id) -> bool {
   if (!context.constant_values().Get(bound_inst_id).is_constant()) {
     CARBON_DIAGNOSTIC(InvalidArrayExpr, Error, "array bound is not a constant");
     context.emitter().Emit(bound_inst_id, InvalidArrayExpr);
-    context.node_stack().Push(node_id, SemIR::ErrorInst::SingletonInstId);
+    context.node_stack().Push(node_id, SemIR::ErrorInst::InstId);
     return true;
   }
 
   bound_inst_id = ConvertToValueOfType(
       context, context.insts().GetLocId(bound_inst_id), bound_inst_id,
-      GetSingletonType(context, SemIR::IntLiteralType::SingletonInstId));
+      GetSingletonType(context, SemIR::IntLiteralType::InstId));
   AddInstAndPush<SemIR::ArrayType>(
       context, node_id,
-      {.type_id = SemIR::TypeType::SingletonTypeId,
+      {.type_id = SemIR::TypeType::TypeId,
        .bound_id = bound_inst_id,
        .element_type_inst_id = element_type.inst_id});
   return true;
diff --git a/toolchain/check/handle_binding_pattern.cpp b/toolchain/check/handle_binding_pattern.cpp
index 7958eab46dc9..5c921015f4fd 100644
--- a/toolchain/check/handle_binding_pattern.cpp
+++ b/toolchain/check/handle_binding_pattern.cpp
@@ -142,8 +142,8 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
           return context.emitter().Build(type_node, AbstractTypeInFieldDecl,
                                          cast_type_id);
         });
-    if (cast_type_id == SemIR::ErrorInst::SingletonTypeId) {
-      cast_type_inst_id = SemIR::ErrorInst::SingletonTypeInstId;
+    if (cast_type_id == SemIR::ErrorInst::TypeId) {
+      cast_type_inst_id = SemIR::ErrorInst::TypeInstId;
     }
     auto binding_id =
         context.parse_tree().As<Parse::VarBindingPatternId>(node_id);
@@ -266,11 +266,11 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
       auto result_inst_id = SemIR::InstId::None;
       if (had_error) {
         if (name_id != SemIR::NameId::Underscore) {
-          AddNameToLookup(context, name_id, SemIR::ErrorInst::SingletonInstId);
+          AddNameToLookup(context, name_id, SemIR::ErrorInst::InstId);
         }
         // Replace the parameter with `ErrorInst` so that we don't try
         // constructing a generic based on it.
-        result_inst_id = SemIR::ErrorInst::SingletonInstId;
+        result_inst_id = SemIR::ErrorInst::InstId;
       } else {
         result_inst_id = make_binding_pattern();
         if (node_kind == Parse::NodeKind::LetBindingPattern) {
@@ -381,8 +381,7 @@ auto HandleParseNode(Context& context, Parse::AddrId node_id) -> bool {
     if (pointer_type) {
       auto addr_pattern_id = AddPatternInst<SemIR::AddrPattern>(
           context, node_id,
-          {.type_id = SemIR::AutoType::SingletonTypeId,
-           .inner_id = param_pattern_id});
+          {.type_id = SemIR::AutoType::TypeId, .inner_id = param_pattern_id});
       context.node_stack().Push(node_id, addr_pattern_id);
     } else {
       CARBON_DIAGNOSTIC(
diff --git a/toolchain/check/handle_choice.cpp b/toolchain/check/handle_choice.cpp
index 97223a7406bc..3bc2ed4221aa 100644
--- a/toolchain/check/handle_choice.cpp
+++ b/toolchain/check/handle_choice.cpp
@@ -50,10 +50,9 @@ auto HandleParseNode(Context& context, Parse::ChoiceDefinitionStartId node_id)
 
   // Choices create a ClassId, since they ultimately turn into a class with
   // methods and some builtin impls.
-  auto class_decl =
-      SemIR::ClassDecl{.type_id = SemIR::TypeType::SingletonTypeId,
-                       .class_id = SemIR::ClassId::None,
-                       .decl_block_id = decl_block_id};
+  auto class_decl = SemIR::ClassDecl{.type_id = SemIR::TypeType::TypeId,
+                                     .class_id = SemIR::ClassId::None,
+                                     .decl_block_id = decl_block_id};
   auto class_decl_id = AddPlaceholderInst(context, node_id, class_decl);
 
   context.decl_name_stack().AddNameOrDiagnose(name_context, class_decl_id,
@@ -281,8 +280,7 @@ auto HandleParseNode(Context& context, Parse::ChoiceDefinitionId node_id)
   auto choice_witness_id = AddInst(
       context, node_id,
       SemIR::CompleteTypeWitness{
-          .type_id =
-              GetSingletonType(context, SemIR::WitnessType::SingletonInstId),
+          .type_id = GetSingletonType(context, SemIR::WitnessType::InstId),
           .object_repr_type_inst_id =
               context.types().GetInstId(GetStructType(context, fields_id))});
   // Note: avoid storing a reference to the returned Class, since it may be
diff --git a/toolchain/check/handle_class.cpp b/toolchain/check/handle_class.cpp
index 478a168183cb..98ce2ea50ea5 100644
--- a/toolchain/check/handle_class.cpp
+++ b/toolchain/check/handle_class.cpp
@@ -208,10 +208,9 @@ static auto BuildClassDecl(Context& context, Parse::AnyClassDeclId node_id,
   auto decl_block_id = context.inst_block_stack().Pop();
 
   // Add the class declaration.
-  auto class_decl =
-      SemIR::ClassDecl{.type_id = SemIR::TypeType::SingletonTypeId,
-                       .class_id = SemIR::ClassId::None,
-                       .decl_block_id = decl_block_id};
+  auto class_decl = SemIR::ClassDecl{.type_id = SemIR::TypeType::TypeId,
+                                     .class_id = SemIR::ClassId::None,
+                                     .decl_block_id = decl_block_id};
   auto class_decl_id = AddPlaceholderInst(context, node_id, class_decl);
 
   // TODO: Store state regarding is_extern.
@@ -388,8 +387,8 @@ auto HandleParseNode(Context& context, Parse::AdaptDeclId node_id) -> bool {
         return context.emitter().Build(node_id, AbstractTypeInAdaptDecl,
                                        adapted_type_inst_id);
       });
-  if (adapted_type_id == SemIR::ErrorInst::SingletonTypeId) {
-    adapted_type_inst_id = SemIR::ErrorInst::SingletonTypeInstId;
+  if (adapted_type_id == SemIR::ErrorInst::TypeId) {
+    adapted_type_inst_id = SemIR::ErrorInst::TypeInstId;
   }
 
   // Build a SemIR representation for the declaration.
@@ -425,10 +424,9 @@ struct BaseInfo {
   SemIR::NameScopeId scope_id;
   SemIR::TypeInstId inst_id;
 };
-constexpr BaseInfo BaseInfo::Error = {
-    .type_id = SemIR::ErrorInst::SingletonTypeId,
-    .scope_id = SemIR::NameScopeId::None,
-    .inst_id = SemIR::ErrorInst::SingletonTypeInstId};
+constexpr BaseInfo BaseInfo::Error = {.type_id = SemIR::ErrorInst::TypeId,
+                                      .scope_id = SemIR::NameScopeId::None,
+                                      .inst_id = SemIR::ErrorInst::TypeInstId};
 }  // namespace
 
 // Diagnoses an attempt to derive from a final type.
@@ -453,7 +451,7 @@ static auto CheckBaseType(Context& context, Parse::NodeId node_id,
                                    base_type_inst_id);
   });
 
-  if (base_type_id == SemIR::ErrorInst::SingletonTypeId) {
+  if (base_type_id == SemIR::ErrorInst::TypeId) {
     return BaseInfo::Error;
   }
 
@@ -530,7 +528,7 @@ auto HandleParseNode(Context& context, Parse::BaseDeclId node_id) -> bool {
                                 .base_type_inst_id = base_info.inst_id,
                                 .index = SemIR::ElementIndex::None});
 
-  if (base_info.type_id != SemIR::ErrorInst::SingletonTypeId) {
+  if (base_info.type_id != SemIR::ErrorInst::TypeId) {
     auto base_class_info = context.classes().Get(
         context.types().GetAs<SemIR::ClassType>(base_info.type_id).class_id);
     class_info.is_dynamic |= base_class_info.is_dynamic;
diff --git a/toolchain/check/handle_function.cpp b/toolchain/check/handle_function.cpp
index cef48efa88e7..d2ddccfaf5b8 100644
--- a/toolchain/check/handle_function.cpp
+++ b/toolchain/check/handle_function.cpp
@@ -560,7 +560,7 @@ static auto CheckFunctionDefinitionSignature(Context& context,
 
   // Check the parameter types are complete.
   for (auto param_ref_id : params_to_complete) {
-    if (param_ref_id == SemIR::ErrorInst::SingletonInstId) {
+    if (param_ref_id == SemIR::ErrorInst::InstId) {
       continue;
     }
 
diff --git a/toolchain/check/handle_if_expr.cpp b/toolchain/check/handle_if_expr.cpp
index 1c319ed57889..c511928c1936 100644
--- a/toolchain/check/handle_if_expr.cpp
+++ b/toolchain/check/handle_if_expr.cpp
@@ -38,7 +38,7 @@ static auto DecayIntLiteralToSizedInt(Context& context, Parse::NodeId node_id,
                                       SemIR::InstId operand_id)
     -> SemIR::InstId {
   if (context.types().GetInstId(context.insts().Get(operand_id).type_id()) ==
-      SemIR::IntLiteralType::SingletonInstId) {
+      SemIR::IntLiteralType::InstId) {
     operand_id = ConvertToValueOfType(
         context, node_id, operand_id,
         MakeIntType(context, node_id, SemIR::IntKind::Signed,
diff --git a/toolchain/check/handle_impl.cpp b/toolchain/check/handle_impl.cpp
index b19f943f5244..587b56519851 100644
--- a/toolchain/check/handle_impl.cpp
+++ b/toolchain/check/handle_impl.cpp
@@ -100,7 +100,7 @@ auto HandleParseNode(Context& context, Parse::DefaultSelfImplAsId node_id)
     CARBON_DIAGNOSTIC(ImplAsOutsideClass, Error,
                       "`impl as` can only be used in a class");
     context.emitter().Emit(node_id, ImplAsOutsideClass);
-    self_type_id = SemIR::ErrorInst::SingletonTypeId;
+    self_type_id = SemIR::ErrorInst::TypeId;
   }
 
   // Build the implicit access to the enclosing `Self`.
@@ -111,7 +111,7 @@ auto HandleParseNode(Context& context, Parse::DefaultSelfImplAsId node_id)
   // handling of the `Self` expression.
   auto self_inst_id = AddTypeInst(
       context, node_id,
-      SemIR::NameRef{.type_id = SemIR::TypeType::SingletonTypeId,
+      SemIR::NameRef{.type_id = SemIR::TypeType::TypeId,
                      .name_id = SemIR::NameId::SelfType,
                      .value_id = context.types().GetInstId(self_type_id)});
 
@@ -184,7 +184,7 @@ static auto ExtendImpl(Context& context, Parse::NodeId extend_node,
   }
 
   const auto& impl = context.impls().Get(impl_id);
-  if (impl.witness_id == SemIR::ErrorInst::SingletonInstId) {
+  if (impl.witness_id == SemIR::ErrorInst::InstId) {
     parent_scope.set_has_error();
   } else {
     bool is_complete = RequireCompleteType(
@@ -227,8 +227,7 @@ static auto PopImplIntroducerAndParamsAsNameComponent(
     CARBON_CHECK(call_params_id == SemIR::InstBlockId::Empty ||
                  llvm::all_of(context.inst_blocks().Get(call_params_id),
                               [](SemIR::InstId inst_id) {
-                                return inst_id ==
-                                       SemIR::ErrorInst::SingletonInstId;
+                                return inst_id == SemIR::ErrorInst::InstId;
                               }));
   }
 
@@ -321,7 +320,7 @@ static auto CheckConstraintIsInterface(Context& context,
                                        SemIR::TypeInstId constraint_id)
     -> SemIR::SpecificInterface {
   auto facet_type_id = context.types().GetTypeIdForTypeInstId(constraint_id);
-  if (facet_type_id == SemIR::ErrorInst::SingletonTypeId) {
+  if (facet_type_id == SemIR::ErrorInst::TypeId) {
     return SemIR::SpecificInterface::None;
   }
   auto facet_type = context.types().TryGetAs<SemIR::FacetType>(facet_type_id);
@@ -415,7 +414,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
       impl_info.witness_id =
           ImplWitnessForDeclaration(context, impl_info, is_definition);
     } else {
-      impl_info.witness_id = SemIR::ErrorInst::SingletonInstId;
+      impl_info.witness_id = SemIR::ErrorInst::InstId;
       // TODO: We might also want to mark that the name scope for the impl has
       // an error -- at least once we start making name lookups within the impl
       // also look into the facet (eg, so you can name associated constants from
@@ -435,14 +434,14 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
     if (name.implicit_param_patterns_id.has_value()) {
       for (auto inst_id :
            context.inst_blocks().Get(name.implicit_param_patterns_id)) {
-        if (inst_id == SemIR::ErrorInst::SingletonInstId) {
+        if (inst_id == SemIR::ErrorInst::InstId) {
           has_error_in_implicit_pattern = true;
           break;
         }
       }
     }
     if (impl_info.generic_id.has_value() && !has_error_in_implicit_pattern &&
-        impl_info.witness_id != SemIR::ErrorInst::SingletonInstId) {
+        impl_info.witness_id != SemIR::ErrorInst::InstId) {
       context.inst_block_stack().Push();
       auto deduced_specific_id = DeduceImplArguments(
           context, node_id,
@@ -471,7 +470,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
         // diagnostics.
         FillImplWitnessWithErrors(context, impl_info);
         context.impls().Get(impl_decl.impl_id).witness_id =
-            SemIR::ErrorInst::SingletonInstId;
+            SemIR::ErrorInst::InstId;
       }
     }
   } else {
@@ -484,13 +483,13 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
   ReplaceInstBeforeConstantUse(context, impl_decl_id, impl_decl);
 
