diff --git a/toolchain/check/call.cpp b/toolchain/check/call.cpp
index cd587f4ac4ae..c25818489142 100644
--- a/toolchain/check/call.cpp
+++ b/toolchain/check/call.cpp
@@ -94,7 +94,7 @@ static auto PerformCallToGenericClass(Context& context, SemIR::LocId loc_id,
                           EntityKind::GenericClass, enclosing_specific_id,
                           /*self_id=*/SemIR::InstId::Invalid, arg_ids);
   if (!callee_specific_id) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
   return context.GetOrAddInst<SemIR::ClassType>(
       loc_id, {.type_id = SemIR::TypeId::TypeType,
@@ -114,7 +114,7 @@ static auto PerformCallToGenericInterface(
                           EntityKind::GenericInterface, enclosing_specific_id,
                           /*self_id=*/SemIR::InstId::Invalid, arg_ids);
   if (!callee_specific_id) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
   return context.GetOrAddInst(loc_id, context.FacetTypeFromInterface(
                                           interface_id, *callee_specific_id));
@@ -144,7 +144,7 @@ auto PerformCall(Context& context, SemIR::LocId loc_id, SemIR::InstId callee_id,
                             "value of type {0} is not callable", TypeOfInstId);
           context.emitter().Emit(loc_id, CallToNonCallable, callee_id);
         }
-        return SemIR::InstId::BuiltinError;
+        return SemIR::InstId::BuiltinErrorInst;
       }
     }
   }
@@ -156,7 +156,7 @@ auto PerformCall(Context& context, SemIR::LocId loc_id, SemIR::InstId callee_id,
       EntityKind::Function, callee_function.enclosing_specific_id,
       callee_function.self_id, arg_ids);
   if (!callee_specific_id) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
   if (callee_specific_id->is_valid()) {
     callee_id = context.GetOrAddInst(
diff --git a/toolchain/check/context.cpp b/toolchain/check/context.cpp
index 83bb60bb9c6e..0959c6fae330 100644
--- a/toolchain/check/context.cpp
+++ b/toolchain/check/context.cpp
@@ -60,7 +60,7 @@ Context::Context(const Lex::TokenizedBuffer& tokens, DiagnosticEmitter& emitter,
   // Map the builtin `<error>` and `type` type constants to their corresponding
   // special `TypeId` values.
   type_ids_for_type_constants_.Insert(
-      SemIR::ConstantId::ForTemplateConstant(SemIR::InstId::BuiltinError),
+      SemIR::ConstantId::ForTemplateConstant(SemIR::InstId::BuiltinErrorInst),
       SemIR::TypeId::Error);
   type_ids_for_type_constants_.Insert(
       SemIR::ConstantId::ForTemplateConstant(SemIR::InstId::BuiltinTypeType),
@@ -345,7 +345,7 @@ auto Context::LookupUnqualifiedName(Parse::NodeId node_id,
   }
 
   return {.specific_id = SemIR::SpecificId::Invalid,
-          .inst_id = SemIR::InstId::BuiltinError};
+          .inst_id = SemIR::InstId::BuiltinErrorInst};
 }
 
 auto Context::LookupNameInExactScope(SemIRLoc loc, SemIR::NameId name_id,
@@ -571,7 +571,7 @@ auto Context::LookupQualifiedName(SemIRLoc loc, SemIR::NameId name_id,
       emitter_->Emit(loc, NameAmbiguousDueToExtend, name_id);
       // TODO: Add notes pointing to the scopes.
       return {.specific_id = SemIR::SpecificId::Invalid,
-              .inst_id = SemIR::InstId::BuiltinError};
+              .inst_id = SemIR::InstId::BuiltinErrorInst};
     }
 
     result.inst_id = scope_result_id;
@@ -597,7 +597,7 @@ auto Context::LookupQualifiedName(SemIRLoc loc, SemIR::NameId name_id,
     }
 
     return {.specific_id = SemIR::SpecificId::Invalid,
-            .inst_id = SemIR::InstId::BuiltinError};
+            .inst_id = SemIR::InstId::BuiltinErrorInst};
   }
 
   return result;
@@ -639,7 +639,7 @@ auto Context::LookupNameInCore(SemIRLoc loc, llvm::StringRef name)
     -> SemIR::InstId {
   auto core_package_id = GetCorePackage(*this, loc);
   if (!core_package_id.is_valid()) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   auto name_id = SemIR::NameId::ForIdentifier(identifiers().Add(name));
@@ -651,7 +651,7 @@ auto Context::LookupNameInCore(SemIRLoc loc, llvm::StringRef name)
         "name `Core.{0}` implicitly referenced here, but not found",
         SemIR::NameId);
     emitter_->Emit(loc, CoreNameNotFound, name_id);
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // Look through import_refs and aliases.
@@ -1006,11 +1006,11 @@ class TypeCompleter {
     switch (builtin.builtin_inst_kind) {
       case SemIR::BuiltinInstKind::TypeType:
       case SemIR::BuiltinInstKind::AutoType:
-      case SemIR::BuiltinInstKind::Error:
+      case SemIR::BuiltinInstKind::ErrorInst:
       case SemIR::BuiltinInstKind::Invalid:
       case SemIR::BuiltinInstKind::BoolType:
       case SemIR::BuiltinInstKind::IntLiteralType:
-      case SemIR::BuiltinInstKind::FloatType:
+      case SemIR::BuiltinInstKind::LegacyFloatType:
       case SemIR::BuiltinInstKind::NamespaceType:
       case SemIR::BuiltinInstKind::BoundMethodType:
       case SemIR::BuiltinInstKind::WitnessType:
diff --git a/toolchain/check/context.h b/toolchain/check/context.h
index 0d28c53f0ad9..a66d527e03a7 100644
--- a/toolchain/check/context.h
+++ b/toolchain/check/context.h
@@ -242,7 +242,7 @@ class Context {
       -> LookupResult;
 
   // Returns the instruction corresponding to a name in the core package, or
-  // BuiltinError if not found.
+  // BuiltinErrorInst if not found.
   auto LookupNameInCore(SemIRLoc loc, llvm::StringRef name) -> SemIR::InstId;
 
