diff --git a/toolchain/check/call.cpp b/toolchain/check/call.cpp
index 5d2dbfebe8e7..c63dc3d49433 100644
--- a/toolchain/check/call.cpp
+++ b/toolchain/check/call.cpp
@@ -12,12 +12,35 @@
 
 namespace Carbon::Check {
 
+// Performs a call where the callee is the name of a generic class, such as
+// `Vector(i32)`.
+static auto PerformCallToGenericClass(Context& context, Parse::NodeId node_id,
+                                      SemIR::ClassId class_id,
+                                      llvm::ArrayRef<SemIR::InstId> arg_ids)
+    -> SemIR::InstId {
+  auto& class_info = context.classes().Get(class_id);
+
+  // Convert the arguments to match the parameters.
+  auto converted_args_id = ConvertCallArgs(
+      context, node_id, /*self_id=*/SemIR::InstId::Invalid, arg_ids,
+      /*return_storage_id=*/SemIR::InstId::Invalid, class_info.decl_id,
+      class_info.implicit_param_refs_id, class_info.param_refs_id);
+  return context.AddInst(
+      {node_id,
+       SemIR::ClassType{SemIR::TypeId::TypeType, class_id, converted_args_id}});
+}
+
 auto PerformCall(Context& context, Parse::NodeId node_id,
                  SemIR::InstId callee_id, llvm::ArrayRef<SemIR::InstId> arg_ids)
     -> SemIR::InstId {
   // Identify the function we're calling.
   auto callee_function = GetCalleeFunction(context.sem_ir(), callee_id);
   if (!callee_function.function_id.is_valid()) {
+    if (auto generic_class = context.types().TryGetAs<SemIR::GenericClassType>(
+            context.insts().Get(callee_id).type_id())) {
+      return PerformCallToGenericClass(context, node_id,
+                                       generic_class->class_id, arg_ids);
+    }
     if (!callee_function.is_error) {
       CARBON_DIAGNOSTIC(CallToNonCallable, Error,
                         "Value of type `{0}` is not callable.", SemIR::TypeId);
diff --git a/toolchain/check/convert.cpp b/toolchain/check/convert.cpp
index fc0c334c339b..4c588e7e1edc 100644
--- a/toolchain/check/convert.cpp
+++ b/toolchain/check/convert.cpp
@@ -1158,8 +1158,9 @@ auto ConvertCallArgs(Context& context, SemIR::LocId call_loc_id,
                      SemIR::InstId return_storage_id, SemIR::InstId callee_id,
                      SemIR::InstBlockId implicit_param_refs_id,
                      SemIR::InstBlockId param_refs_id) -> SemIR::InstBlockId {
-  auto implicit_param_refs = context.inst_blocks().Get(implicit_param_refs_id);
-  auto param_refs = context.inst_blocks().Get(param_refs_id);
+  auto implicit_param_refs =
+      context.inst_blocks().GetOrEmpty(implicit_param_refs_id);
+  auto param_refs = context.inst_blocks().GetOrEmpty(param_refs_id);
 
   // If sizes mismatch, fail early.
   if (arg_refs.size() != param_refs.size()) {
diff --git a/toolchain/check/testdata/class/generic/call.carbon b/toolchain/check/testdata/class/generic/call.carbon
new file mode 100644
index 000000000000..4d3cd46eea3a
--- /dev/null
+++ b/toolchain/check/testdata/class/generic/call.carbon
@@ -0,0 +1,236 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+// --- call.carbon
+
+library "call" api;
+
+class Class(T:! type, N:! i32) {}
+
+var a: Class(i32*, 5);
+
+// Requires an implicit conversion to type `type`.
+var b: Class((), 0);
+
+// --- fail_too_few.carbon
+
+library "too_few" api;
+
+class Class(T:! type, N:! i32) {}
+
+// CHECK:STDERR: fail_too_few.carbon:[[@LINE+7]]:8: ERROR: 1 argument(s) passed to function expecting 2 argument(s).