   // For an `extend impl` declaration, mark the impl as extending this `impl`.
-  if (self_type_id != SemIR::ErrorInst::SingletonTypeId &&
+  if (self_type_id != SemIR::ErrorInst::TypeId &&
       introducer.modifier_set.HasAnyOf(KeywordModifierSet::Extend)) {
     auto extend_node = introducer.modifier_node_id(ModifierOrder::Decl);
     if (impl_info.generic_id.has_value()) {
       constraint_type_inst_id = AddTypeInst<SemIR::SpecificConstant>(
           context, context.insts().GetLocId(constraint_type_inst_id),
-          {.type_id = SemIR::TypeType::SingletonTypeId,
+          {.type_id = SemIR::TypeType::TypeId,
            .inst_id = constraint_type_inst_id,
            .specific_id =
                context.generics().GetSelfSpecific(impl_info.generic_id)});
@@ -501,7 +500,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
       // Don't allow the invalid impl to be used.
       FillImplWitnessWithErrors(context, impl_info);
       context.impls().Get(impl_decl.impl_id).witness_id =
-          SemIR::ErrorInst::SingletonInstId;
+          SemIR::ErrorInst::InstId;
     }
   }
 
@@ -510,7 +509,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
   // there is an error that would prevent the impl from being legal to define.
   if (!is_definition && !invalid_redeclaration &&
       context.impls().Get(impl_decl.impl_id).witness_id !=
-          SemIR::ErrorInst::SingletonInstId) {
+          SemIR::ErrorInst::InstId) {
     context.definitions_required_by_decl().push_back(impl_decl_id);
   }
 
diff --git a/toolchain/check/handle_index.cpp b/toolchain/check/handle_index.cpp
index e7e8d009f110..b53c536c5af0 100644
--- a/toolchain/check/handle_index.cpp
+++ b/toolchain/check/handle_index.cpp
@@ -102,7 +102,7 @@ static auto PerformIndexWith(Context& context, Parse::NodeId node_id,
     CARBON_DIAGNOSTIC(TypeNotIndexable, Error,
                       "type {0} does not support indexing", SemIR::TypeId);
     context.emitter().Emit(node_id, TypeNotIndexable, operand_type_id);
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   Operator op{
@@ -171,8 +171,8 @@ auto HandleParseNode(Context& context, Parse::IndexExprId node_id) -> bool {
     }
 
     default: {
-      auto elem_id = SemIR::ErrorInst::SingletonInstId;
-      if (operand_type_id != SemIR::ErrorInst::SingletonTypeId) {
+      auto elem_id = SemIR::ErrorInst::InstId;
+      if (operand_type_id != SemIR::ErrorInst::TypeId) {
         elem_id = PerformIndexWith(context, node_id, operand_inst_id,
                                    operand_type_id, index_inst_id);
       }
diff --git a/toolchain/check/handle_interface.cpp b/toolchain/check/handle_interface.cpp
index e281dfb60fb4..3b7701a73944 100644
--- a/toolchain/check/handle_interface.cpp
+++ b/toolchain/check/handle_interface.cpp
@@ -52,9 +52,8 @@ static auto BuildInterfaceDecl(Context& context,
   auto decl_block_id = context.inst_block_stack().Pop();
 
   // Add the interface declaration.
-  auto interface_decl =
-      SemIR::InterfaceDecl{SemIR::TypeType::SingletonTypeId,
-                           SemIR::InterfaceId::None, decl_block_id};
+  auto interface_decl = SemIR::InterfaceDecl{
+      SemIR::TypeType::TypeId, SemIR::InterfaceId::None, decl_block_id};
   auto interface_decl_id = AddPlaceholderInst(context, node_id, interface_decl);
 
   SemIR::Interface interface_info = {name_context.MakeEntityWithParamsBase(
diff --git a/toolchain/check/handle_let_and_var.cpp b/toolchain/check/handle_let_and_var.cpp
index 5a277d7d69d5..ffd002e6a66a 100644
--- a/toolchain/check/handle_let_and_var.cpp
+++ b/toolchain/check/handle_let_and_var.cpp
@@ -281,7 +281,7 @@ static auto FinishAssociatedConstant(Context& context, Parse::LetDeclId node_id,
   auto decl = context.insts().TryGetAs<SemIR::AssociatedConstantDecl>(
       decl_info.pattern_id);
   if (!decl) {
-    if (decl_info.pattern_id != SemIR::ErrorInst::SingletonInstId) {
+    if (decl_info.pattern_id != SemIR::ErrorInst::InstId) {
       CARBON_DIAGNOSTIC(ExpectedSymbolicBindingInAssociatedConstant, Error,
                         "pattern in associated constant declaration must be a "
                         "single `:!` binding");
diff --git a/toolchain/check/handle_literal.cpp b/toolchain/check/handle_literal.cpp
index 6a8af3f228b8..4ecba66285ea 100644
--- a/toolchain/check/handle_literal.cpp
+++ b/toolchain/check/handle_literal.cpp
@@ -18,7 +18,7 @@ auto HandleParseNode(Context& context, Parse::BoolLiteralFalseId node_id)
     -> bool {
   AddInstAndPush<SemIR::BoolLiteral>(
       context, node_id,
-      {.type_id = GetSingletonType(context, SemIR::BoolType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::BoolType::InstId),
        .value = SemIR::BoolValue::False});
   return true;
 }
@@ -27,7 +27,7 @@ auto HandleParseNode(Context& context, Parse::BoolLiteralTrueId node_id)
     -> bool {
   AddInstAndPush<SemIR::BoolLiteral>(
       context, node_id,
-      {.type_id = GetSingletonType(context, SemIR::BoolType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::BoolType::InstId),
        .value = SemIR::BoolValue::True});
   return true;
 }
@@ -58,7 +58,7 @@ auto HandleParseNode(Context& context, Parse::RealLiteralId node_id) -> bool {
                       llvm::APSInt);
     context.emitter().Emit(node_id, RealMantissaTooLargeForI64,
                            llvm::APSInt(real_value.mantissa, true));
-    context.node_stack().Push(node_id, SemIR::ErrorInst::SingletonInstId);
+    context.node_stack().Push(node_id, SemIR::ErrorInst::InstId);
     return true;
   }
 
@@ -68,7 +68,7 @@ auto HandleParseNode(Context& context, Parse::RealLiteralId node_id) -> bool {
                       llvm::APSInt);
     context.emitter().Emit(node_id, RealExponentTooLargeForI64,
                            llvm::APSInt(real_value.exponent, false));
-    context.node_stack().Push(node_id, SemIR::ErrorInst::SingletonInstId);
+    context.node_stack().Push(node_id, SemIR::ErrorInst::InstId);
     return true;
   }
 
@@ -79,8 +79,7 @@ auto HandleParseNode(Context& context, Parse::RealLiteralId node_id) -> bool {
   auto float_id = context.sem_ir().floats().Add(llvm::APFloat(double_val));
   AddInstAndPush<SemIR::FloatLiteral>(
       context, node_id,
-      {.type_id =
-           GetSingletonType(context, SemIR::LegacyFloatType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::LegacyFloatType::InstId),
        .float_id = float_id});
   return true;
 }
@@ -88,7 +87,7 @@ auto HandleParseNode(Context& context, Parse::RealLiteralId node_id) -> bool {
 auto HandleParseNode(Context& context, Parse::StringLiteralId node_id) -> bool {
   AddInstAndPush<SemIR::StringLiteral>(
       context, node_id,
-      {.type_id = GetSingletonType(context, SemIR::StringType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::StringType::InstId),
        .string_literal_id = context.tokens().GetStringLiteralValue(
            context.parse_tree().node_token(node_id))});
   return true;
@@ -155,13 +154,13 @@ auto HandleParseNode(Context& context, Parse::FloatTypeLiteralId node_id)
 
 auto HandleParseNode(Context& context, Parse::StringTypeLiteralId node_id)
     -> bool {
-  context.node_stack().Push(node_id, SemIR::StringType::SingletonInstId);
+  context.node_stack().Push(node_id, SemIR::StringType::InstId);
   return true;
 }
 
 auto HandleParseNode(Context& context, Parse::TypeTypeLiteralId node_id)
     -> bool {
-  context.node_stack().Push(node_id, SemIR::TypeType::SingletonInstId);
+  context.node_stack().Push(node_id, SemIR::TypeType::InstId);
   return true;
 }
 
diff --git a/toolchain/check/handle_name.cpp b/toolchain/check/handle_name.cpp
index 06d1abaa6e74..33e1ec68beef 100644
--- a/toolchain/check/handle_name.cpp
+++ b/toolchain/check/handle_name.cpp
@@ -286,8 +286,7 @@ auto HandleParseNode(Context& context, Parse::DesignatorExprId node_id)
 auto HandleParseNode(Context& context, Parse::PackageExprId node_id) -> bool {
   AddInstAndPush<SemIR::NameRef>(
       context, node_id,
-      {.type_id =
-           GetSingletonType(context, SemIR::NamespaceType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::NamespaceType::InstId),
        .name_id = SemIR::NameId::PackageNamespace,
        .value_id = SemIR::Namespace::PackageInstId});
   return true;
diff --git a/toolchain/check/handle_namespace.cpp b/toolchain/check/handle_namespace.cpp
index 32dc9394e942..9aa89bed7623 100644
--- a/toolchain/check/handle_namespace.cpp
+++ b/toolchain/check/handle_namespace.cpp
@@ -39,9 +39,9 @@ auto HandleParseNode(Context& context, Parse::NamespaceId node_id) -> bool {
       context.decl_introducer_state_stack().Pop<Lex::TokenKind::Namespace>();
   LimitModifiersOnDecl(context, introducer, KeywordModifierSet::None);
 
-  auto namespace_inst = SemIR::Namespace{
-      GetSingletonType(context, SemIR::NamespaceType::SingletonInstId),
-      SemIR::NameScopeId::None, SemIR::InstId::None};
+  auto namespace_inst =
+      SemIR::Namespace{GetSingletonType(context, SemIR::NamespaceType::InstId),
+                       SemIR::NameScopeId::None, SemIR::InstId::None};
   auto namespace_id = AddPlaceholderInst(context, node_id, namespace_inst);
 
   SemIR::ScopeLookupResult lookup_result =
diff --git a/toolchain/check/handle_operator.cpp b/toolchain/check/handle_operator.cpp
index 8d1ca9ef9026..31974d004c15 100644
--- a/toolchain/check/handle_operator.cpp
+++ b/toolchain/check/handle_operator.cpp
@@ -221,8 +221,7 @@ auto HandleParseNode(Context& context, Parse::PostfixOperatorStarId node_id)
   auto inner_type = ExprAsType(context, node_id, value_id);
   AddInstAndPush<SemIR::PointerType>(
       context, node_id,
-      {.type_id = SemIR::TypeType::SingletonTypeId,
-       .pointee_id = inner_type.inst_id});
+      {.type_id = SemIR::TypeType::TypeId, .pointee_id = inner_type.inst_id});
   return true;
 }
 
@@ -239,13 +238,13 @@ auto HandleParseNode(Context& context, Parse::PrefixOperatorAmpId node_id)
       CARBON_DIAGNOSTIC(AddrOfEphemeralRef, Error,
                         "cannot take the address of a temporary object");
       context.emitter().Emit(TokenOnly(node_id), AddrOfEphemeralRef);
-      value_id = SemIR::ErrorInst::SingletonInstId;
+      value_id = SemIR::ErrorInst::InstId;
       break;
     default:
       CARBON_DIAGNOSTIC(AddrOfNonRef, Error,
                         "cannot take the address of non-reference expression");
       context.emitter().Emit(TokenOnly(node_id), AddrOfNonRef);
-      value_id = SemIR::ErrorInst::SingletonInstId;
+      value_id = SemIR::ErrorInst::InstId;
       break;
   }
   // TODO: Preserve spelling of type of operand where possible.
@@ -276,9 +275,9 @@ auto HandleParseNode(Context& context, Parse::PrefixOperatorConstId node_id)
     context.emitter().Emit(node_id, RepeatedConst);
   }
   auto inner_type = ExprAsType(context, node_id, value_id);
-  AddInstAndPush<SemIR::ConstType>(context, node_id,
-                                   {.type_id = SemIR::TypeType::SingletonTypeId,
-                                    .inner_id = inner_type.inst_id});
+  AddInstAndPush<SemIR::ConstType>(
+      context, node_id,
+      {.type_id = SemIR::TypeType::TypeId, .inner_id = inner_type.inst_id});
   return true;
 }
 
@@ -330,7 +329,7 @@ auto HandleParseNode(Context& context, Parse::PrefixOperatorStarId node_id)
             TokenOnly(node_id), DerefOfNonPointer, not_pointer_type_id);
 
         // TODO: Check for any facet here, rather than only a type.
-        if (not_pointer_type_id == SemIR::TypeType::SingletonTypeId) {
+        if (not_pointer_type_id == SemIR::TypeType::TypeId) {
           CARBON_DIAGNOSTIC(
               DerefOfType, Note,
               "to form a pointer type, write the `*` after the pointee type");
diff --git a/toolchain/check/handle_struct.cpp b/toolchain/check/handle_struct.cpp
index ce8c83509e19..1e3f3947f5e1 100644
--- a/toolchain/check/handle_struct.cpp
+++ b/toolchain/check/handle_struct.cpp
@@ -142,7 +142,7 @@ auto HandleParseNode(Context& context, Parse::StructLiteralId node_id) -> bool {
 
   if (DiagnoseDuplicateNames(context, field_name_nodes, fields,
                              /*is_struct_type_literal=*/false)) {
-    context.node_stack().Push(node_id, SemIR::ErrorInst::SingletonInstId);
+    context.node_stack().Push(node_id, SemIR::ErrorInst::InstId);
   } else {
     auto type_id = GetStructType(
         context, context.struct_type_fields().AddCanonical(fields));
@@ -168,12 +168,12 @@ auto HandleParseNode(Context& context, Parse::StructTypeLiteralId node_id)
 
   if (DiagnoseDuplicateNames(context, field_name_nodes, fields,
                              /*is_struct_type_literal=*/true)) {
-    context.node_stack().Push(node_id, SemIR::ErrorInst::SingletonInstId);
+    context.node_stack().Push(node_id, SemIR::ErrorInst::InstId);
   } else {
     auto fields_id = context.struct_type_fields().AddCanonical(fields);
     AddInstAndPush<SemIR::StructType>(
         context, node_id,
-        {.type_id = SemIR::TypeType::SingletonTypeId, .fields_id = fields_id});
+        {.type_id = SemIR::TypeType::TypeId, .fields_id = fields_id});
   }
 
   context.struct_type_fields_stack().PopArray();
diff --git a/toolchain/check/handle_where.cpp b/toolchain/check/handle_where.cpp
index 6666313da8ce..676f14c99591 100644
--- a/toolchain/check/handle_where.cpp
+++ b/toolchain/check/handle_where.cpp
@@ -17,12 +17,12 @@ auto HandleParseNode(Context& context, Parse::WhereOperandId node_id) -> bool {
   auto [self_node, self_id] = context.node_stack().PopExprWithNodeId();
   auto self_type_id = ExprAsType(context, self_node, self_id).type_id;
   // Only facet types may have `where` restrictions.
-  if (self_type_id != SemIR::ErrorInst::SingletonTypeId &&
+  if (self_type_id != SemIR::ErrorInst::TypeId &&
       !context.types().IsFacetType(self_type_id)) {
     CARBON_DIAGNOSTIC(WhereOnNonFacetType, Error,
                       "left argument of `where` operator must be a facet type");
     context.emitter().Emit(self_node, WhereOnNonFacetType);
-    self_type_id = SemIR::ErrorInst::SingletonTypeId;
+    self_type_id = SemIR::ErrorInst::TypeId;
   }
 