   // Prints a diagnostic for a duplicate name.
diff --git a/toolchain/check/convert.cpp b/toolchain/check/convert.cpp
index 4eadf9d1118d..7b245836695c 100644
--- a/toolchain/check/convert.cpp
+++ b/toolchain/check/convert.cpp
@@ -247,7 +247,7 @@ static auto ConvertTupleToArray(Context& context, SemIR::TupleType tuple_type,
                                ? ArrayInitFromExprArgCountMismatch
                                : ArrayInitFromLiteralArgCountMismatch,
                            array_bound, tuple_elem_types.size());
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   PendingBlock target_block_storage(context);
@@ -276,8 +276,8 @@ static auto ConvertTupleToArray(Context& context, SemIR::TupleType tuple_type,
             context, value_loc_id, value_id, src_type_id, literal_elems,
             ConversionTarget::FullInitializer, return_slot_id,
             array_type.element_type_id, target_block, i);
-    if (init_id == SemIR::InstId::BuiltinError) {
-      return SemIR::InstId::BuiltinError;
+    if (init_id == SemIR::InstId::BuiltinErrorInst) {
+      return SemIR::InstId::BuiltinErrorInst;
     }
     inits.push_back(init_id);
   }
@@ -325,7 +325,7 @@ static auto ConvertTupleToTuple(Context& context, SemIR::TupleType src_type,
                       IntAsSelect, IntAsSelect);
     context.emitter().Emit(value_loc_id, TupleInitElementCountMismatch,
                            dest_elem_types.size(), src_elem_types.size());
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // If we're forming an initializer, then we want an initializer for each
@@ -357,8 +357,8 @@ static auto ConvertTupleToTuple(Context& context, SemIR::TupleType src_type,
         ConvertAggregateElement<SemIR::TupleAccess, SemIR::TupleAccess>(
             context, value_loc_id, value_id, src_type_id, literal_elems,
             inner_kind, target.init_id, dest_type_id, target.init_block, i);
-    if (init_id == SemIR::InstId::BuiltinError) {
-      return SemIR::InstId::BuiltinError;
+    if (init_id == SemIR::InstId::BuiltinErrorInst) {
+      return SemIR::InstId::BuiltinErrorInst;
     }
     new_block.Set(i, init_id);
   }
@@ -423,7 +423,7 @@ static auto ConvertStructToStructOrClass(Context& context,
     context.emitter().Emit(value_loc_id, StructInitElementCountMismatch,
                            ToClass, dest_elem_fields_size,
                            src_elem_fields.size());
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // Prepare to look up fields in the source by index.
@@ -459,7 +459,7 @@ static auto ConvertStructToStructOrClass(Context& context,
   for (auto [i, dest_field] : llvm::enumerate(dest_elem_fields)) {
     if (dest_field.name_id == SemIR::NameId::Vptr) {
       // TODO: Initialize the vptr to point to a vtable.
-      new_block.Set(i, SemIR::InstId::BuiltinError);
+      new_block.Set(i, SemIR::InstId::BuiltinErrorInst);
       continue;
     }
 
@@ -485,7 +485,7 @@ static auto ConvertStructToStructOrClass(Context& context,
                                  StructInitMissingFieldInConversion, value_id,
                                  target.type_id, dest_field.name_id);
         }
-        return SemIR::InstId::BuiltinError;
+        return SemIR::InstId::BuiltinErrorInst;
       }
     }
     auto src_field = src_elem_fields[src_field_index];
@@ -497,8 +497,8 @@ static auto ConvertStructToStructOrClass(Context& context,
             context, value_loc_id, value_id, src_field.type_id, literal_elems,
             inner_kind, target.init_id, dest_field.type_id, target.init_block,
             src_field_index);
-    if (init_id == SemIR::InstId::BuiltinError) {
-      return SemIR::InstId::BuiltinError;
+    if (init_id == SemIR::InstId::BuiltinErrorInst) {
+      return SemIR::InstId::BuiltinErrorInst;
     }
     new_block.Set(i, init_id);
   }
@@ -548,7 +548,7 @@ static auto ConvertStructToClass(Context& context, SemIR::StructType src_type,
   auto object_repr_id =
       dest_class_info.GetObjectRepr(context.sem_ir(), dest_type.specific_id);
   if (object_repr_id == SemIR::TypeId::Error) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
   auto dest_struct_type =
       context.types().GetAs<SemIR::StructType>(object_repr_id);
@@ -911,7 +911,7 @@ static auto PerformCopy(Context& context, SemIR::InstId expr_id)
   auto expr = context.insts().Get(expr_id);
   auto type_id = expr.type_id();
   if (type_id == SemIR::TypeId::Error) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // TODO: Directly track on the value representation whether it's a copy of
@@ -930,7 +930,7 @@ static auto PerformCopy(Context& context, SemIR::InstId expr_id)
   CARBON_DIAGNOSTIC(CopyOfUncopyableType, Error,
                     "cannot copy value of type {0}", TypeOfInstId);
   context.emitter().Emit(expr_id, CopyOfUncopyableType, expr_id);
-  return SemIR::InstId::BuiltinError;
+  return SemIR::InstId::BuiltinErrorInst;
 }
 
 auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
@@ -942,7 +942,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
   // result is invalid and we shouldn't error.
   if (sem_ir.insts().Get(expr_id).type_id() == SemIR::TypeId::Error ||
       target.type_id == SemIR::TypeId::Error) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   if (SemIR::GetExprCategory(sem_ir, expr_id) == SemIR::ExprCategory::NotExpr) {
@@ -952,7 +952,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
     CARBON_DIAGNOSTIC(UseOfNonExprAsValue, Error,
                       "expression cannot be used as a value");
     context.emitter().Emit(expr_id, UseOfNonExprAsValue);
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // We can only perform initialization for complete types.
@@ -985,12 +985,12 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
             return context.emitter().Build(loc_id, AbstractTypeInInit,
                                            target.type_id);
           })) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // Check whether any builtin conversion applies.
   expr_id = PerformBuiltinConversion(context, loc_id, expr_id, target);
-  if (expr_id == SemIR::InstId::BuiltinError) {
+  if (expr_id == SemIR::InstId::BuiltinErrorInst) {
     return expr_id;
   }
 
