diff --git a/toolchain/check/cpp/generate_ast.cpp b/toolchain/check/cpp/generate_ast.cpp
index 81eb358349bf..1ba626c68344 100644
--- a/toolchain/check/cpp/generate_ast.cpp
+++ b/toolchain/check/cpp/generate_ast.cpp
@@ -468,6 +468,7 @@ auto CarbonExternalASTSource::FindExternalVisibleDeclsByName(
   }
 
   // Map the found Carbon entity to a Clang NamedDecl.
+  // Use the key to reach the owned, mutable copy of decl_context.
   auto* clang_decl = MapInstIdToClangDecl(*decl_context_inst_id.key(), result);
   if (!clang_decl) {
     return false;