   // Introduce a name scope so that we can remove the `.Self` entry we are
@@ -105,13 +105,13 @@ auto HandleParseNode(Context& context, Parse::RequirementImplsId node_id)
   // Check lhs is a facet and rhs is a facet type.
   auto lhs_as_type = ExprAsType(context, lhs_node, lhs_id);
   auto rhs_as_type = ExprAsType(context, rhs_node, rhs_id);
-  if (rhs_as_type.type_id != SemIR::ErrorInst::SingletonTypeId &&
+  if (rhs_as_type.type_id != SemIR::ErrorInst::TypeId &&
       !context.types().IsFacetType(rhs_as_type.type_id)) {
     CARBON_DIAGNOSTIC(
         ImplsOnNonFacetType, Error,
         "right argument of `impls` requirement must be a facet type");
     context.emitter().Emit(rhs_node, ImplsOnNonFacetType);
-    rhs_as_type.inst_id = SemIR::ErrorInst::SingletonTypeInstId;
+    rhs_as_type.inst_id = SemIR::ErrorInst::TypeInstId;
   }
   // TODO: Require that at least one side uses a designator.
   // TODO: For things like `HashSet(.T) as type`, add an implied constraint
@@ -139,7 +139,7 @@ auto HandleParseNode(Context& context, Parse::WhereExprId node_id) -> bool {
       context.node_stack().Pop<Parse::NodeKind::WhereOperand>();
   SemIR::InstBlockId requirements_id = context.args_type_info_stack().Pop();
   AddInstAndPush<SemIR::WhereExpr>(context, node_id,
-                                   {.type_id = SemIR::TypeType::SingletonTypeId,
+                                   {.type_id = SemIR::TypeType::TypeId,
                                     .period_self_id = period_self_id,
                                     .requirements_id = requirements_id});
   return true;
diff --git a/toolchain/check/impl.cpp b/toolchain/check/impl.cpp
index c3b58d906c26..46c43f32e554 100644
--- a/toolchain/check/impl.cpp
+++ b/toolchain/check/impl.cpp
@@ -56,7 +56,7 @@ static auto CheckAssociatedFunctionImplementation(
                            interface_function_type.function_id);
     builder.Emit();
 
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   auto impl_enclosing_specific_id =
@@ -80,7 +80,7 @@ static auto CheckAssociatedFunctionImplementation(
           context.functions().Get(interface_function_type.function_id),
           interface_function_specific_id, /*check_syntax=*/false,
           /*check_self=*/true)) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   return impl_decl_id;
 }
@@ -91,9 +91,9 @@ auto ImplWitnessForDeclaration(Context& context, const SemIR::Impl& impl,
   CARBON_CHECK(!impl.has_definition_started());
 
   auto self_type_id = context.types().GetTypeIdForTypeInstId(impl.self_id);
-  if (self_type_id == SemIR::ErrorInst::SingletonTypeId) {
+  if (self_type_id == SemIR::ErrorInst::TypeId) {
     // When 'impl as' is invalid, the self type is an error.
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   return InitialFacetTypeImplWitness(
@@ -105,7 +105,7 @@ auto ImplWitnessForDeclaration(Context& context, const SemIR::Impl& impl,
 auto ImplWitnessStartDefinition(Context& context, SemIR::Impl& impl) -> void {
   CARBON_CHECK(impl.is_being_defined());
   CARBON_CHECK(impl.witness_id.has_value());
-  if (impl.witness_id == SemIR::ErrorInst::SingletonInstId) {
+  if (impl.witness_id == SemIR::ErrorInst::InstId) {
     return;
   }
   auto witness = context.insts().GetAs<SemIR::ImplWitness>(impl.witness_id);
@@ -143,8 +143,7 @@ auto ImplWitnessStartDefinition(Context& context, SemIR::Impl& impl) -> void {
     CARBON_CHECK(decl_id.has_value(), "Non-constant associated entity");
     if (auto decl =
             context.insts().TryGetAs<SemIR::AssociatedConstantDecl>(decl_id)) {
-      if (witness_value ==
-          SemIR::ImplWitnessTablePlaceholder::SingletonInstId) {
+      if (witness_value == SemIR::ImplWitnessTablePlaceholder::InstId) {
         CARBON_DIAGNOSTIC(ImplAssociatedConstantNeedsValue, Error,
                           "associated constant {0} not given a value in impl "
                           "of interface {1}",
@@ -160,7 +159,7 @@ auto ImplWitnessStartDefinition(Context& context, SemIR::Impl& impl) -> void {
             .Note(assoc_entity, AssociatedConstantHere)
             .Emit();
 
-        witness_value = SemIR::ErrorInst::SingletonInstId;
+        witness_value = SemIR::ErrorInst::InstId;
       }
     }
   }
@@ -171,7 +170,7 @@ auto ImplWitnessStartDefinition(Context& context, SemIR::Impl& impl) -> void {
 auto FinishImplWitness(Context& context, SemIR::Impl& impl) -> void {
   CARBON_CHECK(impl.is_being_defined());
   CARBON_CHECK(impl.witness_id.has_value());
-  if (impl.witness_id == SemIR::ErrorInst::SingletonInstId) {
+  if (impl.witness_id == SemIR::ErrorInst::InstId) {
     return;
   }
   auto witness = context.insts().GetAs<SemIR::ImplWitness>(impl.witness_id);
@@ -195,8 +194,8 @@ auto FinishImplWitness(Context& context, SemIR::Impl& impl) -> void {
     auto decl = context.insts().Get(decl_id);
     CARBON_KIND_SWITCH(decl) {
       case CARBON_KIND(SemIR::StructValue struct_value): {
-        if (struct_value.type_id == SemIR::ErrorInst::SingletonTypeId) {
-          witness_value = SemIR::ErrorInst::SingletonInstId;
+        if (struct_value.type_id == SemIR::ErrorInst::TypeId) {
+          witness_value = SemIR::ErrorInst::InstId;
           break;
         }
         auto type_inst = context.types().GetAsInst(struct_value.type_id);
@@ -224,7 +223,7 @@ auto FinishImplWitness(Context& context, SemIR::Impl& impl) -> void {
           NoteAssociatedFunction(context, builder, fn_type->function_id);
           builder.Emit();
 
-          witness_value = SemIR::ErrorInst::SingletonInstId;
+          witness_value = SemIR::ErrorInst::InstId;
         }
         break;
       }
@@ -233,9 +232,9 @@ auto FinishImplWitness(Context& context, SemIR::Impl& impl) -> void {
         break;
       }
       default:
-        CARBON_CHECK(decl_id == SemIR::ErrorInst::SingletonInstId,
+        CARBON_CHECK(decl_id == SemIR::ErrorInst::InstId,
                      "Unexpected kind of associated entity {0}", decl);
-        witness_value = SemIR::ErrorInst::SingletonInstId;
+        witness_value = SemIR::ErrorInst::InstId;
         break;
     }
   }
@@ -245,15 +244,15 @@ auto FinishImplWitness(Context& context, SemIR::Impl& impl) -> void {
 
 auto FillImplWitnessWithErrors(Context& context, SemIR::Impl& impl) -> void {
   if (impl.witness_id.has_value() &&
-      impl.witness_id != SemIR::ErrorInst::SingletonInstId) {
+      impl.witness_id != SemIR::ErrorInst::InstId) {
     auto witness = context.insts().GetAs<SemIR::ImplWitness>(impl.witness_id);
     auto witness_table = context.insts().GetAs<SemIR::ImplWitnessTable>(
         witness.witness_table_id);
     auto witness_block =
         context.inst_blocks().GetMutable(witness_table.elements_id);
     for (auto& elem : witness_block) {
-      if (elem == SemIR::ImplWitnessTablePlaceholder::SingletonInstId) {
-        elem = SemIR::ErrorInst::SingletonInstId;
+      if (elem == SemIR::ImplWitnessTablePlaceholder::InstId) {
+        elem = SemIR::ErrorInst::InstId;
       }
     }
   }
@@ -261,7 +260,7 @@ auto FillImplWitnessWithErrors(Context& context, SemIR::Impl& impl) -> void {
 
 auto AssignImplIdInWitness(Context& context, SemIR::ImplId impl_id,
                            SemIR::InstId witness_id) -> void {
-  if (witness_id == SemIR::ErrorInst::SingletonInstId) {
+  if (witness_id == SemIR::ErrorInst::InstId) {
     return;
   }
   auto witness = context.insts().GetAs<SemIR::ImplWitness>(witness_id);
diff --git a/toolchain/check/impl_lookup.cpp b/toolchain/check/impl_lookup.cpp
index dadc463d71cb..ea55b3a0fa12 100644
--- a/toolchain/check/impl_lookup.cpp
+++ b/toolchain/check/impl_lookup.cpp
@@ -264,7 +264,7 @@ static auto GetWitnessIdForImpl(Context& context, SemIR::LocId loc_id,
   auto deduced_constraint_id =
       context.constant_values().GetInstId(SemIR::GetConstantValueInSpecific(
           context.sem_ir(), specific_id, impl.constraint_id));
-  if (deduced_constraint_id == SemIR::ErrorInst::SingletonInstId) {
+  if (deduced_constraint_id == SemIR::ErrorInst::InstId) {
     return EvalImplLookupResult::MakeNone();
   }
 
@@ -329,13 +329,12 @@ static auto FindWitnessInFacet(
     for (auto [index, interface] :
          llvm::enumerate(identified.required_interfaces())) {
       if (interface == specific_interface) {
-        auto witness_id =
-            GetOrAddInst(context, loc_id,
-                         SemIR::FacetAccessWitness{
-                             .type_id = GetSingletonType(
-                                 context, SemIR::WitnessType::SingletonInstId),
-                             .facet_value_inst_id = facet_inst_id,
-                             .index = SemIR::ElementIndex(index)});
+        auto witness_id = GetOrAddInst(
+            context, loc_id,
+            SemIR::FacetAccessWitness{.type_id = GetSingletonType(
+                                          context, SemIR::WitnessType::InstId),
+                                      .facet_value_inst_id = facet_inst_id,
+                                      .index = SemIR::ElementIndex(index)});
         return witness_id;
       }
     }
@@ -356,8 +355,7 @@ static auto GetOrAddLookupImplWitness(Context& context, SemIR::LocId loc_id,
   auto witness_const_id = EvalOrAddInst(
       context, loc_id.ToImplicit(),
       SemIR::LookupImplWitness{
-          .type_id =
-              GetSingletonType(context, SemIR::WitnessType::SingletonInstId),
+          .type_id = GetSingletonType(context, SemIR::WitnessType::InstId),
           .query_self_inst_id =
               context.constant_values().GetInstId(query_self_const_id),
           .query_specific_interface_id =
@@ -375,8 +373,8 @@ auto LookupImplWitness(Context& context, SemIR::LocId loc_id,
                        SemIR::ConstantId query_self_const_id,
                        SemIR::ConstantId query_facet_type_const_id)
     -> SemIR::InstBlockIdOrError {
-  if (query_self_const_id == SemIR::ErrorInst::SingletonConstantId ||
-      query_facet_type_const_id == SemIR::ErrorInst::SingletonConstantId) {
+  if (query_self_const_id == SemIR::ErrorInst::ConstantId ||
+      query_facet_type_const_id == SemIR::ErrorInst::ConstantId) {
     return SemIR::InstBlockIdOrError::MakeError();
   }
 
@@ -544,7 +542,7 @@ static auto CollectCandidateImplsForQuery(
     // This check comes first to avoid deduction with an invalid impl. We use
     // an error value to indicate an error during creation of the impl, such
     // as a recursive impl which will cause deduction to recurse infinitely.
-    if (impl.witness_id == SemIR::ErrorInst::SingletonInstId) {
+    if (impl.witness_id == SemIR::ErrorInst::InstId) {
       continue;
     }
     CARBON_CHECK(impl.witness_id.has_value());
diff --git a/toolchain/check/import.cpp b/toolchain/check/import.cpp
index 515a76a02618..e5c20647909d 100644
--- a/toolchain/check/import.cpp
+++ b/toolchain/check/import.cpp
@@ -596,7 +596,7 @@ static auto AddNamespaceFromOtherPackage(Context& context,
                                          SemIR::NameId name_id)
     -> SemIR::InstId {
   auto namespace_type_id =
-      GetSingletonType(context, SemIR::NamespaceType::SingletonInstId);
+      GetSingletonType(context, SemIR::NamespaceType::InstId);
   AddImportNamespaceToScopeResult result = CopySingleNameScopeFromImportIR(
       context, namespace_type_id, /*copied_namespaces=*/nullptr, import_ir_id,
       import_inst_id, import_ns.name_scope_id, parent_scope_id, name_id);
diff --git a/toolchain/check/import_cpp.cpp b/toolchain/check/import_cpp.cpp
index 535f31914967..bb88ac1e4f52 100644
--- a/toolchain/check/import_cpp.cpp
+++ b/toolchain/check/import_cpp.cpp
@@ -171,8 +171,7 @@ static auto AddNamespace(Context& context, PackageNameId cpp_package_id,
   }
 
   return AddImportNamespaceToScope(
-             context,
-             GetSingletonType(context, SemIR::NamespaceType::SingletonInstId),
+             context, GetSingletonType(context, SemIR::NamespaceType::InstId),
              SemIR::NameId::ForPackageName(cpp_package_id),
              SemIR::NameScopeId::Package,
              /*diagnose_duplicate_namespace=*/false,
@@ -273,8 +272,8 @@ static auto MapType(Context& context, clang::QualType type) -> TypeExpr {
       context.ast_context().getTypeSize(type) == 32) {
     return MakeIntType(context, context.ints().Add(32));
   }
-  return {.inst_id = SemIR::ErrorInst::SingletonTypeInstId,
-          .type_id = SemIR::ErrorInst::SingletonTypeId};
+  return {.inst_id = SemIR::ErrorInst::TypeInstId,
+          .type_id = SemIR::ErrorInst::TypeId};
 }
 