@@ -1020,7 +1020,7 @@ 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::InstId::BuiltinError &&
+    if (expr_id != SemIR::InstId::BuiltinErrorInst &&
         CanUseValueOfInitializer(sem_ir, target.type_id, target.kind)) {
       expr_id = context.AddInst<SemIR::ValueOfInitializer>(
           loc_id, {.type_id = target.type_id, .init_id = expr_id});
@@ -1049,7 +1049,7 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
                    sem_ir.insts().Get(expr_id));
 
     case SemIR::ExprCategory::Error:
-      return SemIR::InstId::BuiltinError;
+      return SemIR::InstId::BuiltinErrorInst;
 
     case SemIR::ExprCategory::Initializing:
       if (target.is_initializer()) {
@@ -1202,7 +1202,7 @@ auto ConvertCallArgs(Context& context, SemIR::LocId call_loc_id,
         .Build(call_loc_id, MissingObjectInMethodCall)
         .Note(callee_decl_id, InCallToFunction)
         .Emit();
-    self_id = SemIR::InstId::BuiltinError;
+    self_id = SemIR::InstId::BuiltinErrorInst;
   }
 
   return CallerPatternMatch(context, callee_specific_id, self_param_id,
@@ -1214,7 +1214,7 @@ auto ExprAsType(Context& context, SemIR::LocId loc_id, SemIR::InstId value_id)
     -> TypeExpr {
   auto type_inst_id =
       ConvertToValueOfType(context, loc_id, value_id, SemIR::TypeId::TypeType);
-  if (type_inst_id == SemIR::InstId::BuiltinError) {
+  if (type_inst_id == SemIR::InstId::BuiltinErrorInst) {
     return {.inst_id = type_inst_id, .type_id = SemIR::TypeId::Error};
   }
 
@@ -1223,7 +1223,7 @@ auto ExprAsType(Context& context, SemIR::LocId loc_id, SemIR::InstId value_id)
     CARBON_DIAGNOSTIC(TypeExprEvaluationFailure, Error,
                       "cannot evaluate type expression");
     context.emitter().Emit(loc_id, TypeExprEvaluationFailure);
-    return {.inst_id = SemIR::InstId::BuiltinError,
+    return {.inst_id = SemIR::InstId::BuiltinErrorInst,
             .type_id = SemIR::TypeId::Error};
   }
 
diff --git a/toolchain/check/convert.h b/toolchain/check/convert.h
index 86cf9fdf85d1..5630e68ea820 100644
--- a/toolchain/check/convert.h
+++ b/toolchain/check/convert.h
@@ -102,7 +102,7 @@ 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 `InstId::BuiltinError`.
+  // The converted expression of type `type`, or `InstId::BuiltinErrorInst`.
   SemIR::InstId inst_id;
   // The corresponding type, or `TypeId::Error`.
   SemIR::TypeId type_id;
diff --git a/toolchain/check/deduce.cpp b/toolchain/check/deduce.cpp
index edd57ef5783b..8096f005d649 100644
--- a/toolchain/check/deduce.cpp
+++ b/toolchain/check/deduce.cpp
@@ -322,7 +322,7 @@ auto DeductionContext::Deduce() -> bool {
       DiagnosticAnnotationScope annotate_diagnostics(&context().emitter(),
                                                      note_initializing_param);
       arg_id = ConvertToValueOfType(context(), loc_id_, arg_id, param_type_id);
-      if (arg_id == SemIR::InstId::BuiltinError) {
+      if (arg_id == SemIR::InstId::BuiltinErrorInst) {
         return false;
       }
     }
diff --git a/toolchain/check/eval.cpp b/toolchain/check/eval.cpp
index a8ab7ffc3b08..0615adcc8c27 100644
--- a/toolchain/check/eval.cpp
+++ b/toolchain/check/eval.cpp
@@ -1075,7 +1075,8 @@ static auto MakeConstantForBuiltinCall(Context& context, SemIRLoc loc,
       if (!ValidateFloatBitWidth(context, loc, arg_ids[0])) {
         return SemIR::ConstantId::Error;
       }
-      return context.constant_values().Get(SemIR::InstId::BuiltinFloatType);
+      return context.constant_values().Get(
+          SemIR::InstId::BuiltinLegacyFloatType);
     }
 
     case SemIR::BuiltinFunctionKind::BoolMakeType: {
@@ -1716,7 +1717,8 @@ auto TryEvalBlockForSpecific(Context& context, SemIR::SpecificId specific_id,
     result[i] = context.constant_values().GetInstId(const_id);
 
     // TODO: If this becomes possible through monomorphization failure, produce
-    // a diagnostic and put `SemIR::InstId::BuiltinError` in the table entry.
+    // a diagnostic and put `SemIR::InstId::BuiltinErrorInst` in the table
+    // entry.
     CARBON_CHECK(result[i].is_valid());
   }
 
diff --git a/toolchain/check/handle_alias.cpp b/toolchain/check/handle_alias.cpp
index dd3dd3999e02..1a84eb4cc341 100644
--- a/toolchain/check/handle_alias.cpp
+++ b/toolchain/check/handle_alias.cpp
@@ -62,7 +62,7 @@ auto HandleParseNode(Context& context, Parse::AliasId /*node_id*/) -> bool {
                       "alias initializer must be a name reference");
     context.emitter().Emit(expr_node, AliasRequiresNameRef);
     alias_type_id = SemIR::TypeId::Error;
-    alias_value_id = SemIR::InstId::BuiltinError;
+    alias_value_id = SemIR::InstId::BuiltinErrorInst;
   }
   auto alias_id = context.AddInst<SemIR::BindAlias>(
       name_context.loc_id, {.type_id = alias_type_id,
diff --git a/toolchain/check/handle_array.cpp b/toolchain/check/handle_array.cpp
index c9a6a5470f64..a7b89680611c 100644
--- a/toolchain/check/handle_array.cpp
+++ b/toolchain/check/handle_array.cpp
@@ -46,7 +46,7 @@ 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::InstId::BuiltinError);
+    context.node_stack().Push(node_id, SemIR::InstId::BuiltinErrorInst);
     return true;
   }
 
