diff --git a/toolchain/check/cpp/generate_ast.cpp b/toolchain/check/cpp/generate_ast.cpp
index c62b4bd1b0a2..a42e8c8a0351 100644
--- a/toolchain/check/cpp/generate_ast.cpp
+++ b/toolchain/check/cpp/generate_ast.cpp
@@ -323,7 +323,7 @@ class ShallowCopyCompilerInvocation : public clang::CompilerInvocation {
 };
 
 // Provides clang AST nodes representing Carbon SemIR entities.
-class CarbonExternalASTSource : public clang::ExternalASTSource {
+class CarbonExternalASTSource : public clang::ExternalSemaSource {
  public:
   explicit CarbonExternalASTSource(Context* context) : context_(context) {}
 
@@ -896,13 +896,17 @@ auto GenerateAst(Context& context,
   }
 
   auto& ast = clang_instance.getASTContext();
-  // TODO: Clang's modules support is implemented as an ExternalASTSource
-  // (ASTReader) and there's no multiplexing support for ExternalASTSources at
-  // the moment - so registering CarbonExternalASTSource breaks Clang modules
-  // support. Implement multiplexing support (possibly in Clang) to restore
-  // modules functionality.
-  ast.setExternalSource(
-      llvm::makeIntrusiveRefCnt<CarbonExternalASTSource>(&context));
+  llvm::IntrusiveRefCntPtr<clang::ExternalSemaSource> carbon_source =
+      llvm::makeIntrusiveRefCnt<CarbonExternalASTSource>(&context);
+  if (auto* existing_source = llvm::cast_or_null<clang::ExternalSemaSource>(
+          ast.getExternalSource())) {
+    auto multiplex_source =
+        llvm::makeIntrusiveRefCnt<clang::MultiplexExternalSemaSource>(
+            existing_source, std::move(carbon_source));
+    ast.setExternalSource(std::move(multiplex_source));
+  } else {
+    ast.setExternalSource(std::move(carbon_source));
+  }
 
   if (llvm::Error error = action.Execute()) {
     // `Execute` currently never fails, but its contract allows it to.
diff --git a/toolchain/check/testdata/interop/cpp/modules/import.carbon b/toolchain/check/testdata/interop/cpp/modules/import.carbon
index 6d2488439ad7..ba7a38f471e1 100644
--- a/toolchain/check/testdata/interop/cpp/modules/import.carbon
+++ b/toolchain/check/testdata/interop/cpp/modules/import.carbon
@@ -37,25 +37,13 @@ struct B {};
 
 B *_Nonnull makeB(A *_Nonnull);
 
-// --- fail_todo_use_a_b.carbon
+// --- use_a_b.carbon
 
 library "[[@TEST_NAME]]";
 
 import Cpp library "a.h";
 import Cpp library "b.h";
 
-// CHECK:STDERR: fail_todo_use_a_b.carbon:[[@LINE+4]]:12: error: member name `B` not found in `Cpp` [MemberNameNotFoundInInstScope]
-// CHECK:STDERR: fn Go() -> Cpp.B* {
-// CHECK:STDERR:            ^~~~~
-// CHECK:STDERR:
 fn Go() -> Cpp.B* {
-  // CHECK:STDERR: fail_todo_use_a_b.carbon:[[@LINE+8]]:10: error: member name `makeB` not found in `Cpp` [MemberNameNotFoundInInstScope]
-  // CHECK:STDERR:   return Cpp.makeB(Cpp.makeA());
-  // CHECK:STDERR:          ^~~~~~~~~
-  // CHECK:STDERR:
-  // CHECK:STDERR: fail_todo_use_a_b.carbon:[[@LINE+4]]:20: error: member name `makeA` not found in `Cpp` [MemberNameNotFoundInInstScope]
-  // CHECK:STDERR:   return Cpp.makeB(Cpp.makeA());
-  // CHECK:STDERR:                    ^~~~~~~~~
-  // CHECK:STDERR:
   return Cpp.makeB(Cpp.makeA());
 }