 // Returns a block id for the explicit parameters of the given function
@@ -293,7 +292,7 @@ static auto MakeParamPatternsBlockId(Context& context, SemIR::LocId loc_id,
   for (const clang::ParmVarDecl* param : clang_decl.parameters()) {
     clang::QualType param_type = param->getType().getCanonicalType();
     SemIR::TypeId type_id = MapType(context, param_type).type_id;
-    if (type_id == SemIR::ErrorInst::SingletonTypeId) {
+    if (type_id == SemIR::ErrorInst::TypeId) {
       context.TODO(loc_id, llvm::formatv("Unsupported: parameter type: {0}",
                                          param_type.getAsString()));
       return SemIR::InstBlockId::None;
@@ -336,10 +335,10 @@ static auto GetReturnType(Context& context, SemIR::LocId loc_id,
     return SemIR::InstId::None;
   }
   auto [type_inst_id, type_id] = MapType(context, ret_type);
-  if (type_id == SemIR::ErrorInst::SingletonTypeId) {
+  if (type_id == SemIR::ErrorInst::TypeId) {
     context.TODO(loc_id, llvm::formatv("Unsupported: return type: {0}",
                                        ret_type.getAsString()));
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   SemIR::InstId return_slot_pattern_id = AddInstInNoBlock(
       // TODO: Fill in a location for the return type once available.
@@ -363,24 +362,24 @@ static auto ImportFunctionDecl(Context& context, SemIR::LocId loc_id,
     -> SemIR::InstId {
   if (clang_decl->isVariadic()) {
     context.TODO(loc_id, "Unsupported: Variadic function");
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   if (!clang_decl->isGlobal()) {
     context.TODO(loc_id, "Unsupported: Non-global function");
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   if (clang_decl->getTemplatedKind() != clang::FunctionDecl::TK_NonTemplate) {
     context.TODO(loc_id, "Unsupported: Template function");
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   auto param_patterns_id =
       MakeParamPatternsBlockId(context, loc_id, *clang_decl);
   if (!param_patterns_id.has_value()) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   auto return_slot_pattern_id = GetReturnType(context, loc_id, clang_decl);
-  if (SemIR::ErrorInst::SingletonInstId == return_slot_pattern_id) {
-    return SemIR::ErrorInst::SingletonInstId;
+  if (SemIR::ErrorInst::InstId == return_slot_pattern_id) {
+    return SemIR::ErrorInst::InstId;
   }
 
   auto function_decl = SemIR::FunctionDecl{
@@ -426,8 +425,8 @@ static auto ImportNamespaceDecl(Context& context,
                                 clang::NamespaceDecl* clang_decl)
     -> SemIR::InstId {
   auto result = AddImportNamespace(
-      context, GetSingletonType(context, SemIR::NamespaceType::SingletonInstId),
-      name_id, parent_scope_id, /*import_id=*/SemIR::InstId::None);
+      context, GetSingletonType(context, SemIR::NamespaceType::InstId), name_id,
+      parent_scope_id, /*import_id=*/SemIR::InstId::None);
   context.name_scopes()
       .Get(result.name_scope_id)
       .set_cpp_decl_context(clang_decl);
@@ -440,10 +439,9 @@ static auto BuildClassDecl(Context& context, SemIR::NameScopeId parent_scope_id,
                            SemIR::NameId name_id)
     -> std::tuple<SemIR::ClassId, SemIR::InstId> {
   // Add the class declaration.
-  auto class_decl =
-      SemIR::ClassDecl{.type_id = SemIR::TypeType::SingletonTypeId,
-                       .class_id = SemIR::ClassId::None,
-                       .decl_block_id = SemIR::InstBlockId::None};
+  auto class_decl = SemIR::ClassDecl{.type_id = SemIR::TypeType::TypeId,
+                                     .class_id = SemIR::ClassId::None,
+                                     .decl_block_id = SemIR::InstBlockId::None};
   // TODO: Consider setting a proper location.
   auto class_decl_id =
       AddPlaceholderInst(context, SemIR::LocIdAndInst::NoLoc(class_decl));
@@ -511,12 +509,12 @@ static auto ImportCXXRecordDecl(Context& context, SemIR::LocId loc_id,
   if (!clang_def) {
     context.TODO(loc_id,
                  "Unsupported: Record declarations without a definition");
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   if (clang_def->isDynamicClass()) {
     context.TODO(loc_id, "Unsupported: Dynamic Class");
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   auto [class_id, class_def_id] =
@@ -584,7 +582,7 @@ auto ImportNameFromCpp(Context& context, SemIR::LocId loc_id,
                                "find a single result; LookupResultKind: {0}",
                                lookup->getResultKind())
                      .str());
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   return ImportNameDecl(context, loc_id, scope_id, name_id,
diff --git a/toolchain/check/import_ref.cpp b/toolchain/check/import_ref.cpp
index 0a5680ce756a..0fcd535ebd5d 100644
--- a/toolchain/check/import_ref.cpp
+++ b/toolchain/check/import_ref.cpp
@@ -1245,7 +1245,7 @@ static auto GetLocalNameScopeId(ImportRefResolver& resolver,
       break;
     }
     default: {
-      if (const_inst_id == SemIR::ErrorInst::SingletonInstId) {
+      if (const_inst_id == SemIR::ErrorInst::InstId) {
         return SemIR::NameScopeId::None;
       }
       break;
@@ -1410,7 +1410,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   auto adapted_type_inst_id = resolver.local_ir().types().GetAsTypeInstId(
-      AddLoadedImportRef(resolver, SemIR::TypeType::SingletonTypeId,
+      AddLoadedImportRef(resolver, SemIR::TypeType::TypeId,
                          inst.adapted_type_inst_id, adapted_type_const_id));
 
   // Create a corresponding instruction to represent the declaration.
@@ -1425,7 +1425,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::ArrayType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto element_type_inst_id =
       GetLocalTypeInstId(resolver, inst.element_type_inst_id);
   auto bound_id = GetLocalConstantInstId(resolver, inst.bound_id);
@@ -1434,7 +1434,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::ArrayType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
+      resolver, {.type_id = SemIR::TypeType::TypeId,
                  .bound_id = bound_id,
                  .element_type_inst_id = element_type_inst_id});
 }
@@ -1542,7 +1542,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::AssociatedEntityType inst)
     -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto data =
       GetLocalSpecificInterfaceData(resolver, inst.GetSpecificInterface());
 
@@ -1553,7 +1553,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   auto specific_interface =
       GetLocalSpecificInterface(resolver, inst.GetSpecificInterface(), data);
   return ResolveAs<SemIR::AssociatedEntityType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
+      resolver, {.type_id = SemIR::TypeType::TypeId,
                  .interface_id = specific_interface.interface_id,
                  .interface_specific_id = specific_interface.specific_id});
 }
@@ -1569,7 +1569,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   auto base_type_inst_id = resolver.local_ir().types().GetAsTypeInstId(
-      AddLoadedImportRef(resolver, SemIR::TypeType::SingletonTypeId,
+      AddLoadedImportRef(resolver, SemIR::TypeType::TypeId,
                          inst.base_type_inst_id, base_type_const_id));
 
   // Create a corresponding instruction to represent the declaration.
@@ -1659,7 +1659,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::BoundMethod inst) -> ResolveResult {
   CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
-               SemIR::BoundMethodType::SingletonInstId);
+               SemIR::BoundMethodType::InstId);
   auto object_id = GetLocalConstantInstId(resolver, inst.object_id);
   auto function_decl_id =
       GetLocalConstantInstId(resolver, inst.function_decl_id);
@@ -1669,11 +1669,10 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::BoundMethod>(
-      resolver,
-      {.type_id = GetSingletonType(resolver.local_context(),
-                                   SemIR::BoundMethodType::SingletonInstId),
-       .object_id = object_id,
-       .function_decl_id = function_decl_id});
+      resolver, {.type_id = GetSingletonType(resolver.local_context(),
+                                             SemIR::BoundMethodType::InstId),
+                 .object_id = object_id,
+                 .function_decl_id = function_decl_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver, SemIR::Call inst)
@@ -1701,7 +1700,7 @@ static auto MakeIncompleteClass(ImportContext& context,
                                 const SemIR::Class& import_class,
                                 SemIR::SpecificId enclosing_specific_id)
     -> std::pair<SemIR::ClassId, SemIR::ConstantId> {
-  SemIR::ClassDecl class_decl = {.type_id = SemIR::TypeType::SingletonTypeId,
+  SemIR::ClassDecl class_decl = {.type_id = SemIR::TypeType::TypeId,
                                  .class_id = SemIR::ClassId::None,
                                  .decl_block_id = SemIR::InstBlockId::Empty};
   auto class_decl_id = AddPlaceholderInstInNoBlock(
@@ -1850,8 +1849,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   if (import_class.is_complete()) {
     auto complete_type_witness_id = AddLoadedImportRef(
         resolver,
-        GetSingletonType(resolver.local_context(),
-                         SemIR::WitnessType::SingletonInstId),
+        GetSingletonType(resolver.local_context(), SemIR::WitnessType::InstId),
         import_class.complete_type_witness_id, complete_type_witness_const_id);
     AddClassDefinition(resolver, import_class, new_class,
                        complete_type_witness_id, base_id, adapt_id, vtable_id);
@@ -1862,7 +1860,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::ClassType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto class_const_id = GetLocalConstantId(
       resolver,
       resolver.import_classes().Get(inst.class_id).first_owning_decl_id);
@@ -1884,10 +1882,10 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
             class_const_inst.type_id());
     auto specific_id =
         GetOrAddLocalSpecific(resolver, inst.specific_id, specific_data);
-    return ResolveAs<SemIR::ClassType>(
-        resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
-                   .class_id = generic_class_type.class_id,
-                   .specific_id = specific_id});
+    return ResolveAs<SemIR::ClassType>(resolver,
+                                       {.type_id = SemIR::TypeType::TypeId,
+                                        .class_id = generic_class_type.class_id,
+                                        .specific_id = specific_id});
   }
 }
 
@@ -1895,29 +1893,27 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::CompleteTypeWitness inst)
     -> ResolveResult {
   CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
-               SemIR::WitnessType::SingletonInstId);
+               SemIR::WitnessType::InstId);
   auto object_repr_type_inst_id =
       GetLocalTypeInstId(resolver, inst.object_repr_type_inst_id);
   if (resolver.HasNewWork()) {
     return ResolveResult::Retry();
   }
   return ResolveAs<SemIR::CompleteTypeWitness>(
-      resolver,
-      {.type_id = GetSingletonType(resolver.local_context(),
-                                   SemIR::WitnessType::SingletonInstId),
-       .object_repr_type_inst_id = object_repr_type_inst_id});
+      resolver, {.type_id = GetSingletonType(resolver.local_context(),
+                                             SemIR::WitnessType::InstId),
+                 .object_repr_type_inst_id = object_repr_type_inst_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::ConstType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto inner_id = GetLocalTypeInstId(resolver, inst.inner_id);
   if (resolver.HasNewWork()) {
     return ResolveResult::Retry();
   }
   return ResolveAs<SemIR::ConstType>(
-      resolver,
-      {.type_id = SemIR::TypeType::SingletonTypeId, .inner_id = inner_id});
+      resolver, {.type_id = SemIR::TypeType::TypeId, .inner_id = inner_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
@@ -2056,7 +2052,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::FunctionType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto fn_val_id = GetLocalConstantInstId(
       resolver,
       resolver.import_functions().Get(inst.function_id).first_decl_id());
@@ -2066,7 +2062,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
   auto fn_type_id = resolver.local_insts().Get(fn_val_id).type_id();
   return ResolveAs<SemIR::FunctionType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
+      resolver, {.type_id = SemIR::TypeType::TypeId,
                  .function_id = resolver.local_types()
                                     .GetAs<SemIR::FunctionType>(fn_type_id)
                                     .function_id,
@@ -2077,7 +2073,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::FunctionTypeWithSelfType inst)
     -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto interface_function_type_id =
       GetLocalTypeInstId(resolver, inst.interface_function_type_id);
   auto self_id = GetLocalConstantInstId(resolver, inst.self_id);
@@ -2086,14 +2082,14 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::FunctionTypeWithSelfType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
+      resolver, {.type_id = SemIR::TypeType::TypeId,
                  .interface_function_type_id = interface_function_type_id,
                  .self_id = self_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::GenericClassType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto class_val_id = GetLocalConstantInstId(
       resolver,
       resolver.import_classes().Get(inst.class_id).first_owning_decl_id);
@@ -2110,7 +2106,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::GenericInterfaceType inst)
     -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto interface_val_id = GetLocalConstantInstId(
       resolver,
       resolver.import_interfaces().Get(inst.interface_id).first_owning_decl_id);
@@ -2236,10 +2232,10 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   // Create instructions for self and constraint to hold the symbolic constant
   // value for a generic impl.
   new_impl.self_id = resolver.local_ir().types().GetAsTypeInstId(
-      AddLoadedImportRef(resolver, SemIR::TypeType::SingletonTypeId,
-                         import_impl.self_id, self_const_id));
+      AddLoadedImportRef(resolver, SemIR::TypeType::TypeId, import_impl.self_id,
+                         self_const_id));
   new_impl.constraint_id = resolver.local_ir().types().GetAsTypeInstId(
-      AddLoadedImportRef(resolver, SemIR::TypeType::SingletonTypeId,
+      AddLoadedImportRef(resolver, SemIR::TypeType::TypeId,
                          import_impl.constraint_id, constraint_const_id));
   new_impl.interface = GetLocalSpecificInterface(
       resolver, import_impl.interface, specific_interface_data);
@@ -2282,7 +2278,7 @@ static auto MakeInterfaceDecl(ImportContext& context,
                               SemIR::SpecificId enclosing_specific_id)
     -> std::pair<SemIR::InterfaceId, SemIR::ConstantId> {
   SemIR::InterfaceDecl interface_decl = {
-      .type_id = SemIR::TypeType::SingletonTypeId,
+      .type_id = SemIR::TypeType::TypeId,
       .interface_id = SemIR::InterfaceId::None,
       .decl_block_id = SemIR::InstBlockId::Empty};
   auto interface_decl_id = AddPlaceholderInstInNoBlock(
@@ -2429,7 +2425,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::FacetAccessType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
+      resolver, {.type_id = SemIR::TypeType::TypeId,
                  .facet_value_inst_id = facet_value_inst_id});
 }
 
