diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -647,7 +647,7 @@
     return;
   }
 
-  Server->addDocument(File, *Contents, WantDiags);
+  Server->addDocument(File, *Contents, WantDiags, Params.forceRebuild);
 }
 
 void ClangdLSPServer::onFileEvent(const DidChangeWatchedFilesParams &Params) {
diff --git a/clang-tools-extra/clangd/ClangdServer.h b/clang-tools-extra/clangd/ClangdServer.h
--- a/clang-tools-extra/clangd/ClangdServer.h
+++ b/clang-tools-extra/clangd/ClangdServer.h
@@ -172,7 +172,8 @@
   /// separate thread. When the parsing is complete, DiagConsumer passed in
   /// constructor will receive onDiagnosticsReady callback.
   void addDocument(PathRef File, StringRef Contents,
-                   WantDiagnostics WD = WantDiagnostics::Auto);
+                   WantDiagnostics WD = WantDiagnostics::Auto,
+                   bool ForceRebuild = false);
 
   /// Get the contents of \p File, which should have been added.
   llvm::StringRef getDocument(PathRef File) const;
diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -170,7 +170,7 @@
 }
 
 void ClangdServer::addDocument(PathRef File, llvm::StringRef Contents,
-                               WantDiagnostics WantDiags) {
+                               WantDiagnostics WantDiags, bool ForceRebuild) {
   auto FS = FSProvider.getFileSystem();
 
   ParseOptions Opts;
@@ -184,6 +184,7 @@
   ParseInputs Inputs;
   Inputs.FS = FS;
   Inputs.Contents = std::string(Contents);
+  Inputs.ForceRebuild = ForceRebuild;
   Inputs.Opts = std::move(Opts);
   Inputs.Index = Index;
   bool NewFile = WorkScheduler.update(File, Inputs, WantDiags);
diff --git a/clang-tools-extra/clangd/Compiler.h b/clang-tools-extra/clangd/Compiler.h
--- a/clang-tools-extra/clangd/Compiler.h
+++ b/clang-tools-extra/clangd/Compiler.h
@@ -45,6 +45,9 @@
   tooling::CompileCommand CompileCommand;
   IntrusiveRefCntPtr<llvm::vfs::FileSystem> FS;
   std::string Contents;
+  // Prevent reuse of the cached preamble/AST. Slow! Useful to workaround
+  // clangd's assumption that missing header files will stay missing.
+  bool ForceRebuild = false;
   // Used to recover from diagnostics (e.g. find missing includes for symbol).
   const SymbolIndex *Index = nullptr;
   ParseOptions Opts;
diff --git a/clang-tools-extra/clangd/Protocol.h b/clang-tools-extra/clangd/Protocol.h
--- a/clang-tools-extra/clangd/Protocol.h
+++ b/clang-tools-extra/clangd/Protocol.h
@@ -652,6 +652,12 @@
   /// either they will be provided for this version or some subsequent one.
   /// This is a clangd extension.
   llvm::Optional<bool> wantDiagnostics;
+
+  /// Force a complete rebuild of the file, ignoring all cached state. Slow!
+  /// This is useful to defeat clangd's assumption that missing headers will
+  /// stay missing.
+  /// This is a clangd extension.
+  bool forceRebuild = false;
 };
 bool fromJSON(const llvm::json::Value &, DidChangeTextDocumentParams &);
 
diff --git a/clang-tools-extra/clangd/Protocol.cpp b/clang-tools-extra/clangd/Protocol.cpp
--- a/clang-tools-extra/clangd/Protocol.cpp
+++ b/clang-tools-extra/clangd/Protocol.cpp
@@ -430,7 +430,10 @@
 