+// CHECK:STDERR: var a: Class(i32*);
+// CHECK:STDERR:        ^~~~~~
+// CHECK:STDERR: fail_too_few.carbon:[[@LINE-5]]:1: Calling function declared here.
+// CHECK:STDERR: class Class(T:! type, N:! i32) {}
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+var a: Class(i32*);
+
+// --- fail_too_many.carbon
+
+library "too_many" api;
+
+class Class(T:! type, N:! i32) {}
+
+// CHECK:STDERR: fail_too_many.carbon:[[@LINE+7]]:8: ERROR: 3 argument(s) passed to function expecting 2 argument(s).
+// CHECK:STDERR: var a: Class(i32*, 1, 2);
+// CHECK:STDERR:        ^~~~~~
+// CHECK:STDERR: fail_too_many.carbon:[[@LINE-5]]:1: Calling function declared here.
+// CHECK:STDERR: class Class(T:! type, N:! i32) {}
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+var a: Class(i32*, 1, 2);
+
+// --- fail_no_conversion.carbon
+
+library "no_conversion" api;
+
+class Class(T:! type, N:! i32) {}
+
+// CHECK:STDERR: fail_no_conversion.carbon:[[@LINE+6]]:8: ERROR: Cannot implicitly convert from `i32` to `type`.
+// CHECK:STDERR: var a: Class(5, i32*);
+// CHECK:STDERR:        ^~~~~~
+// CHECK:STDERR: fail_no_conversion.carbon:[[@LINE-5]]:1: Initializing parameter 1 of function declared here.
+// CHECK:STDERR: class Class(T:! type, N:! i32) {}
+// CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+var a: Class(5, i32*);
+
+// CHECK:STDOUT: --- call.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %T: type = bind_symbolic_name T 0 [symbolic]
+// CHECK:STDOUT:   %N: i32 = bind_symbolic_name N 1 [symbolic]
+// CHECK:STDOUT:   %Class.1: type = generic_class_type @Class [template]
+// CHECK:STDOUT:   %.1: type = tuple_type () [template]
+// CHECK:STDOUT:   %struct: Class = struct_value () [template]
+// CHECK:STDOUT:   %Class.2: type = class_type @Class [template]
+// CHECK:STDOUT:   %.2: type = struct_type {} [template]
+// CHECK:STDOUT:   %.3: type = ptr_type i32 [template]
+// CHECK:STDOUT:   %.4: i32 = int_literal 5 [template]
+// CHECK:STDOUT:   %Class.3: type = class_type @Class, (file.%.loc6_17, file.%.loc6_20) [template]
+// CHECK:STDOUT:   %.5: type = ptr_type {} [template]
+// CHECK:STDOUT:   %.6: i32 = int_literal 0 [template]
+// CHECK:STDOUT:   %Class.4: type = class_type @Class, (file.%.loc9_13, file.%.loc9_18) [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .Core = %Core
+// CHECK:STDOUT:     .Class = %Class.decl
+// CHECK:STDOUT:     .a = %a
+// CHECK:STDOUT:     .b = %b
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core: <namespace> = namespace [template] {}
+// CHECK:STDOUT:   %Class.decl: Class = class_decl @Class [template = constants.%struct] {
+// CHECK:STDOUT:     %T.loc4_13.1: type = param T
+// CHECK:STDOUT:     %T.loc4_13.2: type = bind_symbolic_name T 0, %T.loc4_13.1 [symbolic = constants.%T]
+// CHECK:STDOUT:     %N.loc4_23.1: i32 = param N
+// CHECK:STDOUT:     %N.loc4_23.2: i32 = bind_symbolic_name N 1, %N.loc4_23.1 [symbolic = constants.%N]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Class.ref.loc6: Class = name_ref Class, %Class.decl [template = constants.%struct]
+// CHECK:STDOUT:   %.loc6_17: type = ptr_type i32 [template = constants.%.3]
+// CHECK:STDOUT:   %.loc6_20: i32 = int_literal 5 [template = constants.%.4]
+// CHECK:STDOUT:   %Class.loc6: type = class_type @Class, (%.loc6_17, %.loc6_20) [template = constants.%Class.3]
+// CHECK:STDOUT:   %a.var: ref Class = var a
+// CHECK:STDOUT:   %a: ref Class = bind_name a, %a.var
+// CHECK:STDOUT:   %Class.ref.loc9: Class = name_ref Class, %Class.decl [template = constants.%struct]