@@ -2443,16 +2439,15 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::FacetAccessWitness>(
-      resolver,
-      {.type_id = GetSingletonType(resolver.local_context(),
-                                   SemIR::WitnessType::SingletonInstId),
-       .facet_value_inst_id = facet_value_inst_id,
-       .index = inst.index});
+      resolver, {.type_id = GetSingletonType(resolver.local_context(),
+                                             SemIR::WitnessType::InstId),
+                 .facet_value_inst_id = facet_value_inst_id,
+                 .index = inst.index});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::FacetType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
 
   const SemIR::FacetTypeInfo& import_facet_type_info =
       resolver.import_facet_types().Get(inst.facet_type_id);
@@ -2501,8 +2496,8 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   SemIR::FacetTypeId facet_type_id =
       resolver.local_facet_types().Add(std::move(local_facet_type_info));
   return ResolveAs<SemIR::FacetType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
-                 .facet_type_id = facet_type_id});
+      resolver,
+      {.type_id = SemIR::TypeType::TypeId, .facet_type_id = facet_type_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
@@ -2527,7 +2522,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::LookupImplWitness inst)
     -> ResolveResult {
   CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
-               SemIR::WitnessType::SingletonInstId);
+               SemIR::WitnessType::InstId);
 
   auto query_self_inst_id =
       GetLocalConstantInstId(resolver, inst.query_self_inst_id);
@@ -2547,17 +2542,16 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   auto query_specific_interface_id =
       resolver.local_specific_interfaces().Add(specific_interface);
   return ResolveAs<SemIR::LookupImplWitness>(
-      resolver,
-      {.type_id = GetSingletonType(resolver.local_context(),
-                                   SemIR::WitnessType::SingletonInstId),
-       .query_self_inst_id = query_self_inst_id,
-       .query_specific_interface_id = query_specific_interface_id});
+      resolver, {.type_id = GetSingletonType(resolver.local_context(),
+                                             SemIR::WitnessType::InstId),
+                 .query_self_inst_id = query_self_inst_id,
+                 .query_specific_interface_id = query_specific_interface_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::ImplWitness inst) -> ResolveResult {
   CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
-               SemIR::WitnessType::SingletonInstId);
+               SemIR::WitnessType::InstId);
 
   auto specific_data = GetLocalSpecificData(resolver, inst.specific_id);
   auto witness_table_id =
@@ -2569,11 +2563,10 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   auto specific_id =
       GetOrAddLocalSpecific(resolver, inst.specific_id, specific_data);
   return ResolveAs<SemIR::ImplWitness>(
-      resolver,
-      {.type_id = GetSingletonType(resolver.local_context(),
-                                   SemIR::WitnessType::SingletonInstId),
-       .witness_table_id = witness_table_id,
-       .specific_id = specific_id});
+      resolver, {.type_id = GetSingletonType(resolver.local_context(),
+                                             SemIR::WitnessType::InstId),
+                 .witness_table_id = witness_table_id,
+                 .specific_id = specific_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
@@ -2634,14 +2627,14 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::IntType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto bit_width_id = GetLocalConstantInstId(resolver, inst.bit_width_id);
   if (resolver.HasNewWork()) {
     return ResolveResult::Retry();
   }
 
   return ResolveAs<SemIR::IntType>(resolver,
-                                   {.type_id = SemIR::TypeType::SingletonTypeId,
+                                   {.type_id = SemIR::TypeType::TypeId,
                                     .int_kind = inst.int_kind,
                                     .bit_width_id = bit_width_id});
 }
@@ -2662,8 +2655,8 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
     return ResolveResult::Retry();
   }
 
-  auto namespace_type_id = GetSingletonType(
-      resolver.local_context(), SemIR::NamespaceType::SingletonInstId);
+  auto namespace_type_id =
+      GetSingletonType(resolver.local_context(), SemIR::NamespaceType::InstId);
   auto namespace_decl =
       SemIR::Namespace{.type_id = namespace_type_id,
                        .name_scope_id = SemIR::NameScopeId::None,
@@ -2689,22 +2682,21 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::PointerType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto pointee_id = GetLocalTypeInstId(resolver, inst.pointee_id);
   if (resolver.HasNewWork()) {
     return ResolveResult::Retry();
   }
 
   return ResolveAs<SemIR::PointerType>(
-      resolver,
-      {.type_id = SemIR::TypeType::SingletonTypeId, .pointee_id = pointee_id});
+      resolver, {.type_id = SemIR::TypeType::TypeId, .pointee_id = pointee_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::RequireCompleteType inst)
     -> ResolveResult {
   CARBON_CHECK(resolver.import_types().GetInstId(inst.type_id) ==
-               SemIR::WitnessType::SingletonInstId);
+               SemIR::WitnessType::InstId);
 
   auto complete_type_inst_id =
       GetLocalTypeInstId(resolver, inst.complete_type_inst_id);
@@ -2713,10 +2705,9 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::RequireCompleteType>(
-      resolver,
-      {.type_id = GetSingletonType(resolver.local_context(),
-                                   SemIR::WitnessType::SingletonInstId),
-       .complete_type_inst_id = complete_type_inst_id});
+      resolver, {.type_id = GetSingletonType(resolver.local_context(),
+                                             SemIR::WitnessType::InstId),
+                 .complete_type_inst_id = complete_type_inst_id});
 }
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
@@ -2747,7 +2738,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   auto type_id = GetSingletonType(resolver.local_context(),
-                                  SemIR::SpecificFunctionType::SingletonInstId);
+                                  SemIR::SpecificFunctionType::InstId);
   auto specific_id =
       GetOrAddLocalSpecific(resolver, inst.specific_id, specific_data);
   return ResolveAs<SemIR::SpecificImplFunction>(
@@ -2757,7 +2748,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::StructType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto orig_fields = resolver.import_struct_type_fields().Get(inst.fields_id);
   llvm::SmallVector<SemIR::TypeInstId> field_type_inst_ids;
   field_type_inst_ids.reserve(orig_fields.size());
@@ -2780,7 +2771,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::StructType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
+      resolver, {.type_id = SemIR::TypeType::TypeId,
                  .fields_id = resolver.local_struct_type_fields().AddCanonical(
                      new_fields)});
 }
@@ -2803,7 +2794,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::TupleType inst) -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
 
   auto orig_type_inst_ids =
       resolver.import_inst_blocks().Get(inst.type_elements_id);
@@ -2820,7 +2811,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::TupleType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
+      resolver, {.type_id = SemIR::TypeType::TypeId,
                  .type_elements_id = GetLocalCanonicalInstBlockId(
                      resolver, inst.type_elements_id, type_inst_ids)});
 }
@@ -2844,7 +2835,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
 static auto TryResolveTypedInst(ImportRefResolver& resolver,
                                 SemIR::UnboundElementType inst)
     -> ResolveResult {
-  CARBON_CHECK(inst.type_id == SemIR::TypeType::SingletonTypeId);
+  CARBON_CHECK(inst.type_id == SemIR::TypeType::TypeId);
   auto class_const_inst_id =
       GetLocalTypeInstId(resolver, inst.class_type_inst_id);
   auto elem_const_inst_id =
@@ -2854,7 +2845,7 @@ static auto TryResolveTypedInst(ImportRefResolver& resolver,
   }
 
   return ResolveAs<SemIR::UnboundElementType>(
-      resolver, {.type_id = SemIR::TypeType::SingletonTypeId,
+      resolver, {.type_id = SemIR::TypeType::TypeId,
                  .class_type_inst_id = class_const_inst_id,
                  .element_type_inst_id = elem_const_inst_id});
 }
diff --git a/toolchain/check/interface.cpp b/toolchain/check/interface.cpp
index ea8519c1e736..49663ae9a2fa 100644
--- a/toolchain/check/interface.cpp
+++ b/toolchain/check/interface.cpp
@@ -22,7 +22,7 @@ auto BuildAssociatedEntity(Context& context, SemIR::InterfaceId interface_id,
     // This should only happen if the interface is erroneously defined more than
     // once.
     // TODO: Find a way to CHECK this.
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // This associated entity is being declared as a member of the self specific
diff --git a/toolchain/check/literal.cpp b/toolchain/check/literal.cpp
index afc928abd4be..c107c26a7b19 100644
--- a/toolchain/check/literal.cpp
+++ b/toolchain/check/literal.cpp
@@ -16,8 +16,7 @@ auto MakeIntLiteral(Context& context, Parse::NodeId node_id, IntId int_id)
     -> SemIR::InstId {
   return AddInst<SemIR::IntValue>(
       context, node_id,
-      {.type_id =
-           GetSingletonType(context, SemIR::IntLiteralType::SingletonInstId),
+      {.type_id = GetSingletonType(context, SemIR::IntLiteralType::InstId),
        .int_id = int_id});
 }
 
diff --git a/toolchain/check/member_access.cpp b/toolchain/check/member_access.cpp
index 9c1c4187bc8d..51b6dfc4995e 100644
--- a/toolchain/check/member_access.cpp
+++ b/toolchain/check/member_access.cpp
@@ -150,17 +150,17 @@ static auto AccessMemberOfImplWitness(Context& context, SemIR::LocId loc_id,
     -> SemIR::InstId {
   auto member_value_id = context.constant_values().GetConstantInstId(member_id);
   if (!member_value_id.has_value()) {
-    if (member_value_id != SemIR::ErrorInst::SingletonInstId) {
+    if (member_value_id != SemIR::ErrorInst::InstId) {
       context.TODO(member_id, "non-constant associated entity");
     }
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   auto assoc_entity =
       context.insts().TryGetAs<SemIR::AssociatedEntity>(member_value_id);
   if (!assoc_entity) {
     context.TODO(member_id, "unexpected value for associated entity");
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // Substitute the interface specific and `Self` type into the type of the
@@ -188,7 +188,7 @@ static auto GetWitnessFromSingleImplLookupResult(
     -> SemIR::InstId {
   auto witness_id = SemIR::InstId::None;
   if (lookup_result.has_error_value()) {
-    witness_id = SemIR::ErrorInst::SingletonInstId;
+    witness_id = SemIR::ErrorInst::InstId;
   } else {
     auto witnesses = context.inst_blocks().Get(lookup_result.inst_block_id());
     CARBON_CHECK(witnesses.size() == 1);
@@ -228,7 +228,7 @@ static auto PerformImplLookup(
       context.emitter().Emit(loc_id, MissingImplInMemberAccess,
                              interface_type_id, self_type_id);
     }
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   auto witness_id =
@@ -256,7 +256,7 @@ static auto LookupMemberNameInScope(Context& context, SemIR::LocId loc_id,
       context, loc_id, name_id, lookup_scopes, required, access_info);
 
   if (!result.scope_result.is_found()) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // TODO: This duplicates the work that HandleNameAsExpr does. Factor this out.
@@ -358,13 +358,13 @@ static auto LookupMemberNameInScope(Context& context, SemIR::LocId loc_id,
                llvm::enumerate(identified.required_interfaces())) {
             // Get the witness that `T` implements `base_type_id`.
             if (base_interface == assoc_interface) {
-              witness_inst_id = GetOrAddInst(
-                  context, loc_id,
-                  SemIR::FacetAccessWitness{
-                      .type_id = GetSingletonType(
-                          context, SemIR::WitnessType::SingletonInstId),
-                      .facet_value_inst_id = base_id,
-                      .index = SemIR::ElementIndex(index)});
+              witness_inst_id =
+                  GetOrAddInst(context, loc_id,
+                               SemIR::FacetAccessWitness{
+                                   .type_id = GetSingletonType(
+                                       context, SemIR::WitnessType::InstId),
+                                   .facet_value_inst_id = base_id,
+                                   .index = SemIR::ElementIndex(index)});
               break;
             }
           }
@@ -375,7 +375,7 @@ static auto LookupMemberNameInScope(Context& context, SemIR::LocId loc_id,
           context.TODO(member_id,
                        "associated entity not found in facet type, need to do "
                        "impl lookup");
-          return SemIR::ErrorInst::SingletonInstId;
+          return SemIR::ErrorInst::InstId;
         }
 
         member_id = AccessMemberOfImplWitness(
@@ -417,8 +417,7 @@ static auto PerformInstanceBinding(Context& context, SemIR::LocId loc_id,
 
     return GetOrAddInst<SemIR::BoundMethod>(
         context, loc_id,
-        {.type_id =
-             GetSingletonType(context, SemIR::BoundMethodType::SingletonInstId),
+        {.type_id = GetSingletonType(context, SemIR::BoundMethodType::InstId),
          .object_id = base_id,
          .function_decl_id = member_id});
   }
@@ -493,13 +492,13 @@ auto PerformMemberAccess(Context& context, SemIR::LocId loc_id,
   if (required) {
     return HandleAction<SemIR::AccessMemberAction>(
         context, loc_id,
-        {.type_id = SemIR::InstType::SingletonTypeId,
+        {.type_id = SemIR::InstType::TypeId,
          .base_id = base_id,
          .name_id = name_id});
   } else {
     return HandleAction<SemIR::AccessOptionalMemberAction>(
         context, loc_id,
-        {.type_id = SemIR::InstType::SingletonTypeId,
+        {.type_id = SemIR::InstType::TypeId,
          .base_id = base_id,
          .name_id = name_id});
   }
@@ -532,7 +531,7 @@ static auto PerformActionHelper(Context& context, SemIR::LocId loc_id,
         return context.emitter().Build(base_id, IncompleteTypeInMemberAccess,
                                        base_id);
       })) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // Materialize a temporary for the base expression if necessary.
@@ -567,19 +566,19 @@ static auto PerformActionHelper(Context& context, SemIR::LocId loc_id,
                           SemIR::NameId);
         context.emitter().Emit(loc_id, QualifiedExprNameNotFound, base_id,
                                name_id);
-        return SemIR::ErrorInst::SingletonInstId;
+        return SemIR::ErrorInst::InstId;
       } else {
         return SemIR::InstId::None;
       }
     }
 