diff --git a/toolchain/check/handle_binding_pattern.cpp b/toolchain/check/handle_binding_pattern.cpp
index f8d777926ce6..5dc845637ec0 100644
--- a/toolchain/check/handle_binding_pattern.cpp
+++ b/toolchain/check/handle_binding_pattern.cpp
@@ -201,10 +201,10 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
           break;
       }
       if (had_error) {
-        context.AddNameToLookup(name_id, SemIR::InstId::BuiltinError);
+        context.AddNameToLookup(name_id, SemIR::InstId::BuiltinErrorInst);
         // Replace the parameter with an invalid instruction so that we don't
         // try constructing a generic based on it.
-        param_pattern_id = SemIR::InstId::BuiltinError;
+        param_pattern_id = SemIR::InstId::BuiltinErrorInst;
       } else {
         auto bind_id = context.AddInstInNoBlock(
             make_bind_name(cast_type_id, SemIR::InstId::Invalid));
diff --git a/toolchain/check/handle_class.cpp b/toolchain/check/handle_class.cpp
index b9092cd5e89a..df002374430b 100644
--- a/toolchain/check/handle_class.cpp
+++ b/toolchain/check/handle_class.cpp
@@ -569,7 +569,7 @@ static auto CheckCompleteAdapterClassType(Context& context,
         .Build(class_info.adapt_id, AdaptWithBase)
         .Note(class_info.base_id, AdaptWithBaseHere)
         .Emit();
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   if (auto fields = context.struct_type_fields().Get(fields_id);
@@ -584,7 +584,7 @@ static auto CheckCompleteAdapterClassType(Context& context,
         .Build(class_info.adapt_id, AdaptWithFields)
         .Note(first_field_inst_id, AdaptWithFieldHere)
         .Emit();
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   for (auto inst_id : context.inst_block_stack().PeekCurrentBlockContents()) {
@@ -601,7 +601,7 @@ static auto CheckCompleteAdapterClassType(Context& context,
             .Build(class_info.adapt_id, AdaptWithVirtual)
             .Note(inst_id, AdaptWithVirtualHere)
             .Emit();
-        return SemIR::InstId::BuiltinError;
+        return SemIR::InstId::BuiltinErrorInst;
       }
     }
   }
diff --git a/toolchain/check/handle_function.cpp b/toolchain/check/handle_function.cpp
index 5326dd396cb4..a819acab08d2 100644
--- a/toolchain/check/handle_function.cpp
+++ b/toolchain/check/handle_function.cpp
@@ -345,7 +345,7 @@ static auto HandleFunctionDefinitionAfterSignature(
   for (auto param_ref_id : llvm::concat<const SemIR::InstId>(
            context.inst_blocks().GetOrEmpty(function.implicit_param_refs_id),
            context.inst_blocks().GetOrEmpty(function.param_refs_id))) {
-    if (param_ref_id == SemIR::InstId::BuiltinError) {
+    if (param_ref_id == SemIR::InstId::BuiltinErrorInst) {
       continue;
     }
     auto param_info =
diff --git a/toolchain/check/handle_index.cpp b/toolchain/check/handle_index.cpp
index ef3de125a6f2..667be17ff6a5 100644
--- a/toolchain/check/handle_index.cpp
+++ b/toolchain/check/handle_index.cpp
@@ -97,7 +97,7 @@ 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::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   Operator op{
@@ -160,7 +160,7 @@ auto HandleParseNode(Context& context, Parse::IndexExprId node_id) -> bool {
     }
 
     default: {
-      auto elem_id = SemIR::InstId::BuiltinError;
+      auto elem_id = SemIR::InstId::BuiltinErrorInst;
       if (operand_type_id != SemIR::TypeId::Error) {
         elem_id = PerformIndexWith(context, node_id, operand_inst_id,
                                    operand_type_id, index_inst_id);
diff --git a/toolchain/check/handle_let_and_var.cpp b/toolchain/check/handle_let_and_var.cpp
index cc6e34e2e9df..7d830c155338 100644
--- a/toolchain/check/handle_let_and_var.cpp
+++ b/toolchain/check/handle_let_and_var.cpp
@@ -245,8 +245,8 @@ auto HandleParseNode(Context& context, Parse::LetDeclId node_id) -> bool {
   auto bind_name = pattern.inst.As<SemIR::AnyBindName>();
   CARBON_CHECK(!bind_name.value_id.is_valid(),
                "Binding should not already have a value!");
-  bind_name.value_id =
-      decl_info->init_id ? *decl_info->init_id : SemIR::InstId::BuiltinError;
+  bind_name.value_id = decl_info->init_id ? *decl_info->init_id
+                                          : SemIR::InstId::BuiltinErrorInst;
   context.ReplaceInstBeforeConstantUse(decl_info->pattern_id, bind_name);
   context.inst_block_stack().AddInstId(decl_info->pattern_id);
 
diff --git a/toolchain/check/handle_literal.cpp b/toolchain/check/handle_literal.cpp
index 7e3237e14d60..b5e00f709171 100644
--- a/toolchain/check/handle_literal.cpp
+++ b/toolchain/check/handle_literal.cpp
@@ -64,7 +64,7 @@ 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::InstId::BuiltinError);
+    context.node_stack().Push(node_id, SemIR::InstId::BuiltinErrorInst);
     return true;
   }
 
@@ -74,7 +74,7 @@ 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::InstId::BuiltinError);
+    context.node_stack().Push(node_id, SemIR::InstId::BuiltinErrorInst);
     return true;
   }
 
@@ -84,9 +84,9 @@ auto HandleParseNode(Context& context, Parse::RealLiteralId node_id) -> bool {
 
   auto float_id = context.sem_ir().floats().Add(llvm::APFloat(double_val));
   context.AddInstAndPush<SemIR::FloatLiteral>(
-      node_id,
-      {.type_id = context.GetBuiltinType(SemIR::BuiltinInstKind::FloatType),
-       .float_id = float_id});
+      node_id, {.type_id = context.GetBuiltinType(
+                    SemIR::BuiltinInstKind::LegacyFloatType),
+                .float_id = float_id});
   return true;
 }
 
diff --git a/toolchain/check/handle_operator.cpp b/toolchain/check/handle_operator.cpp
index d0e3e8aea854..20b48691fb25 100644
--- a/toolchain/check/handle_operator.cpp
+++ b/toolchain/check/handle_operator.cpp
@@ -233,13 +233,13 @@ 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::InstId::BuiltinError;
+      value_id = SemIR::InstId::BuiltinErrorInst;
       break;
     default:
       CARBON_DIAGNOSTIC(AddrOfNonRef, Error,
                         "cannot take the address of non-reference expression");
       context.emitter().Emit(TokenOnly(node_id), AddrOfNonRef);
-      value_id = SemIR::InstId::BuiltinError;
+      value_id = SemIR::InstId::BuiltinErrorInst;
       break;
   }
   context.AddInstAndPush<SemIR::AddrOf>(
diff --git a/toolchain/check/handle_struct.cpp b/toolchain/check/handle_struct.cpp
index 6579d06680cd..53bc187831e9 100644
--- a/toolchain/check/handle_struct.cpp
+++ b/toolchain/check/handle_struct.cpp
@@ -139,7 +139,7 @@ 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::InstId::BuiltinError);
+    context.node_stack().Push(node_id, SemIR::InstId::BuiltinErrorInst);
   } else {
     auto type_id = context.GetStructType(
         context.struct_type_fields().AddCanonical(fields));
@@ -165,7 +165,7 @@ 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::InstId::BuiltinError);
+    context.node_stack().Push(node_id, SemIR::InstId::BuiltinErrorInst);
   } else {
     auto fields_id = context.struct_type_fields().AddCanonical(fields);
     context.AddInstAndPush<SemIR::StructType>(
diff --git a/toolchain/check/handle_where.cpp b/toolchain/check/handle_where.cpp
index 65b2b3a6bab6..d7918925c27d 100644
--- a/toolchain/check/handle_where.cpp
+++ b/toolchain/check/handle_where.cpp
@@ -103,7 +103,7 @@ auto HandleParseNode(Context& context, Parse::RequirementImplsId node_id)
         ImplsOnNonFacetType, Error,
         "right argument of `impls` requirement must be a facet type");
     context.emitter().Emit(rhs_node, ImplsOnNonFacetType);
-    rhs_as_type.inst_id = SemIR::InstId::BuiltinError;
+    rhs_as_type.inst_id = SemIR::InstId::BuiltinErrorInst;
   }
   // TODO: Require that at least one side uses a designator.
 