+// CHECK:STDOUT:   %.loc9_15: () = tuple_literal ()
+// CHECK:STDOUT:   %.loc9_18: i32 = int_literal 0 [template = constants.%.6]
+// CHECK:STDOUT:   %.loc9_13: type = converted %.loc9_15, constants.%.1 [template = constants.%.1]
+// CHECK:STDOUT:   %Class.loc9: type = class_type @Class, (%.loc9_13, %.loc9_18) [template = constants.%Class.4]
+// CHECK:STDOUT:   %b.var: ref Class = var b
+// CHECK:STDOUT:   %b: ref Class = bind_name b, %b.var
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Class {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Class.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_too_few.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %T: type = bind_symbolic_name T 0 [symbolic]
+// CHECK:STDOUT:   %N: i32 = bind_symbolic_name N 1 [symbolic]
+// CHECK:STDOUT:   %Class.1: type = generic_class_type @Class [template]
+// CHECK:STDOUT:   %.1: type = tuple_type () [template]
+// CHECK:STDOUT:   %struct: Class = struct_value () [template]
+// CHECK:STDOUT:   %Class.2: type = class_type @Class [template]
+// CHECK:STDOUT:   %.2: type = struct_type {} [template]
+// CHECK:STDOUT:   %.3: type = ptr_type i32 [template]
+// CHECK:STDOUT:   %.4: type = ptr_type {} [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .Core = %Core
+// CHECK:STDOUT:     .Class = %Class.decl
+// CHECK:STDOUT:     .a = %a
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core: <namespace> = namespace [template] {}
+// CHECK:STDOUT:   %Class.decl: Class = class_decl @Class [template = constants.%struct] {
+// CHECK:STDOUT:     %T.loc4_13.1: type = param T
+// CHECK:STDOUT:     %T.loc4_13.2: type = bind_symbolic_name T 0, %T.loc4_13.1 [symbolic = constants.%T]
+// CHECK:STDOUT:     %N.loc4_23.1: i32 = param N
+// CHECK:STDOUT:     %N.loc4_23.2: i32 = bind_symbolic_name N 1, %N.loc4_23.1 [symbolic = constants.%N]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Class.ref: Class = name_ref Class, %Class.decl [template = constants.%struct]
+// CHECK:STDOUT:   %.loc13: type = ptr_type i32 [template = constants.%.3]
+// CHECK:STDOUT:   %Class: type = class_type @Class [template = constants.%Class.2]
+// CHECK:STDOUT:   %a.var: ref Class = var a
+// CHECK:STDOUT:   %a: ref Class = bind_name a, %a.var
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Class {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Class.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_too_many.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %T: type = bind_symbolic_name T 0 [symbolic]
+// CHECK:STDOUT:   %N: i32 = bind_symbolic_name N 1 [symbolic]
+// CHECK:STDOUT:   %Class.1: type = generic_class_type @Class [template]
+// CHECK:STDOUT:   %.1: type = tuple_type () [template]
+// CHECK:STDOUT:   %struct: Class = struct_value () [template]
+// CHECK:STDOUT:   %Class.2: type = class_type @Class [template]
+// CHECK:STDOUT:   %.2: type = struct_type {} [template]
+// CHECK:STDOUT:   %.3: type = ptr_type i32 [template]
+// CHECK:STDOUT:   %.4: i32 = int_literal 1 [template]
+// CHECK:STDOUT:   %.5: i32 = int_literal 2 [template]
+// CHECK:STDOUT:   %.6: type = ptr_type {} [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .Core = %Core
+// CHECK:STDOUT:     .Class = %Class.decl
+// CHECK:STDOUT:     .a = %a
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core: <namespace> = namespace [template] {}
+// CHECK:STDOUT:   %Class.decl: Class = class_decl @Class [template = constants.%struct] {
+// CHECK:STDOUT:     %T.loc4_13.1: type = param T