-    if (base_type_id != SemIR::ErrorInst::SingletonTypeId) {
+    if (base_type_id != SemIR::ErrorInst::TypeId) {
       CARBON_DIAGNOSTIC(QualifiedExprUnsupported, Error,
                         "type {0} does not support qualified expressions",
                         TypeOfInstId);
       context.emitter().Emit(loc_id, QualifiedExprUnsupported, base_id);
     }
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // Perform lookup into the base type.
@@ -625,32 +624,30 @@ static auto GetAssociatedValueImpl(Context& context, SemIR::LocId loc_id,
       GetInterfaceType(context, interface.interface_id, interface.specific_id);
   auto facet_inst_id =
       ConvertToValueOfType(context, loc_id, base_id, interface_type_id);
-  if (facet_inst_id == SemIR::ErrorInst::SingletonInstId) {
-    return SemIR::ErrorInst::SingletonInstId;
+  if (facet_inst_id == SemIR::ErrorInst::InstId) {
+    return SemIR::ErrorInst::InstId;
   }
   // That facet value has both the self type we need below and the witness
   // we are going to use to look up the value of the associated member.
   auto self_type_const_id = TryEvalInst(
-      context,
-      SemIR::FacetAccessType{.type_id = SemIR::TypeType::SingletonTypeId,
-                             .facet_value_inst_id = facet_inst_id});
+      context, SemIR::FacetAccessType{.type_id = SemIR::TypeType::TypeId,
+                                      .facet_value_inst_id = facet_inst_id});
   // TODO: We should be able to lookup constant associated values from runtime
   // facet values by using their FacetType only, but we assume constant values
   // for impl lookup at the moment.
   if (!self_type_const_id.is_constant()) {
     context.TODO(loc_id, "associated value lookup on runtime facet value");
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   auto self_type_id =
       context.types().GetTypeIdForTypeConstantId(self_type_const_id);
-  auto witness_id =
-      GetOrAddInst(context, loc_id,
-                   SemIR::FacetAccessWitness{
-                       .type_id = GetSingletonType(
-                           context, SemIR::WitnessType::SingletonInstId),
-                       .facet_value_inst_id = facet_inst_id,
-                       // There's only one interface in this facet type.
-                       .index = SemIR::ElementIndex(0)});
+  auto witness_id = GetOrAddInst(
+      context, loc_id,
+      SemIR::FacetAccessWitness{
+          .type_id = GetSingletonType(context, SemIR::WitnessType::InstId),
+          .facet_value_inst_id = facet_inst_id,
+          // There's only one interface in this facet type.
+          .index = SemIR::ElementIndex(0)});
   // Before we can access the element of the witness, we need to figure out
   // the type of that element. It depends on the self type and the specific
   // interface.
@@ -709,7 +706,7 @@ auto PerformCompoundMemberAccess(Context& context, SemIR::LocId loc_id,
     // > evaluation is required to succeed [...]
     if (!value_inst_id.has_value()) {
       context.TODO(loc_id, "Non-constant associated entity value");
-      return SemIR::ErrorInst::SingletonInstId;
+      return SemIR::ErrorInst::InstId;
     }
     auto assoc_entity =
         context.insts().GetAs<SemIR::AssociatedEntity>(value_inst_id);
@@ -743,7 +740,7 @@ auto PerformCompoundMemberAccess(Context& context, SemIR::LocId loc_id,
   // If we didn't perform impl lookup or instance binding, that's an error
   // because the base expression is not used for anything.
   if (member_id == member_expr_id &&
-      member.type_id() != SemIR::ErrorInst::SingletonTypeId) {
+      member.type_id() != SemIR::ErrorInst::TypeId) {
     CARBON_DIAGNOSTIC(CompoundMemberAccessDoesNotUseBase, Error,
                       "member name of type {0} in compound member access is "
                       "not an instance member or an interface member",
@@ -768,7 +765,7 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
                       "tuples can be indexed that way",
                       TypeOfInstId);
     context.emitter().Emit(loc_id, TupleIndexOnANonTupleType, tuple_inst_id);
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   auto diag_non_constant_index = [&] {
@@ -776,7 +773,7 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
     CARBON_DIAGNOSTIC(TupleIndexNotConstant, Error,
                       "tuple index must be a constant");
     context.emitter().Emit(loc_id, TupleIndexNotConstant);
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   };
   // Diagnose a non-constant index prior to conversion to IntLiteral, because
   // the conversion will fail if the index is not constant.
@@ -784,14 +781,14 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
     return diag_non_constant_index();
   }
 
-  SemIR::TypeId element_type_id = SemIR::ErrorInst::SingletonTypeId;
+  SemIR::TypeId element_type_id = SemIR::ErrorInst::TypeId;
   auto index_node_id = context.insts().GetLocId(index_inst_id);
   index_inst_id = ConvertToValueOfType(
       context, index_node_id, index_inst_id,
-      GetSingletonType(context, SemIR::IntLiteralType::SingletonInstId));
+      GetSingletonType(context, SemIR::IntLiteralType::InstId));
   auto index_const_id = context.constant_values().Get(index_inst_id);
-  if (index_const_id == SemIR::ErrorInst::SingletonConstantId) {
-    return SemIR::ErrorInst::SingletonInstId;
+  if (index_const_id == SemIR::ErrorInst::ConstantId) {
+    return SemIR::ErrorInst::InstId;
   } else if (!index_const_id.is_concrete()) {
     return diag_non_constant_index();
   }
@@ -802,7 +799,7 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
   std::optional<llvm::APInt> index_val = ValidateTupleIndex(
       context, loc_id, tuple_inst_id, index_literal, type_block.size());
   if (!index_val) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // TODO: Handle the case when `index_val->getZExtValue()` has too many bits.
diff --git a/toolchain/check/merge.cpp b/toolchain/check/merge.cpp
index 4a27abf8802a..6d2f84392076 100644
--- a/toolchain/check/merge.cpp
+++ b/toolchain/check/merge.cpp
@@ -190,7 +190,7 @@ static auto EntityHasParamError(Context& context, const DeclParams& info)
         param_patterns_id != SemIR::InstBlockId::Empty) {
       for (auto param_id : context.inst_blocks().Get(param_patterns_id)) {
         if (context.insts().Get(param_id).type_id() ==
-            SemIR::ErrorInst::SingletonTypeId) {
+            SemIR::ErrorInst::TypeId) {
           return true;
         }
       }
diff --git a/toolchain/check/name_lookup.cpp b/toolchain/check/name_lookup.cpp
index b3e015cbfdfb..620f1aa43e0b 100644
--- a/toolchain/check/name_lookup.cpp
+++ b/toolchain/check/name_lookup.cpp
@@ -334,7 +334,7 @@ auto AppendLookupScopesForConstant(Context& context, SemIR::LocId loc_id,
     }
     return true;
   }
-  if (base_const_id == SemIR::ErrorInst::SingletonConstantId) {
+  if (base_const_id == SemIR::ErrorInst::ConstantId) {
     // Lookup into this scope should fail without producing an error.
     scopes->push_back(LookupScope{.name_scope_id = SemIR::NameScopeId::None,
                                   .specific_id = SemIR::SpecificId::None});
@@ -523,7 +523,7 @@ auto LookupNameInCore(Context& context, SemIR::LocId loc_id,
                       llvm::StringRef name) -> SemIR::InstId {
   auto core_package_id = GetCorePackage(context, loc_id, name);
   if (!core_package_id.has_value()) {
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   auto name_id = SemIR::NameId::ForIdentifier(context.identifiers().Add(name));
@@ -536,7 +536,7 @@ auto LookupNameInCore(Context& context, SemIR::LocId loc_id,
         "name `Core.{0}` implicitly referenced here, but not found",
         SemIR::NameId);
     context.emitter().Emit(loc_id, CoreNameNotFound, name_id);
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
 
   // Look through import_refs and aliases.
diff --git a/toolchain/check/operator.cpp b/toolchain/check/operator.cpp
index fb15f5f2eeb2..f974dd1f0a12 100644
--- a/toolchain/check/operator.cpp
+++ b/toolchain/check/operator.cpp
@@ -40,8 +40,8 @@ auto BuildUnaryOperator(Context& context, SemIR::LocId loc_id, Operator op,
   // Form `operand.(Op)`.
   auto bound_op_id = PerformCompoundMemberAccess(context, loc_id, operand_id,
                                                  op_fn, missing_impl_diagnoser);
-  if (bound_op_id == SemIR::ErrorInst::SingletonInstId) {
-    return SemIR::ErrorInst::SingletonInstId;
+  if (bound_op_id == SemIR::ErrorInst::InstId) {
+    return SemIR::ErrorInst::InstId;
   }
 
   // Form `bound_op()`.
@@ -58,8 +58,8 @@ auto BuildBinaryOperator(Context& context, SemIR::LocId loc_id, Operator op,
   // Form `lhs.(Op)`.
   auto bound_op_id = PerformCompoundMemberAccess(context, loc_id, lhs_id, op_fn,
                                                  missing_impl_diagnoser);
-  if (bound_op_id == SemIR::ErrorInst::SingletonInstId) {
-    return SemIR::ErrorInst::SingletonInstId;
+  if (bound_op_id == SemIR::ErrorInst::InstId) {
+    return SemIR::ErrorInst::InstId;
   }
 
   // Form `bound_op(rhs)`.
diff --git a/toolchain/check/pattern_match.cpp b/toolchain/check/pattern_match.cpp
index 2194028f0757..54040e54b808 100644
--- a/toolchain/check/pattern_match.cpp
+++ b/toolchain/check/pattern_match.cpp
@@ -182,7 +182,7 @@ static auto InsertHere(Context& context, SemIR::ExprRegionId region_id)
     context.TODO(loc_id,
                  "Control flow expressions are currently only supported inside "
                  "functions.");
-    return SemIR::ErrorInst::SingletonInstId;
+    return SemIR::ErrorInst::InstId;
   }
   AddInst(context, SemIR::LocIdAndInst::NoLoc<SemIR::Branch>(
                        {.target_id = region.block_ids.front()}));
@@ -291,8 +291,8 @@ auto MatchContext::DoEmitPatternMatch(Context& context,
           "Parameters out of order; expecting {0} but got {1}", results_.size(),
           param_pattern.index.index);
       CARBON_CHECK(entry.scrutinee_id.has_value());
-      if (entry.scrutinee_id == SemIR::ErrorInst::SingletonInstId) {
-        results_.push_back(SemIR::ErrorInst::SingletonInstId);
+      if (entry.scrutinee_id == SemIR::ErrorInst::InstId) {
+        results_.push_back(SemIR::ErrorInst::InstId);
       } else {
         results_.push_back(ConvertToValueOfType(
             context, context.insts().GetLocId(entry.scrutinee_id),
@@ -479,7 +479,7 @@ auto MatchContext::DoEmitPatternMatch(Context& context,
                                       SemIR::TuplePattern tuple_pattern,
                                       SemIR::InstId pattern_inst_id,
                                       WorkItem entry) -> void {
-  if (tuple_pattern.type_id == SemIR::ErrorInst::SingletonTypeId) {
+  if (tuple_pattern.type_id == SemIR::ErrorInst::TypeId) {
     return;
   }
   auto subpattern_ids = context.inst_blocks().Get(tuple_pattern.elements_id);
@@ -544,7 +544,7 @@ auto MatchContext::DoEmitPatternMatch(Context& context,
 
 auto MatchContext::EmitPatternMatch(Context& context,
                                     MatchContext::WorkItem entry) -> void {
-  if (entry.pattern_id == SemIR::ErrorInst::SingletonInstId) {
+  if (entry.pattern_id == SemIR::ErrorInst::InstId) {
     return;
   }
   Diagnostics::AnnotationScope annotate_diagnostics(
diff --git a/toolchain/check/pointer_dereference.cpp b/toolchain/check/pointer_dereference.cpp
index d5c5c4acb2ab..4c2ff82d64a8 100644
--- a/toolchain/check/pointer_dereference.cpp
+++ b/toolchain/check/pointer_dereference.cpp
@@ -26,12 +26,12 @@ auto PerformPointerDereference(
   base_id = ConvertToValueExpr(context, base_id);
   auto type_id = context.types().GetUnqualifiedType(
       context.insts().Get(base_id).type_id());
-  auto result_type_id = SemIR::ErrorInst::SingletonTypeId;
+  auto result_type_id = SemIR::ErrorInst::TypeId;
   if (auto pointer_type =
           context.types().TryGetAs<SemIR::PointerType>(type_id)) {
     result_type_id =
         context.types().GetTypeIdForTypeInstId(pointer_type->pointee_id);
-  } else if (type_id != SemIR::ErrorInst::SingletonTypeId) {
+  } else if (type_id != SemIR::ErrorInst::TypeId) {
     diagnose_not_pointer(type_id);
   }
   return AddInst<SemIR::Deref>(
diff --git a/toolchain/check/return.cpp b/toolchain/check/return.cpp
index f96e5a150f1c..128b9486f094 100644
--- a/toolchain/check/return.cpp
+++ b/toolchain/check/return.cpp
@@ -145,7 +145,7 @@ auto BuildReturnWithExpr(Context& context, Parse::ReturnStatementId node_id,
     auto diag = context.emitter().Build(node_id, ReturnStatementDisallowExpr);
     NoteNoReturnTypeProvided(diag, function);
     diag.Emit();
-    expr_id = SemIR::ErrorInst::SingletonInstId;
+    expr_id = SemIR::ErrorInst::InstId;
   } else if (returned_var_id.has_value()) {
     CARBON_DIAGNOSTIC(
         ReturnExprWithReturnedVar, Error,
@@ -153,11 +153,11 @@ auto BuildReturnWithExpr(Context& context, Parse::ReturnStatementId node_id,
     auto diag = context.emitter().Build(node_id, ReturnExprWithReturnedVar);
     NoteReturnedVar(diag, returned_var_id);
     diag.Emit();
-    expr_id = SemIR::ErrorInst::SingletonInstId;
+    expr_id = SemIR::ErrorInst::InstId;
   } else if (!return_info.is_valid()) {
     // We already diagnosed that the return type is invalid. Don't try to
     // convert to it.
-    expr_id = SemIR::ErrorInst::SingletonInstId;
+    expr_id = SemIR::ErrorInst::InstId;
   } else if (return_info.has_return_slot()) {
     return_slot_id = GetCurrentReturnSlot(context);
     CARBON_CHECK(return_slot_id.has_value());
@@ -181,7 +181,7 @@ auto BuildReturnVar(Context& context, Parse::ReturnStatementId node_id)
     CARBON_DIAGNOSTIC(ReturnVarWithNoReturnedVar, Error,
                       "`return var;` with no `returned var` in scope");
     context.emitter().Emit(node_id, ReturnVarWithNoReturnedVar);
-    returned_var_id = SemIR::ErrorInst::SingletonInstId;
+    returned_var_id = SemIR::ErrorInst::InstId;
   }
 
   auto return_slot_id = GetCurrentReturnSlot(context);
diff --git a/toolchain/check/scope_stack.cpp b/toolchain/check/scope_stack.cpp
index f49f78e38beb..5b5903f56cc7 100644
--- a/toolchain/check/scope_stack.cpp
+++ b/toolchain/check/scope_stack.cpp
@@ -138,7 +138,7 @@ auto ScopeStack::LookupInLexicalScopes(SemIR::NameId name_id)
 