diff --git a/toolchain/check/impl.cpp b/toolchain/check/impl.cpp
index beede7a696dc..b62deebcdc4c 100644
--- a/toolchain/check/impl.cpp
+++ b/toolchain/check/impl.cpp
@@ -78,7 +78,7 @@ static auto GetSelfSpecificForInterfaceMemberWithSelfType(
 // Checks that `impl_function_id` is a valid implementation of the function
 // described in the interface as `interface_function_id`. Returns the value to
 // put into the corresponding slot in the witness table, which can be
-// `BuiltinError` if the function is not usable.
+// `BuiltinErrorInst` if the function is not usable.
 static auto CheckAssociatedFunctionImplementation(
     Context& context, SemIR::FunctionType interface_function_type,
     SemIR::InstId impl_decl_id, SemIR::TypeId self_type_id) -> SemIR::InstId {
@@ -95,7 +95,7 @@ static auto CheckAssociatedFunctionImplementation(
                            interface_function_type.function_id);
     builder.Emit();
 
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // Map from the specific for the function type to the specific for the
@@ -117,7 +117,7 @@ static auto CheckAssociatedFunctionImplementation(
           context.functions().Get(interface_function_type.function_id),
           interface_function_specific_id,
           /*check_syntax=*/false)) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
   return impl_decl_id;
 }
@@ -138,7 +138,7 @@ static auto BuildInterfaceWitness(
         return context.emitter().Build(
             impl.definition_id, ImplOfUndefinedInterface, interface.name_id);
       })) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   auto& impl_scope = context.name_scopes().Get(impl.scope_id);
@@ -159,7 +159,7 @@ static auto BuildInterfaceWitness(
     CARBON_KIND_SWITCH(decl) {
       case CARBON_KIND(SemIR::StructValue struct_value): {
         if (struct_value.type_id == SemIR::TypeId::Error) {
-          return SemIR::InstId::BuiltinError;
+          return SemIR::InstId::BuiltinErrorInst;
         }
         auto type_inst = context.types().GetAsInst(struct_value.type_id);
         auto fn_type = type_inst.TryAs<SemIR::FunctionType>();
@@ -184,7 +184,7 @@ static auto BuildInterfaceWitness(
           NoteAssociatedFunction(context, builder, fn_type->function_id);
           builder.Emit();
 
-          table.push_back(SemIR::InstId::BuiltinError);
+          table.push_back(SemIR::InstId::BuiltinErrorInst);
         }
         break;
       }
@@ -192,11 +192,11 @@ static auto BuildInterfaceWitness(
         // TODO: Check we have a value for this constant in the constraint.
         context.TODO(impl.definition_id,
                      "impl of interface with associated constant");
-        return SemIR::InstId::BuiltinError;
+        return SemIR::InstId::BuiltinErrorInst;
       default:
-        CARBON_CHECK(decl_id == SemIR::InstId::BuiltinError,
+        CARBON_CHECK(decl_id == SemIR::InstId::BuiltinErrorInst,
                      "Unexpected kind of associated entity {0}", decl);
-        table.push_back(SemIR::InstId::BuiltinError);
+        table.push_back(SemIR::InstId::BuiltinErrorInst);
         break;
     }
   }
@@ -215,13 +215,13 @@ auto BuildImplWitness(Context& context, SemIR::ImplId impl_id)
 
   auto facet_type_id = context.GetTypeIdForTypeInst(impl.constraint_id);
   if (facet_type_id == SemIR::TypeId::Error) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
   auto facet_type = context.types().TryGetAs<SemIR::FacetType>(facet_type_id);
   if (!facet_type) {
     CARBON_DIAGNOSTIC(ImplAsNonFacetType, Error, "impl as non-facet-type");
     context.emitter().Emit(impl.definition_id, ImplAsNonFacetType);
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
   const SemIR::FacetTypeInfo& facet_type_info =
       context.facet_types().Get(facet_type->facet_type_id);
@@ -229,7 +229,7 @@ auto BuildImplWitness(Context& context, SemIR::ImplId impl_id)
   auto interface = facet_type_info.TryAsSingleInterface();
   if (!interface) {
     context.TODO(impl.definition_id, "impl as not 1 interface");
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   llvm::SmallVector<SemIR::InstId> used_decl_ids;
diff --git a/toolchain/check/import_ref.cpp b/toolchain/check/import_ref.cpp
index e64b542641c3..38f78f33b826 100644
--- a/toolchain/check/import_ref.cpp
+++ b/toolchain/check/import_ref.cpp
@@ -993,7 +993,7 @@ class ImportRefResolver {
         break;
       }
       default: {
-        if (const_inst_id == SemIR::InstId::BuiltinError) {
+        if (const_inst_id == SemIR::InstId::BuiltinErrorInst) {
           return SemIR::NameScopeId::Invalid;
         }
         break;
diff --git a/toolchain/check/interface.cpp b/toolchain/check/interface.cpp
index 5bf4fca1cee0..03cb7b44e169 100644
--- a/toolchain/check/interface.cpp
+++ b/toolchain/check/interface.cpp
@@ -18,7 +18,7 @@ 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::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // The interface type is the type of `Self`.
diff --git a/toolchain/check/member_access.cpp b/toolchain/check/member_access.cpp
index 7a5bb6d87320..cce4dd077265 100644
--- a/toolchain/check/member_access.cpp
+++ b/toolchain/check/member_access.cpp
@@ -140,7 +140,7 @@ static auto PerformImplLookup(
     context.TODO(loc_id,
                  "Lookup of impl witness not yet supported except for a single "
                  "interface");
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   auto witness_id =
@@ -168,22 +168,22 @@ static auto PerformImplLookup(
                              interface_type_id,
                              context.GetTypeIdForTypeConstant(type_const_id));
     }
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   auto member_value_id = context.constant_values().GetConstantInstId(member_id);
   if (!member_value_id.is_valid()) {
-    if (member_value_id != SemIR::InstId::BuiltinError) {
+    if (member_value_id != SemIR::InstId::BuiltinErrorInst) {
       context.TODO(member_id, "non-constant associated entity");
     }
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   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::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // TODO: This produces the type of the associated entity with no value for
@@ -200,7 +200,7 @@ static auto PerformImplLookup(
 