+// CHECK:STDOUT:     %T.loc4_13.2: type = bind_symbolic_name T 0, %T.loc4_13.1 [symbolic = constants.%T]
+// CHECK:STDOUT:     %N.loc4_23.1: i32 = param N
+// CHECK:STDOUT:     %N.loc4_23.2: i32 = bind_symbolic_name N 1, %N.loc4_23.1 [symbolic = constants.%N]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Class.ref: Class = name_ref Class, %Class.decl [template = constants.%struct]
+// CHECK:STDOUT:   %.loc13_17: type = ptr_type i32 [template = constants.%.3]
+// CHECK:STDOUT:   %.loc13_20: i32 = int_literal 1 [template = constants.%.4]
+// CHECK:STDOUT:   %.loc13_23: i32 = int_literal 2 [template = constants.%.5]
+// CHECK:STDOUT:   %Class: type = class_type @Class [template = constants.%Class.2]
+// CHECK:STDOUT:   %a.var: ref Class = var a
+// CHECK:STDOUT:   %a: ref Class = bind_name a, %a.var
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Class {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Class.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_no_conversion.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %T: type = bind_symbolic_name T 0 [symbolic]
+// CHECK:STDOUT:   %N: i32 = bind_symbolic_name N 1 [symbolic]
+// CHECK:STDOUT:   %Class.1: type = generic_class_type @Class [template]
+// CHECK:STDOUT:   %.1: type = tuple_type () [template]
+// CHECK:STDOUT:   %struct: Class = struct_value () [template]
+// CHECK:STDOUT:   %Class.2: type = class_type @Class [template]
+// CHECK:STDOUT:   %.2: type = struct_type {} [template]
+// CHECK:STDOUT:   %.3: i32 = int_literal 5 [template]
+// CHECK:STDOUT:   %.4: type = ptr_type i32 [template]
+// CHECK:STDOUT:   %.5: type = ptr_type {} [template]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [template] {
+// CHECK:STDOUT:     .Core = %Core
+// CHECK:STDOUT:     .Class = %Class.decl
+// CHECK:STDOUT:     .a = %a
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core: <namespace> = namespace [template] {}
+// CHECK:STDOUT:   %Class.decl: Class = class_decl @Class [template = constants.%struct] {
+// CHECK:STDOUT:     %T.loc4_13.1: type = param T
+// CHECK:STDOUT:     %T.loc4_13.2: type = bind_symbolic_name T 0, %T.loc4_13.1 [symbolic = constants.%T]
+// CHECK:STDOUT:     %N.loc4_23.1: i32 = param N
+// CHECK:STDOUT:     %N.loc4_23.2: i32 = bind_symbolic_name N 1, %N.loc4_23.1 [symbolic = constants.%N]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Class.ref: Class = name_ref Class, %Class.decl [template = constants.%struct]
+// CHECK:STDOUT:   %.loc12_14: i32 = int_literal 5 [template = constants.%.3]
+// CHECK:STDOUT:   %.loc12_20: type = ptr_type i32 [template = constants.%.4]
+// CHECK:STDOUT:   %Class: type = class_type @Class [template = constants.%Class.2]
+// CHECK:STDOUT:   %a.var: ref Class = var a
+// CHECK:STDOUT:   %a: ref Class = bind_name a, %a.var
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @Class {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%Class.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
diff --git a/toolchain/check/testdata/class/generic/fail_todo_use.carbon b/toolchain/check/testdata/class/generic/fail_todo_use.carbon
index ad39ef7668b8..11d608997829 100644
--- a/toolchain/check/testdata/class/generic/fail_todo_use.carbon
+++ b/toolchain/check/testdata/class/generic/fail_todo_use.carbon
@@ -14,10 +14,21 @@ class Class(T:! type) {
 
 // TODO: The following should work.
 fn Run() -> i32 {
-  // CHECK:STDERR: fail_todo_use.carbon:[[@LINE+3]]:10: ERROR: Value of type `Class` is not callable.