   // If we have no lexical results, check all non-lexical scopes.
   if (lexical_results.empty()) {
-    return {LexicalLookupHasLoadError() ? SemIR::ErrorInst::SingletonInstId
+    return {LexicalLookupHasLoadError() ? SemIR::ErrorInst::InstId
                                         : SemIR::InstId::None,
             non_lexical_scope_stack_};
   }
diff --git a/toolchain/check/type.cpp b/toolchain/check/type.cpp
index ceb8d88d97b8..2b6e070bcd04 100644
--- a/toolchain/check/type.cpp
+++ b/toolchain/check/type.cpp
@@ -72,7 +72,7 @@ auto ValidateFloatType(Context& context, SemIR::LocId loc_id,
 template <typename InstT, typename... EachArgT>
 static auto GetTypeImpl(Context& context, EachArgT... each_arg)
     -> SemIR::TypeId {
-  InstT inst = {SemIR::TypeType::SingletonTypeId, each_arg...};
+  InstT inst = {SemIR::TypeType::TypeId, each_arg...};
   return context.types().GetTypeIdForTypeConstantId(TryEvalInst(context, inst));
 }
 
diff --git a/toolchain/check/type_completion.cpp b/toolchain/check/type_completion.cpp
index b16336528b50..af1cfddaf106 100644
--- a/toolchain/check/type_completion.cpp
+++ b/toolchain/check/type_completion.cpp
@@ -227,7 +227,7 @@ auto TypeCompleter::ProcessStep() -> bool {
       }
       // For a pointer representation, the pointee also needs to be complete.
       if (info.value_repr.kind == SemIR::ValueRepr::Pointer) {
-        if (info.value_repr.type_id == SemIR::ErrorInst::SingletonTypeId) {
+        if (info.value_repr.type_id == SemIR::ErrorInst::TypeId) {
           break;
         }
         auto pointee_type_id =
@@ -561,8 +561,7 @@ auto RequireCompleteType(Context& context, SemIR::TypeId type_id,
     AddInstInNoBlock(
         context, loc_id,
         SemIR::RequireCompleteType{
-            .type_id =
-                GetSingletonType(context, SemIR::WitnessType::SingletonInstId),
+            .type_id = GetSingletonType(context, SemIR::WitnessType::InstId),
             .complete_type_inst_id = context.types().GetInstId(type_id)});
   }
 
@@ -645,7 +644,7 @@ auto AsCompleteType(Context& context, SemIR::TypeId type_id,
     -> SemIR::TypeId {
   return RequireCompleteType(context, type_id, loc_id, diagnoser)
              ? type_id
-             : SemIR::ErrorInst::SingletonTypeId;
+             : SemIR::ErrorInst::TypeId;
 }
 
 // Returns the type `type_id` if it is a concrete type, or produces an
@@ -658,7 +657,7 @@ auto AsConcreteType(Context& context, SemIR::TypeId type_id,
   return RequireConcreteType(context, type_id, loc_id, diagnoser,
                              abstract_diagnoser)
              ? type_id
-             : SemIR::ErrorInst::SingletonTypeId;
+             : SemIR::ErrorInst::TypeId;
 }
 
 auto NoteIncompleteClass(Context& context, SemIR::ClassId class_id,
diff --git a/toolchain/check/type_structure.cpp b/toolchain/check/type_structure.cpp
index b1fd6c81c8eb..ae12604f0af0 100644
--- a/toolchain/check/type_structure.cpp
+++ b/toolchain/check/type_structure.cpp
@@ -354,7 +354,7 @@ class TypeStructureBuilder {
       return SymbolicType();
     }
     // Non-type values are concrete, only types are symbolic.
-    if (type_id_of_inst_id != SemIR::TypeType::SingletonTypeId) {
+    if (type_id_of_inst_id != SemIR::TypeType::TypeId) {
       return SemIR::TypeId::None;
     }
     return context_->types().GetTypeIdForTypeInstId(inst_id);
diff --git a/toolchain/docs/adding_features.md b/toolchain/docs/adding_features.md
index 787f4e94bf1c..79a4b3ceff61 100644
--- a/toolchain/docs/adding_features.md
+++ b/toolchain/docs/adding_features.md
@@ -282,12 +282,12 @@ If the resulting SemIR needs a new instruction:
 
         -   Set `.is_type = InstIsType::Always` in its `Kind` definition.
         -   When constructing instructions of this kind, pass
-            `SemIR::TypeType::SingletonTypeId` in as the value of the `type_id`
-            field, as in:
+            `SemIR::TypeType::TypeId` in as the value of the `type_id` field, as
+            in:
 
             ```
             SemIR::InstId inst_id = AddInst<SemIR::NewInstKindName>(context,
-                node_id, {.type_id = SemIR::TypeType::SingletonTypeId, ...});
+                node_id, {.type_id = SemIR::TypeType::TypeId, ...});
             ```
 
     -   Although most instructions have distinct types represented by
@@ -299,11 +299,11 @@ If the resulting SemIR needs a new instruction:
         constructed as a special-case in
         [`File` construction](/toolchain/sem_ir/file.cpp). To get a type id for
         one of these builtin types, use something like
-        `GetSingletonType(context,SemIR::WitnessType::SingletonInstId)`, as in:
+        `GetSingletonType(context,SemIR::WitnessType::InstId)`, as in:
 
         ```
         SemIR::TypeId witness_type_id =
-            GetSingletonType(context, SemIR::WitnessType::SingletonInstId);
+            GetSingletonType(context, SemIR::WitnessType::InstId);
         SemIR::InstId inst_id = AddInst<SemIR::NewInstKindName>(
             context, node_id, {.type_id = witness_type_id, ...});
         ```
diff --git a/toolchain/lower/handle.cpp b/toolchain/lower/handle.cpp
index b21cc8434800..6906e99a85e2 100644
--- a/toolchain/lower/handle.cpp
+++ b/toolchain/lower/handle.cpp
@@ -36,7 +36,7 @@ auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
   llvm::Value* index;
   if (context.sem_ir().types().GetInstId(
           context.sem_ir().insts().Get(inst.index_id).type_id()) ==
-      SemIR::IntLiteralType::SingletonInstId) {
+      SemIR::IntLiteralType::InstId) {
     auto value = context.sem_ir().insts().GetAs<SemIR::IntValue>(
         context.sem_ir().constant_values().GetConstantInstId(inst.index_id));
     index = llvm::ConstantInt::get(context.llvm_context(),
@@ -73,7 +73,7 @@ auto HandleInst(FunctionContext& context, SemIR::InstId /*inst_id*/,
 auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
                 SemIR::BindAlias inst) -> void {
   auto type_inst_id = context.sem_ir().types().GetInstId(inst.type_id);
-  if (type_inst_id == SemIR::NamespaceType::SingletonInstId) {
+  if (type_inst_id == SemIR::NamespaceType::InstId) {
     return;
   }
 
@@ -83,7 +83,7 @@ auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
 auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
                 SemIR::ExportDecl inst) -> void {
   auto type_inst_id = context.sem_ir().types().GetInstId(inst.type_id);
-  if (type_inst_id == SemIR::NamespaceType::SingletonInstId) {
+  if (type_inst_id == SemIR::NamespaceType::InstId) {
     return;
   }
 
@@ -191,7 +191,7 @@ auto HandleInst(FunctionContext& /*context*/, SemIR::InstId /*inst_id*/,
 auto HandleInst(FunctionContext& context, SemIR::InstId inst_id,
                 SemIR::NameRef inst) -> void {
   auto type_inst_id = context.sem_ir().types().GetInstId(inst.type_id);
-  if (type_inst_id == SemIR::NamespaceType::SingletonInstId) {
+  if (type_inst_id == SemIR::NamespaceType::InstId) {
     return;
   }
 
diff --git a/toolchain/sem_ir/builtin_function_kind.cpp b/toolchain/sem_ir/builtin_function_kind.cpp
index d40d95d34f65..93c3056d1b9a 100644
--- a/toolchain/sem_ir/builtin_function_kind.cpp
+++ b/toolchain/sem_ir/builtin_function_kind.cpp
@@ -77,7 +77,7 @@ struct NoReturn {
 };
 
 // Constraint that a type is `bool`.
-using Bool = BuiltinType<BoolType::SingletonInstId>;
+using Bool = BuiltinType<BoolType::InstId>;
 
 // Constraint that requires the type to be a sized integer type.
 struct AnySizedInt {
@@ -92,8 +92,7 @@ struct AnyInt {
   static auto Check(const File& sem_ir, ValidateState& state, TypeId type_id)
       -> bool {
     return AnySizedInt::Check(sem_ir, state, type_id) ||
-           BuiltinType<IntLiteralType::SingletonInstId>::Check(sem_ir, state,
-                                                               type_id);
+           BuiltinType<IntLiteralType::InstId>::Check(sem_ir, state, type_id);
   }
 };
 
@@ -101,8 +100,7 @@ struct AnyInt {
 struct AnyFloat {
   static auto Check(const File& sem_ir, ValidateState& state, TypeId type_id)
       -> bool {
-    if (BuiltinType<LegacyFloatType::SingletonInstId>::Check(sem_ir, state,
-                                                             type_id)) {
+    if (BuiltinType<LegacyFloatType::InstId>::Check(sem_ir, state, type_id)) {
       return true;
     }
     return sem_ir.types().Is<FloatType>(type_id);
@@ -110,18 +108,17 @@ struct AnyFloat {
 };
 
 // Constraint that requires the type to be the type type.
-using Type = BuiltinType<TypeType::SingletonInstId>;
+using Type = BuiltinType<TypeType::InstId>;
 
 // Constraint that requires the type to be a type value, whose type is type
 // type. Also accepts symbolic constant value types.
 struct AnyType {
   static auto Check(const File& sem_ir, ValidateState& state, TypeId type_id)
       -> bool {
-    if (BuiltinType<TypeType::SingletonInstId>::Check(sem_ir, state, type_id)) {
+    if (BuiltinType<TypeType::InstId>::Check(sem_ir, state, type_id)) {
       return true;
     }
-    return sem_ir.types().GetAsInst(type_id).type_id() ==
-           TypeType::SingletonTypeId;
+    return sem_ir.types().GetAsInst(type_id).type_id() == TypeType::TypeId;
   }
 };
 
diff --git a/toolchain/sem_ir/class.cpp b/toolchain/sem_ir/class.cpp
index 1bb31427fc56..aed7198731d8 100644
--- a/toolchain/sem_ir/class.cpp
+++ b/toolchain/sem_ir/class.cpp
@@ -16,8 +16,8 @@ static auto GetFoundationType(const File& file, SpecificId specific_id,
   if (!inst_id.has_value()) {
     return TypeId::None;
   }
-  if (inst_id == SemIR::ErrorInst::SingletonInstId) {
-    return ErrorInst::SingletonTypeId;
+  if (inst_id == SemIR::ErrorInst::InstId) {
+    return ErrorInst::TypeId;
   }
   return TypeId::ForTypeConstant(GetConstantValueInSpecific(
       file, specific_id,
@@ -41,8 +41,8 @@ auto Class::GetObjectRepr(const File& file, SpecificId specific_id) const
   }
   auto witness_id =
       GetConstantValueInSpecific(file, specific_id, complete_type_witness_id);
-  if (witness_id == ErrorInst::SingletonConstantId) {
-    return ErrorInst::SingletonTypeId;
+  if (witness_id == ErrorInst::ConstantId) {
+    return ErrorInst::TypeId;
   }
   return file.types().GetTypeIdForTypeInstId(
       file.insts()
diff --git a/toolchain/sem_ir/file.cpp b/toolchain/sem_ir/file.cpp
index 923e2c0c1285..1d57897bde61 100644
--- a/toolchain/sem_ir/file.cpp
+++ b/toolchain/sem_ir/file.cpp
@@ -35,12 +35,12 @@ File::File(const Parse::Tree* parse_tree, CheckIRId check_ir_id,
       inst_blocks_(allocator_),
       constants_(this) {
   // `type` and the error type are both complete & concrete types.
-  types_.SetComplete(TypeType::SingletonTypeId,
-                     {.value_repr = {.kind = ValueRepr::Copy,
-                                     .type_id = TypeType::SingletonTypeId}});
-  types_.SetComplete(ErrorInst::SingletonTypeId,
-                     {.value_repr = {.kind = ValueRepr::Copy,
-                                     .type_id = ErrorInst::SingletonTypeId}});
+  types_.SetComplete(
+      TypeType::TypeId,
+      {.value_repr = {.kind = ValueRepr::Copy, .type_id = TypeType::TypeId}});
+  types_.SetComplete(
+      ErrorInst::TypeId,
+      {.value_repr = {.kind = ValueRepr::Copy, .type_id = ErrorInst::TypeId}});
 
   insts_.Reserve(SingletonInstKinds.size());
   for (auto kind : SingletonInstKinds) {
diff --git a/toolchain/sem_ir/ids.cpp b/toolchain/sem_ir/ids.cpp
index cdc9502b2def..87517323dab8 100644
--- a/toolchain/sem_ir/ids.cpp
+++ b/toolchain/sem_ir/ids.cpp
@@ -132,11 +132,11 @@ auto InstBlockId::Print(llvm::raw_ostream& out) const -> void {
 
 auto TypeId::Print(llvm::raw_ostream& out) const -> void {
   out << Label << "(";
-  if (*this == TypeType::SingletonTypeId) {
+  if (*this == TypeType::TypeId) {
     out << "TypeType";
-  } else if (*this == AutoType::SingletonTypeId) {
+  } else if (*this == AutoType::TypeId) {
     out << "AutoType";
-  } else if (*this == ErrorInst::SingletonTypeId) {
+  } else if (*this == ErrorInst::TypeId) {
     out << "Error";
   } else {
     AsConstantId().Print(out, /*disambiguate=*/false);
diff --git a/toolchain/sem_ir/inst.h b/toolchain/sem_ir/inst.h
index d536a2373ed9..d443869df97d 100644
--- a/toolchain/sem_ir/inst.h
+++ b/toolchain/sem_ir/inst.h
@@ -164,8 +164,8 @@ class Inst : public Printable<Inst> {
     // Error uses a self-referential type so that it's not accidentally treated
     // as a normal type. Every other builtin is a type, including the
     // self-referential TypeType.
-    auto type_id = kind == InstKind::ErrorInst ? ErrorInst::SingletonTypeId
-                                               : TypeType::SingletonTypeId;
+    auto type_id =
+        kind == InstKind::ErrorInst ? ErrorInst::TypeId : TypeType::TypeId;
     return Inst(kind, type_id, InstId::NoneIndex, InstId::NoneIndex);
   }
 
diff --git a/toolchain/sem_ir/inst_fingerprinter.cpp b/toolchain/sem_ir/inst_fingerprinter.cpp
index a7442fb2ff54..57dc3338d0ac 100644
--- a/toolchain/sem_ir/inst_fingerprinter.cpp
+++ b/toolchain/sem_ir/inst_fingerprinter.cpp
@@ -404,8 +404,8 @@ struct Worklist {
 