 // Performs a member name lookup into the specified scope, including performing
 // impl lookup if necessary. If the scope is invalid, assume an error has
-// already been diagnosed, and return BuiltinError.
+// already been diagnosed, and return BuiltinErrorInst.
 static auto LookupMemberNameInScope(Context& context, SemIR::LocId loc_id,
                                     SemIR::InstId /*base_id*/,
                                     SemIR::NameId name_id,
@@ -217,7 +217,7 @@ static auto LookupMemberNameInScope(Context& context, SemIR::LocId loc_id,
                                   /*required=*/true, access_info);
 
   if (!result.inst_id.is_valid()) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // TODO: This duplicates the work that HandleNameAsExpr does. Factor this out.
@@ -355,7 +355,7 @@ auto PerformMemberAccess(Context& context, SemIR::LocId loc_id,
         return context.emitter().Build(base_id, IncompleteTypeInMemberAccess,
                                        base_id);
       })) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // Materialize a temporary for the base expression if necessary.
@@ -387,7 +387,7 @@ auto PerformMemberAccess(Context& context, SemIR::LocId loc_id,
                         SemIR::NameId);
       context.emitter().Emit(loc_id, QualifiedExprNameNotFound, base_id,
                              name_id);
-      return SemIR::InstId::BuiltinError;
+      return SemIR::InstId::BuiltinErrorInst;
     }
 
     if (base_type_id != SemIR::TypeId::Error) {
@@ -396,7 +396,7 @@ auto PerformMemberAccess(Context& context, SemIR::LocId loc_id,
                         TypeOfInstId);
       context.emitter().Emit(loc_id, QualifiedExprUnsupported, base_id);
     }
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // Perform lookup into the base type.
@@ -461,7 +461,7 @@ 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::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   auto diag_non_constant_index = [&] {
@@ -469,7 +469,7 @@ 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::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   };
   // Diagnose a non-constant index prior to conversion to IntLiteral, because
   // the conversion will fail if the index is not constant.
@@ -484,7 +484,7 @@ auto PerformTupleAccess(Context& context, SemIR::LocId loc_id,
       context.GetBuiltinType(SemIR::BuiltinInstKind::IntLiteralType));
   auto index_const_id = context.constant_values().Get(index_inst_id);
   if (index_const_id == SemIR::ConstantId::Error) {
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   } else if (!index_const_id.is_template()) {
     return diag_non_constant_index();
   }
@@ -495,7 +495,7 @@ 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::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // TODO: Handle the case when `index_val->getZExtValue()` has too many bits.
diff --git a/toolchain/check/operator.cpp b/toolchain/check/operator.cpp
index 7936f66c5d53..7132fecaa4b1 100644
--- a/toolchain/check/operator.cpp
+++ b/toolchain/check/operator.cpp
@@ -39,8 +39,8 @@ 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::InstId::BuiltinError) {
-    return SemIR::InstId::BuiltinError;
+  if (bound_op_id == SemIR::InstId::BuiltinErrorInst) {
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // Form `bound_op()`.
@@ -57,8 +57,8 @@ 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::InstId::BuiltinError) {
-    return SemIR::InstId::BuiltinError;
+  if (bound_op_id == SemIR::InstId::BuiltinErrorInst) {
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // Form `bound_op(rhs)`.
diff --git a/toolchain/check/pattern_match.cpp b/toolchain/check/pattern_match.cpp
index 1c96590de27d..41952d4a3f05 100644
--- a/toolchain/check/pattern_match.cpp
+++ b/toolchain/check/pattern_match.cpp
@@ -133,8 +133,8 @@ auto MatchContext::DoWork(Context& context) -> SemIR::InstBlockId {
 
 auto MatchContext::EmitPatternMatch(Context& context,
                                     MatchContext::WorkItem entry) -> void {
-  if (entry.pattern_id == SemIR::InstId::BuiltinError) {
-    results_.push_back(SemIR::InstId::BuiltinError);
+  if (entry.pattern_id == SemIR::InstId::BuiltinErrorInst) {
+    results_.push_back(SemIR::InstId::BuiltinErrorInst);
     return;
   }
   DiagnosticAnnotationScope annotate_diagnostics(
@@ -187,7 +187,7 @@ auto MatchContext::EmitPatternMatch(Context& context,
           context.emitter().Emit(
               TokenOnly(context.insts().GetLocId(entry.scrutinee_id)),
               AddrSelfIsNonRef);
-          results_.push_back(SemIR::InstId::BuiltinError);
+          results_.push_back(SemIR::InstId::BuiltinErrorInst);
           return;
       }
       auto scrutinee_ref = context.insts().Get(scrutinee_ref_id);
@@ -208,8 +208,8 @@ auto MatchContext::EmitPatternMatch(Context& context,
       switch (kind_) {
         case MatchKind::Caller: {
           CARBON_CHECK(entry.scrutinee_id.is_valid());
-          if (entry.scrutinee_id == SemIR::InstId::BuiltinError) {
-            results_.push_back(SemIR::InstId::BuiltinError);
+          if (entry.scrutinee_id == SemIR::InstId::BuiltinErrorInst) {
+            results_.push_back(SemIR::InstId::BuiltinErrorInst);
           } else {
             results_.push_back(ConvertToValueOfType(
                 context, context.insts().GetLocId(entry.scrutinee_id),
diff --git a/toolchain/check/return.cpp b/toolchain/check/return.cpp
index 9341810a8610..8966471bce08 100644
--- a/toolchain/check/return.cpp
+++ b/toolchain/check/return.cpp
@@ -78,7 +78,7 @@ auto CheckReturnedVar(Context& context, Parse::NodeId returned_node,
         context.emitter().Build(returned_node, ReturnedVarWithNoReturnType);
     NoteNoReturnTypeProvided(diag, function);
     diag.Emit();
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // The declared type of the var must match the return type of the function.
@@ -91,7 +91,7 @@ auto CheckReturnedVar(Context& context, Parse::NodeId returned_node,
         context.emitter().Build(type_node, ReturnedVarWrongType, type_id);
     NoteReturnType(context, diag, function);
     diag.Emit();
-    return SemIR::InstId::BuiltinError;
+    return SemIR::InstId::BuiltinErrorInst;
   }
 