 bool fromJSON(const llvm::json::Value &Params, DidChangeTextDocumentParams &R) {
   llvm::json::ObjectMapper O(Params);
-  return O && O.map("textDocument", R.textDocument) &&
+  if (!O)
+    return false;
+  O.map("forceRebuild", R.forceRebuild);  // Optional clangd extension.
+  return O.map("textDocument", R.textDocument) &&
          O.map("contentChanges", R.contentChanges) &&
          O.map("wantDiagnostics", R.wantDiagnostics);
 }
diff --git a/clang-tools-extra/clangd/TUScheduler.cpp b/clang-tools-extra/clangd/TUScheduler.cpp
--- a/clang-tools-extra/clangd/TUScheduler.cpp
+++ b/clang-tools-extra/clangd/TUScheduler.cpp
@@ -436,7 +436,8 @@
     }
 
     std::shared_ptr<const PreambleData> OldPreamble =
-        getPossiblyStalePreamble();
+        Inputs.ForceRebuild ? std::shared_ptr<const PreambleData>()
+                            : getPossiblyStalePreamble();
     std::shared_ptr<const PreambleData> NewPreamble = buildPreamble(
         FileName, *Invocation, OldPreamble, OldCommand, Inputs,
         StorePreambleInMemory,
diff --git a/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp b/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
--- a/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
+++ b/clang-tools-extra/clangd/unittests/TUSchedulerTests.cpp
@@ -616,6 +616,53 @@
   ASSERT_FALSE(DoUpdate(OtherSourceContents));
 }
 
+TEST_F(TUSchedulerTests, ForceRebuild) {
+  TUScheduler S(CDB, optsForTest(), captureDiags());
+
+  auto Source = testPath("foo.cpp");
+  auto Header = testPath("foo.h");
+
+  auto SourceContents = R"cpp(
+      #include "foo.h"
+      int b = a;
+    )cpp";
+
+  ParseInputs Inputs = getInputs(Source, SourceContents);
+
+  // Update the source contents, which should trigger an initial build with
+  // the header file missing.
+  updateWithDiags(S, Source, Inputs, WantDiagnostics::Yes,
+                  [](std::vector<Diag> Diags) {
+    EXPECT_THAT(
+        Diags,
+        ElementsAre(
+            Field(&Diag::Message, "'foo.h' file not found"),
+            Field(&Diag::Message, "use of undeclared identifier 'a'")));
+  });
+
+  // Add the header file. We need to recreate the inputs since we changed a
+  // file from underneath the test FS.
+  Files[Header] = "int a;";
+  Timestamps[Header] = time_t(1);
+  Inputs = getInputs(Source, SourceContents);
+
+  // The addition of the missing header file shouldn't trigger a rebuild since
+  // we don't track missing files.
+  updateWithDiags(S, Source, Inputs, WantDiagnostics::Yes,
+                  [](std::vector<Diag> Diags) {
+    ADD_FAILURE() << "Did not expect diagnostics for missing header update";
+  });
+
+  // Forcing the reload should should cause a rebuild which no longer has any
+  // errors.
+  Inputs.ForceRebuild = true;
+  updateWithDiags(S, Source, Inputs, WantDiagnostics::Yes,
+                  [](std::vector<Diag> Diags) {
+    EXPECT_THAT(Diags, IsEmpty());
+  });
+
+  ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(10)));
+}
 TEST_F(TUSchedulerTests, NoChangeDiags) {
   TUScheduler S(CDB, optsForTest(), captureDiags());
 
@@ -722,7 +769,8 @@
   TUScheduler S(CDB, optsForTest(), captureDiags());
   std::vector<Diag> Diagnostics;
   updateWithDiags(S, testPath("foo.cpp"), "void test() {}",
-                  WantDiagnostics::Yes, [&](std::vector<Diag> D) {
+                  WantDiagnostics::Yes,
+                  [&](std::vector<Diag> D) {
                     Diagnostics = std::move(D);
                     Ready.notify();
                   });
@@ -746,7 +794,8 @@
   TUScheduler S(CDB, optsForTest(), captureDiags());
   std::vector<Diag> Diagnostics;
   updateWithDiags(S, testPath("foo.cpp"), "void test() {}",
-                  WantDiagnostics::Yes, [&](std::vector<Diag> D) {
+                  WantDiagnostics::Yes,
+                  [&](std::vector<Diag> D) {
                     Diagnostics = std::move(D);
                     Ready.notify();
                   });