       // Don't include the type if it's `type` or `<error>`, because those types
       // are self-referential.
-      if (inst.type_id() != TypeType::SingletonTypeId &&
-          inst.type_id() != ErrorInst::SingletonTypeId) {
+      if (inst.type_id() != TypeType::TypeId &&
+          inst.type_id() != ErrorInst::TypeId) {
         Add(inst.type_id());
       }
 
diff --git a/toolchain/sem_ir/name_scope.h b/toolchain/sem_ir/name_scope.h
index 1a044c4df536..34a771d1ff96 100644
--- a/toolchain/sem_ir/name_scope.h
+++ b/toolchain/sem_ir/name_scope.h
@@ -35,7 +35,7 @@ enum class AccessKind : int8_t {
 // - Represent that an error has occurred during lookup. This is still
 //   considered found and the error `InstId` is considered existing. Can be
 //   constructed using `MakeError()` or using `MakeWrappedLookupResult()` with
-//   `ErrorInst::SingletonInstId`.
+//   `ErrorInst::InstId`.
 class ScopeLookupResult {
  public:
   static auto MakeFound(InstId target_inst_id, AccessKind access_kind)
@@ -53,7 +53,7 @@ class ScopeLookupResult {
   }
 
   static auto MakeError() -> ScopeLookupResult {
-    return MakeFound(ErrorInst::SingletonInstId, AccessKind::Public);
+    return MakeFound(ErrorInst::InstId, AccessKind::Public);
   }
 
   static auto MakeWrappedLookupResult(InstId target_inst_id,
diff --git a/toolchain/sem_ir/name_scope_test.cpp b/toolchain/sem_ir/name_scope_test.cpp
index 204eb0e0554d..cebc72c1378d 100644
--- a/toolchain/sem_ir/name_scope_test.cpp
+++ b/toolchain/sem_ir/name_scope_test.cpp
@@ -39,12 +39,12 @@ TEST(ScopeLookupResult, MakeWrappedLookupResultUsingNoneInstId) {
 }
 
 TEST(ScopeLookupResult, MakeWrappedLookupResultUsingErrorInst) {
-  auto result = ScopeLookupResult::MakeWrappedLookupResult(
-      ErrorInst::SingletonInstId, AccessKind::Private);
+  auto result = ScopeLookupResult::MakeWrappedLookupResult(ErrorInst::InstId,
+                                                           AccessKind::Private);
 
   EXPECT_FALSE(result.is_poisoned());
   EXPECT_TRUE(result.is_found());
-  EXPECT_EQ(result.target_inst_id(), ErrorInst::SingletonInstId);
+  EXPECT_EQ(result.target_inst_id(), ErrorInst::InstId);
   EXPECT_DEATH(result.poisoning_loc_id(), "is_poisoned");
   EXPECT_EQ(result.access_kind(), AccessKind::Private);
   EXPECT_TRUE(result == result);
@@ -96,7 +96,7 @@ TEST(ScopeLookupResult, MakeError) {
 
   EXPECT_FALSE(result.is_poisoned());
   EXPECT_TRUE(result.is_found());
-  EXPECT_EQ(result.target_inst_id(), ErrorInst::SingletonInstId);
+  EXPECT_EQ(result.target_inst_id(), ErrorInst::InstId);
   EXPECT_DEATH(result.poisoning_loc_id(), "is_poisoned");
   EXPECT_EQ(result.access_kind(), AccessKind::Public);
   EXPECT_TRUE(result == result);
diff --git a/toolchain/sem_ir/singleton_insts.h b/toolchain/sem_ir/singleton_insts.h
index fec05850ac04..30f6eb32cbd9 100644
--- a/toolchain/sem_ir/singleton_insts.h
+++ b/toolchain/sem_ir/singleton_insts.h
@@ -33,7 +33,7 @@ static constexpr std::array SingletonInstKinds = {
 constexpr auto IsSingletonInstKind(InstKind kind) -> bool;
 
 // Provides the InstId for singleton instructions. These are exposed as
-// `InstT::SingletonInstId` in `typed_insts.h`.
+// `InstT::InstId` in `typed_insts.h`.
 template <InstKind::RawEnumType Kind>
   requires(IsSingletonInstKind(InstKind::Make(Kind)))
 constexpr auto MakeSingletonInstId() -> TypeInstId;
diff --git a/toolchain/sem_ir/stringify.cpp b/toolchain/sem_ir/stringify.cpp
index 28be48bdf141..bbbe580140f4 100644
--- a/toolchain/sem_ir/stringify.cpp
+++ b/toolchain/sem_ir/stringify.cpp
@@ -706,10 +706,9 @@ auto StringifySpecific(const File& sem_ir, SpecificId specific_id)
       // TODO: This duplicates work done in StringifyInst for ClassType.
       const auto& class_info = sem_ir.classes().Get(class_decl.class_id);
       if (auto literal_info = NumericTypeLiteralInfo::ForType(
-              sem_ir,
-              SemIR::ClassType{.type_id = SemIR::TypeType::SingletonTypeId,
-                               .class_id = class_decl.class_id,
-                               .specific_id = specific_id});
+              sem_ir, SemIR::ClassType{.type_id = SemIR::TypeType::TypeId,
+                                       .class_id = class_decl.class_id,
+                                       .specific_id = specific_id});
           literal_info.is_valid()) {
         RawStringOstream out;
         literal_info.PrintLiteral(sem_ir, out);
diff --git a/toolchain/sem_ir/type.cpp b/toolchain/sem_ir/type.cpp
index 9ae6de37fe3b..8b95785b0129 100644
--- a/toolchain/sem_ir/type.cpp
+++ b/toolchain/sem_ir/type.cpp
@@ -14,10 +14,10 @@ static void CheckTypeOfConstantIsTypeType(File& file, ConstantId constant_id) {
                "Canonicalizing non-constant type: {0}", constant_id);
   auto type_id =
       file.insts().Get(file.constant_values().GetInstId(constant_id)).type_id();
-  CARBON_CHECK(type_id == TypeType::SingletonTypeId ||
-                   constant_id == ErrorInst::SingletonConstantId,
-               "Forming type ID for non-type constant of type {0}",
-               file.types().GetAsInst(type_id));
+  CARBON_CHECK(
+      type_id == TypeType::TypeId || constant_id == ErrorInst::ConstantId,
+      "Forming type ID for non-type constant of type {0}",
+      file.types().GetAsInst(type_id));
 }
 
 auto TypeStore::GetTypeIdForTypeConstantId(ConstantId constant_id) const
@@ -80,7 +80,7 @@ static auto TryGetIntTypeInfo(const File& file, TypeId type_id)
     return std::nullopt;
   }
   auto inst_id = file.types().GetInstId(object_repr_id);
-  if (inst_id == IntLiteralType::SingletonInstId) {
+  if (inst_id == IntLiteralType::InstId) {
     // `Core.IntLiteral` has an unknown bit-width.
     return TypeStore::IntTypeInfo{.is_signed = true, .bit_width = IntId::None};
   }
diff --git a/toolchain/sem_ir/type.h b/toolchain/sem_ir/type.h
index 4ea54174539d..6773b6e0a33e 100644
--- a/toolchain/sem_ir/type.h
+++ b/toolchain/sem_ir/type.h
@@ -167,8 +167,7 @@ class TypeStore : public Yaml::Printable<TypeStore> {
 
   // Returns whether `type_id` represents a facet type.
   auto IsFacetType(SemIR::TypeId type_id) const -> bool {
-    return type_id == SemIR::TypeType::SingletonTypeId ||
-           Is<SemIR::FacetType>(type_id);
+    return type_id == SemIR::TypeType::TypeId || Is<SemIR::FacetType>(type_id);
   }
 
   // Returns a list of types that were completed in this file, in the order in
diff --git a/toolchain/sem_ir/typed_insts.h b/toolchain/sem_ir/typed_insts.h
index e3d77e607ac8..5ee088626362 100644
--- a/toolchain/sem_ir/typed_insts.h
+++ b/toolchain/sem_ir/typed_insts.h
@@ -17,8 +17,9 @@
 //
 // - Either a `Kind` constant, or a `Kinds` constant and an `InstKind kind;`
 //   member. These are described below.
-// - Optionally, a `SingletonInstId` if it is a singleton instruction.
-//   Similarly, there may be `SingletonConstantId` and `SingletonTypeId`.
+// - Optionally, a `InstId` if it is a singleton instruction. Similarly, there
+//   may be `ConstantId` and `TypeId`.
+//   - These are named based on the `Id` types they represent.
 // - Optionally, a `TypeId type_id;` member, for instructions that produce a
 //   value. This includes instructions that produce an abstract value, such as a
 //   `Namespace`, for which a placeholder type should be used.
@@ -272,11 +273,11 @@ struct AutoType {
       {.ir_name = "auto",
        .is_type = InstIsType::Always,
        .constant_kind = InstConstantKind::Always});
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
-  static constexpr auto SingletonTypeId =
-      TypeId::ForTypeConstant(ConstantId::ForConcreteConstant(SingletonInstId));
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
+  static constexpr auto TypeId =
+      TypeId::ForTypeConstant(ConstantId::ForConcreteConstant(InstId));
 
-  TypeId type_id;
+  SemIR::TypeId type_id;
 };
 
 // A base in a class, of the form `base: base_type;`. A base class is an
@@ -427,7 +428,7 @@ struct BoolType {
        .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -458,7 +459,7 @@ struct BoundMethodType {
            .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -660,15 +661,12 @@ struct ErrorInst {
       {.ir_name = "<error>",
        .is_type = InstIsType::Always,
        .constant_kind = InstConstantKind::Always});
-  static constexpr InstId SingletonInstId = MakeSingletonInstId<Kind>();
-  static constexpr auto SingletonConstantId =
-      ConstantId::ForConcreteConstant(SingletonInstId);
-  static constexpr auto SingletonTypeId =
-      TypeId::ForTypeConstant(SingletonConstantId);
-  static constexpr auto SingletonTypeInstId =
-      TypeInstId::UnsafeMake(SingletonInstId);
+  static constexpr SemIR::InstId InstId = MakeSingletonInstId<Kind>();
+  static constexpr auto ConstantId = ConstantId::ForConcreteConstant(InstId);
+  static constexpr auto TypeId = TypeId::ForTypeConstant(ConstantId);
+  static constexpr auto TypeInstId = TypeInstId::UnsafeMake(InstId);
 
-  TypeId type_id;
+  SemIR::TypeId type_id;
 };
 
 // An `export bind_name` declaration.
@@ -795,7 +793,7 @@ struct LegacyFloatType {
            .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -1001,7 +999,7 @@ struct ImplWitnessTablePlaceholder {
           {.ir_name = "impl_witness_table_placeholder",
            .constant_kind = InstConstantKind::Unique,
            .is_lowered = false});
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -1080,11 +1078,11 @@ struct InstType {
       {.ir_name = "<instruction>",
        .is_type = InstIsType::Always,
        .constant_kind = InstConstantKind::Always});
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
-  static constexpr auto SingletonTypeId =
-      TypeId::ForTypeConstant(ConstantId::ForConcreteConstant(SingletonInstId));
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
+  static constexpr auto TypeId =
+      TypeId::ForTypeConstant(ConstantId::ForConcreteConstant(InstId));
 
-  TypeId type_id;
+  SemIR::TypeId type_id;
 };
 
 // A value of type `InstType` that refers to an instruction. This is used to
@@ -1135,7 +1133,7 @@ struct IntLiteralType {
            .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -1235,7 +1233,7 @@ struct NamespaceType {
            .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -1540,7 +1538,7 @@ struct SpecificFunctionType {
            .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -1615,7 +1613,7 @@ struct StringType {
        .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -1792,11 +1790,11 @@ struct TypeType {
       {.ir_name = "type",
        .is_type = InstIsType::Always,
        .constant_kind = InstConstantKind::Always});
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
-  static constexpr auto SingletonTypeId =
-      TypeId::ForTypeConstant(ConstantId::ForConcreteConstant(SingletonInstId));
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
+  static constexpr auto TypeId =
+      TypeId::ForTypeConstant(ConstantId::ForConcreteConstant(InstId));
 
-  TypeId type_id;
+  SemIR::TypeId type_id;
 };
 
 // The `not` operator, such as `not operand`.
@@ -1887,7 +1885,7 @@ struct VtableType {
        .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };
@@ -1936,7 +1934,7 @@ struct WitnessType {
        .constant_kind = InstConstantKind::Always});
   // This is a singleton instruction. However, it may still evolve into a more
   // standard type and be removed.
-  static constexpr auto SingletonInstId = MakeSingletonInstId<Kind>();
+  static constexpr auto InstId = MakeSingletonInstId<Kind>();
 
   TypeId type_id;
 };