   // The variable aliases the return slot if there is one. If not, it has its
@@ -146,7 +146,7 @@ auto BuildReturnWithExpr(Context& context, Parse::ReturnStatementId node_id,
     auto diag = context.emitter().Build(node_id, ReturnStatementDisallowExpr);
     NoteNoReturnTypeProvided(diag, function);
     diag.Emit();
-    expr_id = SemIR::InstId::BuiltinError;
+    expr_id = SemIR::InstId::BuiltinErrorInst;
   } else if (returned_var_id.is_valid()) {
     CARBON_DIAGNOSTIC(
         ReturnExprWithReturnedVar, Error,
@@ -154,11 +154,11 @@ 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::InstId::BuiltinError;
+    expr_id = SemIR::InstId::BuiltinErrorInst;
   } 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::InstId::BuiltinError;
+    expr_id = SemIR::InstId::BuiltinErrorInst;
   } else if (return_info.has_return_slot()) {
     return_slot_id = function.return_slot_id;
     // Note that this can import a function and invalidate `function`.
@@ -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::InstId::BuiltinError;
+    returned_var_id = SemIR::InstId::BuiltinErrorInst;
   }
 
   auto return_slot_id = function.return_slot_id;
diff --git a/toolchain/check/scope_stack.cpp b/toolchain/check/scope_stack.cpp
index f75a3a4895f3..22ac659f9c00 100644
--- a/toolchain/check/scope_stack.cpp
+++ b/toolchain/check/scope_stack.cpp
@@ -122,7 +122,7 @@ 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::InstId::BuiltinError
+    return {LexicalLookupHasLoadError() ? SemIR::InstId::BuiltinErrorInst
                                         : SemIR::InstId::Invalid,
             non_lexical_scope_stack_};
   }
diff --git a/toolchain/check/testdata/basics/builtin_insts.carbon b/toolchain/check/testdata/basics/builtin_insts.carbon
index 9147454378d3..6f1cad7c2f01 100644
--- a/toolchain/check/testdata/basics/builtin_insts.carbon
+++ b/toolchain/check/testdata/basics/builtin_insts.carbon
@@ -33,11 +33,11 @@
 // CHECK:STDOUT:     type_block0:     {}
 // CHECK:STDOUT:   insts:
 // CHECK:STDOUT:     instTypeType:    {kind: BuiltinInst, arg0: TypeType, type: typeTypeType}
-// CHECK:STDOUT:     instError:       {kind: BuiltinInst, arg0: Error, type: typeError}
+// CHECK:STDOUT:     instErrorInst:   {kind: BuiltinInst, arg0: ErrorInst, type: typeError}
 // CHECK:STDOUT:     instAutoType:    {kind: BuiltinInst, arg0: AutoType, type: typeTypeType}
 // CHECK:STDOUT:     instBoolType:    {kind: BuiltinInst, arg0: BoolType, type: typeTypeType}
 // CHECK:STDOUT:     instIntLiteralType: {kind: BuiltinInst, arg0: IntLiteralType, type: typeTypeType}
-// CHECK:STDOUT:     instFloatType:   {kind: BuiltinInst, arg0: FloatType, type: typeTypeType}
+// CHECK:STDOUT:     instLegacyFloatType: {kind: BuiltinInst, arg0: LegacyFloatType, type: typeTypeType}
 // CHECK:STDOUT:     instStringType:  {kind: BuiltinInst, arg0: StringType, type: typeTypeType}
 // CHECK:STDOUT:     instBoundMethodType: {kind: BuiltinInst, arg0: BoundMethodType, type: typeTypeType}
 // CHECK:STDOUT:     instSpecificFunctionType: {kind: BuiltinInst, arg0: SpecificFunctionType, type: typeTypeType}
@@ -47,11 +47,11 @@
 // CHECK:STDOUT:     'inst+0':          {kind: Namespace, arg0: name_scope0, arg1: inst<invalid>, type: type(instNamespaceType)}
 // CHECK:STDOUT:   constant_values:
 // CHECK:STDOUT:     instTypeType:    templateConstant(instTypeType)
-// CHECK:STDOUT:     instError:       templateConstant(instError)
+// CHECK:STDOUT:     instErrorInst:   templateConstant(instErrorInst)
 // CHECK:STDOUT:     instAutoType:    templateConstant(instAutoType)
 // CHECK:STDOUT:     instBoolType:    templateConstant(instBoolType)
 // CHECK:STDOUT:     instIntLiteralType: templateConstant(instIntLiteralType)
-// CHECK:STDOUT:     instFloatType:   templateConstant(instFloatType)
+// CHECK:STDOUT:     instLegacyFloatType: templateConstant(instLegacyFloatType)
 // CHECK:STDOUT:     instStringType:  templateConstant(instStringType)
 // CHECK:STDOUT:     instBoundMethodType: templateConstant(instBoundMethodType)
 // CHECK:STDOUT:     instSpecificFunctionType: templateConstant(instSpecificFunctionType)
diff --git a/toolchain/lower/file_context.cpp b/toolchain/lower/file_context.cpp
index 835a213b14f6..9e065e7d79dd 100644
--- a/toolchain/lower/file_context.cpp
+++ b/toolchain/lower/file_context.cpp
@@ -473,12 +473,12 @@ static auto BuildTypeForInst(FileContext& context, SemIR::BuiltinInst inst)
     case SemIR::BuiltinInstKind::Invalid:
     case SemIR::BuiltinInstKind::AutoType:
       CARBON_FATAL("Unexpected builtin type in lowering: {0}", inst);
-    case SemIR::BuiltinInstKind::Error:
+    case SemIR::BuiltinInstKind::ErrorInst:
       // This is a complete type but uses of it should never be lowered.
       return nullptr;
     case SemIR::BuiltinInstKind::TypeType:
       return context.GetTypeType();
-    case SemIR::BuiltinInstKind::FloatType:
+    case SemIR::BuiltinInstKind::LegacyFloatType:
       return llvm::Type::getDoubleTy(context.llvm_context());
     case SemIR::BuiltinInstKind::BoolType:
       // TODO: We may want to have different representations for `bool`
diff --git a/toolchain/sem_ir/builtin_function_kind.cpp b/toolchain/sem_ir/builtin_function_kind.cpp
index 7c3c53532c70..fc36b1e0d887 100644
--- a/toolchain/sem_ir/builtin_function_kind.cpp
+++ b/toolchain/sem_ir/builtin_function_kind.cpp
@@ -91,7 +91,8 @@ struct AnyInt {
 struct AnyFloat {
   static auto Check(const File& sem_ir, ValidateState& state, TypeId type_id)
       -> bool {
-    if (BuiltinType<InstId::BuiltinFloatType>::Check(sem_ir, state, type_id)) {
+    if (BuiltinType<InstId::BuiltinLegacyFloatType>::Check(sem_ir, state,
+                                                           type_id)) {
       return true;
     }
     return sem_ir.types().Is<FloatType>(type_id);
diff --git a/toolchain/sem_ir/builtin_inst_kind.def b/toolchain/sem_ir/builtin_inst_kind.def
index 8d4e2b7fa3f0..6167216b3c37 100644
--- a/toolchain/sem_ir/builtin_inst_kind.def
+++ b/toolchain/sem_ir/builtin_inst_kind.def
@@ -44,7 +44,7 @@ CARBON_SEM_IR_BUILTIN_INST_KIND(TypeType, "type")
 // required. For example, when there is a type checking issue, this will be used
 // in the type_id. It's typically used as a cue that semantic checking doesn't
 // need to issue further diagnostics.
-CARBON_SEM_IR_BUILTIN_INST_KIND(Error, "<error>")
+CARBON_SEM_IR_BUILTIN_INST_KIND(ErrorInst, "<error>")
 
