diff --git a/clang-tools-extra/clangd/CodeComplete.cpp b/clang-tools-extra/clangd/CodeComplete.cpp
index 22f75cc05dd3..4789a8094c5d 100644
--- a/clang-tools-extra/clangd/CodeComplete.cpp
+++ b/clang-tools-extra/clangd/CodeComplete.cpp
@@ -1380,6 +1380,7 @@ public:
 
     CodeCompleteResult Output = toCodeCompleteResult(mergeResults(
         /*SemaResults=*/{}, IndexResults, IdentifierResults));
+    Output.RanParser = false;
     logResults(Output, Tracer);
     return Output;
   }
diff --git a/clang-tools-extra/clangd/CodeComplete.h b/clang-tools-extra/clangd/CodeComplete.h
index 9d43107a01ea..2dc7f22c14dd 100644
--- a/clang-tools-extra/clangd/CodeComplete.h
+++ b/clang-tools-extra/clangd/CodeComplete.h
@@ -207,6 +207,9 @@ struct CodeCompleteResult {
   std::vector<CodeCompletion> Completions;
   bool HasMore = false;
   CodeCompletionContext::Kind Context = CodeCompletionContext::CCC_Other;
+  // Usually the source will be parsed with a real C++ parser.
+  // But heuristics may be used instead if e.g. the preamble is not ready.
+  bool RanParser = true;
 };
 raw_ostream &operator<<(raw_ostream &, const CodeCompleteResult &);
 
diff --git a/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp b/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
index 509b35e9144d..e584597e7a90 100644
--- a/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
+++ b/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp
@@ -235,6 +235,7 @@ void TestAfterDotCompletion(clangd::CodeCompleteOptions Opts) {
       )cpp",
       {cls("IndexClass"), var("index_var"), func("index_func")}, Opts);
 
+  EXPECT_TRUE(Results.RanParser);
   // Class members. The only items that must be present in after-dot
   // completion.
   EXPECT_THAT(Results.Completions,
@@ -284,6 +285,7 @@ void TestGlobalScopeCompletion(clangd::CodeCompleteOptions Opts) {
       )cpp",
       {cls("IndexClass"), var("index_var"), func("index_func")}, Opts);
 
+  EXPECT_TRUE(Results.RanParser);
   // Class members. Should never be present in global completions.
   EXPECT_THAT(Results.Completions,
               Not(AnyOf(Has("method"), Has("method()"), Has("field"))));
@@ -2459,6 +2461,7 @@ TEST(NoCompileCompletionTest, Basic) {
       ^
     }
   )cpp");
+  EXPECT_FALSE(Results.RanParser);
   EXPECT_THAT(Results.Completions,
               UnorderedElementsAre(Named("void"), Named("func"), Named("int"),
                                    Named("xyz"), Named("abc")));