+  // CHECK:STDERR: fail_todo_use.carbon:[[@LINE+4]]:23: ERROR: Cannot implicitly convert from `i32` to `T`.
   // CHECK:STDERR:   var v: Class(i32) = {.k = 0};
-  // CHECK:STDERR:          ^~~~~~
+  // CHECK:STDERR:                       ^~~~~~~~
+  // CHECK:STDERR:
   var v: Class(i32) = {.k = 0};
+  // CHECK:STDERR: fail_todo_use.carbon:[[@LINE+10]]:3: ERROR: Cannot implicitly convert from `T*` to `i32`.
+  // CHECK:STDERR:   return v.Get();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  // CHECK:STDERR: fail_todo_use.carbon:[[@LINE+6]]:10: ERROR: Cannot implicitly convert from `Class*` to `Class*`.
+  // CHECK:STDERR:   return v.Get();
+  // CHECK:STDERR:          ^~~~~~
+  // CHECK:STDERR: fail_todo_use.carbon:[[@LINE-21]]:15: Initializing `addr self` parameter of method declared here.
+  // CHECK:STDERR:   fn Get[addr self: Self*]() -> T* {
+  // CHECK:STDERR:               ^~~~
   return v.Get();
 }
 
@@ -38,8 +49,10 @@ fn Run() -> i32 {
 // CHECK:STDOUT:   %.6: type = ptr_type {.k: T} [symbolic]
 // CHECK:STDOUT:   %Run: type = fn_type @Run [template]
 // CHECK:STDOUT:   %struct.3: Run = struct_value () [template]
+// CHECK:STDOUT:   %Class.3: type = class_type @Class, (i32) [template]
 // CHECK:STDOUT:   %.7: i32 = int_literal 0 [template]
 // CHECK:STDOUT:   %.8: type = struct_type {.k: i32} [template]
+// CHECK:STDOUT:   %.9: type = ptr_type Class [template]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -91,12 +104,17 @@ fn Run() -> i32 {
 // CHECK:STDOUT: fn @Run() -> i32 {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %Class.ref: Class = name_ref Class, file.%Class.decl [template = constants.%struct.1]
-// CHECK:STDOUT:   %v.var: ref <error> = var v
-// CHECK:STDOUT:   %v: ref <error> = bind_name v, %v.var
-// CHECK:STDOUT:   %.loc20_29: i32 = int_literal 0 [template = constants.%.7]
-// CHECK:STDOUT:   %.loc20_30: {.k: i32} = struct_literal (%.loc20_29)
+// CHECK:STDOUT:   %Class: type = class_type @Class, (i32) [template = constants.%Class.3]
+// CHECK:STDOUT:   %v.var: ref Class = var v
+// CHECK:STDOUT:   %v: ref Class = bind_name v, %v.var
+// CHECK:STDOUT:   %.loc21_29: i32 = int_literal 0 [template = constants.%.7]
+// CHECK:STDOUT:   %.loc21_30: {.k: i32} = struct_literal (%.loc21_29)
 // CHECK:STDOUT:   assign %v.var, <error>
-// CHECK:STDOUT:   %v.ref: ref <error> = name_ref v, %v
+// CHECK:STDOUT:   %v.ref: ref Class = name_ref v, %v
+// CHECK:STDOUT:   %Get.ref: Get = name_ref Get, @Class.%Get.decl [template = constants.%struct.2]
+// CHECK:STDOUT:   %.loc32_11: <bound method> = bound_method %v.ref, %Get.ref
+// CHECK:STDOUT:   %.loc32_10: Class* = addr_of %v.ref
+// CHECK:STDOUT:   %Get.call: init T* = call %.loc32_11(<invalid>) [template = <error>]
 // CHECK:STDOUT:   return <error>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/sem_ir/formatter.cpp b/toolchain/sem_ir/formatter.cpp
index 16029d593431..f5edafa7e5d1 100644
--- a/toolchain/sem_ir/formatter.cpp
+++ b/toolchain/sem_ir/formatter.cpp
@@ -604,6 +604,14 @@ class Formatter {
     FormatTrailingBlock(inst.decl_block_id);
   }
 
+  auto FormatInstructionRHS(ClassType inst) -> void {
+    if (inst.args_id.is_valid()) {
+      FormatArgs(inst.class_id, inst.args_id);
+    } else {
+      FormatArgs(inst.class_id);
+    }
+  }
+
   auto FormatInstructionRHS(ImplDecl inst) -> void {
     FormatArgs(inst.impl_id);
     FormatTrailingBlock(inst.decl_block_id);
diff --git a/toolchain/sem_ir/inst.h b/toolchain/sem_ir/inst.h
index 28bb3a5dc4c4..7cdcf78391b0 100644
--- a/toolchain/sem_ir/inst.h
+++ b/toolchain/sem_ir/inst.h
@@ -437,6 +437,12 @@ class InstBlockStore : public BlockValueStore<InstBlockId> {
     CARBON_CHECK(block_id != InstBlockId::Unreachable);
     BlockValueStore<InstBlockId>::Set(block_id, content);
   }
+
+  // Returns the contents of the specified block, or an empty array if the block
+  // is empty.
