Index: clangd/CodeComplete.cpp
===================================================================
--- clangd/CodeComplete.cpp
+++ clangd/CodeComplete.cpp
@@ -644,8 +644,10 @@
   Result.IncludeGlobals = IncludeGlobals;
   Result.IncludeBriefComments = IncludeBriefComments;
 
-  // Enable index-based code completion when Index is provided.
-  Result.IncludeNamespaceLevelDecls = !Index;
+  // When an is used, Sema is responsible for completing the main file,
+  // the index can provide results from the preamble.
+  // Tell Sema not to deserialize the preamble to look for results.
+  Result.LoadExternal = !Index;
 
   return Result;
 }
Index: unittests/clangd/CodeCompleteTests.cpp
===================================================================
--- unittests/clangd/CodeCompleteTests.cpp
+++ unittests/clangd/CodeCompleteTests.cpp
@@ -57,6 +57,7 @@
 using ::testing::Each;
 using ::testing::ElementsAre;
 using ::testing::Not;
+using ::testing::UnorderedElementsAre;
 
 class IgnoreDiagnostics : public DiagnosticsConsumer {
   void
@@ -482,11 +483,11 @@
   Opts.Index = nullptr;
 
   auto Results = completions(R"cpp(
-      namespace ns { class No {}; }
+      namespace ns { class Local {}; }
       void f() { ns::^ }
   )cpp",
                              Opts);
-  EXPECT_THAT(Results.items, Has("No"));
+  EXPECT_THAT(Results.items, Has("Local"));
 }
 
 TEST(CompletionTest, StaticAndDynamicIndex) {
@@ -514,13 +515,13 @@
   Opts.Index = I.get();
 
   auto Results = completions(R"cpp(
-      namespace ns { class No {}; }
+      namespace ns { class Local {}; }
       void f() { ns::^ }
   )cpp",
                              Opts);
   EXPECT_THAT(Results.items, Has("XYZ", CompletionItemKind::Class));
   EXPECT_THAT(Results.items, Has("foo", CompletionItemKind::Function));
-  EXPECT_THAT(Results.items, Not(Has("No")));
+  EXPECT_THAT(Results.items, Has("Local"));
 }
 
 TEST(CompletionTest, IndexBasedWithFilter) {
@@ -561,6 +562,41 @@
   EXPECT_THAT(Results.items, Has("XYZ", CompletionItemKind::Class));
 }
 
+TEST(CompletionTest, IndexSuppressesPreambleCompletions) {
+  MockFSProvider FS;
+  MockCompilationDatabase CDB;
+  IgnoreDiagnostics DiagConsumer;
+  ClangdServer Server(CDB, DiagConsumer, FS, getDefaultAsyncThreadsCount(),
+                      /*StorePreamblesInMemory=*/true);
+
+  FS.Files[getVirtualTestFilePath("bar.h")] =
+      R"cpp(namespace ns { int preamble; })cpp";
+  auto File = getVirtualTestFilePath("foo.cpp");
+  Annotations Test(R"cpp(
+      #include "bar.h"
+      namespace ns { int local; }
+      void f() { ns::^ }
+  )cpp");
+  Server.addDocument(Context::empty(), File, Test.code()).wait();
+  clangd::CodeCompleteOptions Opts = {};
+
+  auto WithoutIndex =
+      Server.codeComplete(Context::empty(), File, Test.point(), Opts)
+          .get()
+          .second.Value;
+  EXPECT_THAT(WithoutIndex.items,
+              UnorderedElementsAre(Named("local"), Named("preamble")));
+
+  auto I = simpleIndexFromSymbols({{"ns::index", index::SymbolKind::Variable}});
+  Opts.Index = I.get();
+  auto WithIndex =
+      Server.codeComplete(Context::empty(), File, Test.point(), Opts)
+          .get()
+          .second.Value;
+  EXPECT_THAT(WithIndex.items,
+              UnorderedElementsAre(Named("local"), Named("index")));
+}
+
 TEST(CompletionTest, ASTIndexMultiFile) {
   MockFSProvider FS;
   MockCompilationDatabase CDB;