 // Used for the type of patterns that do not match a fixed type.
 CARBON_SEM_IR_BUILTIN_INST_KIND(AutoType, "auto")
@@ -64,8 +64,10 @@ CARBON_SEM_IR_BUILTIN_INST_KIND(BoolType, "bool")
 // runtime.
 CARBON_SEM_IR_BUILTIN_INST_KIND(IntLiteralType, "Core.IntLiteral")
 
-// The type of floating point values and real literals, currently always f64.
-CARBON_SEM_IR_BUILTIN_INST_KIND(FloatType, "f64")
+// The legacy float type. This is currently used for real literals, and is
+// treated as f64. It's separate from `FloatType`, and should change to mirror
+// integers, likely replacing this with a `FloatLiteralType`.
+CARBON_SEM_IR_BUILTIN_INST_KIND(LegacyFloatType, "f64")
 
 // The type of string values and String literals.
 CARBON_SEM_IR_BUILTIN_INST_KIND(StringType, "String")
diff --git a/toolchain/sem_ir/file.cpp b/toolchain/sem_ir/file.cpp
index ac93b8d3714d..af24e0a020c3 100644
--- a/toolchain/sem_ir/file.cpp
+++ b/toolchain/sem_ir/file.cpp
@@ -43,11 +43,11 @@ File::File(CheckIRId check_ir_id, IdentifierId package_id,
 // 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.
-#define CARBON_SEM_IR_BUILTIN_INST_KIND(Name, ...)                \
-  insts_.AddInNoBlock(LocIdAndInst::NoLoc<BuiltinInst>(           \
-      {.type_id = BuiltinInstKind::Name == BuiltinInstKind::Error \
-                      ? TypeId::Error                             \
-                      : TypeId::TypeType,                         \
+#define CARBON_SEM_IR_BUILTIN_INST_KIND(Name, ...)                    \
+  insts_.AddInNoBlock(LocIdAndInst::NoLoc<BuiltinInst>(               \
+      {.type_id = BuiltinInstKind::Name == BuiltinInstKind::ErrorInst \
+                      ? TypeId::Error                                 \
+                      : TypeId::TypeType,                             \
        .builtin_inst_kind = BuiltinInstKind::Name}));
 #include "toolchain/sem_ir/builtin_inst_kind.def"
   CARBON_CHECK(insts_.size() == BuiltinInstKind::ValidCount,
@@ -288,7 +288,7 @@ auto GetExprCategory(const File& file, InstId inst_id) -> ExprCategory {
         return value_category;
 
       case CARBON_KIND(BuiltinInst inst): {
-        if (inst.builtin_inst_kind == BuiltinInstKind::Error) {
+        if (inst.builtin_inst_kind == BuiltinInstKind::ErrorInst) {
           return ExprCategory::Error;
         }
         return value_category;
diff --git a/toolchain/sem_ir/function.cpp b/toolchain/sem_ir/function.cpp
index c0955e4c4618..fb2c1aeef5a7 100644
--- a/toolchain/sem_ir/function.cpp
+++ b/toolchain/sem_ir/function.cpp
@@ -86,7 +86,7 @@ auto Function::GetNameFromPatternId(const File& sem_ir, InstId pattern_id)
     inst = sem_ir.insts().Get(inst_id);
   }
 
-  if (inst_id == SemIR::InstId::BuiltinError) {
+  if (inst_id == SemIR::InstId::BuiltinErrorInst) {
     return SemIR::NameId::Invalid;
   }
 
diff --git a/toolchain/sem_ir/function.h b/toolchain/sem_ir/function.h
index 6c5a779fa2ac..7995adb7f85a 100644
--- a/toolchain/sem_ir/function.h
+++ b/toolchain/sem_ir/function.h
@@ -88,7 +88,7 @@ struct Function : public EntityWithParamsBase,
       -> ParamPatternInfo;
 
   // Gets the name from the name binding instruction, or invalid if this pattern
-  // has been replaced with BuiltinError.
+  // has been replaced with BuiltinErrorInst.
   static auto GetNameFromPatternId(const File& sem_ir, InstId param_pattern_id)
       -> SemIR::NameId;
 
diff --git a/toolchain/sem_ir/ids.h b/toolchain/sem_ir/ids.h
index d8b3e6baebf2..2b1e45af4f2f 100644
--- a/toolchain/sem_ir/ids.h
+++ b/toolchain/sem_ir/ids.h
@@ -212,7 +212,7 @@ struct ConstantId : public IdBase, public Printable<ConstantId> {
 
 constexpr ConstantId ConstantId::NotConstant = ConstantId(NotConstantIndex);
 constexpr ConstantId ConstantId::Error =
-    ConstantId::ForTemplateConstant(InstId::BuiltinError);
+    ConstantId::ForTemplateConstant(InstId::BuiltinErrorInst);
 constexpr ConstantId ConstantId::Invalid = ConstantId(InvalidIndex);
 
 // The ID of a EntityName.
diff --git a/toolchain/sem_ir/impl.h b/toolchain/sem_ir/impl.h
index 17356885ce79..2e478e8ca226 100644
--- a/toolchain/sem_ir/impl.h
+++ b/toolchain/sem_ir/impl.h
@@ -31,7 +31,7 @@ struct ImplFields {
 
   // The following members are set at the `}` of the impl definition.
 
-  // The witness for the impl. This can be `BuiltinError`.
+  // The witness for the impl. This can be `BuiltinErrorInst`.
   InstId witness_id = InstId::Invalid;
 };