+  auto GetOrEmpty(InstBlockId block_id) -> llvm::ArrayRef<InstId> {
+    return block_id.is_valid() ? Get(block_id) : llvm::ArrayRef<InstId>();
+  }
 };
 
 }  // namespace Carbon::SemIR
diff --git a/toolchain/sem_ir/inst_profile.cpp b/toolchain/sem_ir/inst_profile.cpp
index 4e2e34f73c35..86006c2b51ec 100644
--- a/toolchain/sem_ir/inst_profile.cpp
+++ b/toolchain/sem_ir/inst_profile.cpp
@@ -35,7 +35,15 @@ static auto DefaultProfileArgFunction(llvm::FoldingSetNodeID& id,
 static auto InstBlockProfileArgFunction(llvm::FoldingSetNodeID& id,
                                         const File& sem_ir, int32_t arg)
     -> void {
-  for (auto inst_id : sem_ir.inst_blocks().Get(InstBlockId(arg))) {
+  auto inst_block_id = InstBlockId(arg);
+  if (!inst_block_id.is_valid()) {
+    id.AddInteger(-1);
+    return;
+  }
+
+  auto inst_block = sem_ir.inst_blocks().Get(inst_block_id);
+  id.AddInteger(inst_block.size());
+  for (auto inst_id : inst_block) {
     id.AddInteger(inst_id.index);
   }
 }
@@ -45,7 +53,15 @@ static auto InstBlockProfileArgFunction(llvm::FoldingSetNodeID& id,
 static auto TypeBlockProfileArgFunction(llvm::FoldingSetNodeID& id,
                                         const File& sem_ir, int32_t arg)
     -> void {
-  for (auto type_id : sem_ir.type_blocks().Get(TypeBlockId(arg))) {
+  auto type_block_id = TypeBlockId(arg);
+  if (!type_block_id.is_valid()) {
+    id.AddInteger(-1);
+    return;
+  }
+
+  auto type_block = sem_ir.type_blocks().Get(type_block_id);
+  id.AddInteger(type_block.size());
+  for (auto type_id : type_block) {
     id.AddInteger(type_id.index);
   }
 }
diff --git a/toolchain/sem_ir/typed_insts.h b/toolchain/sem_ir/typed_insts.h
index a6817d183e4e..d36f0ec24b27 100644
--- a/toolchain/sem_ir/typed_insts.h
+++ b/toolchain/sem_ir/typed_insts.h
@@ -402,11 +402,11 @@ struct ClassInit {
 
 struct ClassType {
   static constexpr auto Kind =
-      InstKind::ClassType.Define<Parse::AnyClassDeclId>("class_type");
+      InstKind::ClassType.Define<Parse::NodeId>("class_type");
 
   TypeId type_id;
   ClassId class_id;
-  // TODO: Once we support generic classes, include the class's arguments here.
+  InstBlockId args_id = InstBlockId::Invalid;
 };
 
 struct ConstType {