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;
@@ -179,6 +179,7 @@
   if (GetClangTidyOptions)
     Opts.ClangTidyOpts = GetClangTidyOptions(*FS, File);
   Opts.SuggestMissingIncludes = SuggestMissingIncludes;
+  Opts.ForceRebuild = ForceRebuild;
 
   // Compile command is set asynchronously during update, as it can be slow.
   ParseInputs Inputs;
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
@@ -38,6 +38,7 @@
 struct ParseOptions {
   tidy::ClangTidyOptions ClangTidyOpts;
   bool SuggestMissingIncludes = false;
+  bool ForceRebuild = false;
 };
 
 /// Information required to run clang, e.g. to parse AST or do code completion.
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!
+  /// Disabled by default. This is useful to defeat clangd's assumption that
+  /// missing headers stay missing.
+  /// This is a clangd extension.
+  bool forceRebuild;
 };
 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
@@ -432,7 +432,8 @@
     }
 
     std::shared_ptr<const PreambleData> OldPreamble =
-        getPossiblyStalePreamble();
+        Inputs.Opts.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
@@ -615,6 +615,46 @@
   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";
+
+  // Return value indicates if the updated callback was received.
+  auto DoUpdate = [&](std::string Contents, bool ForceRebuild) -> bool {
+    std::atomic<bool> Updated(false);
+    Updated = false;
+    ParseInputs Inputs = getInputs(Source, std::string(Contents));
+    Inputs.Opts.ForceRebuild = ForceRebuild;
+    updateWithDiags(S, Source, Inputs, WantDiagnostics::Yes,
+                    [&Updated](std::vector<Diag>) { Updated = true; });
+    bool UpdateFinished = S.blockUntilIdle(timeoutSeconds(10));
+    if (!UpdateFinished)
+      ADD_FAILURE() << "Updated has not finished in one second. Threading bug?";
+    return Updated;
+  };
+
+  // Update the source contents, which should trigger an initial build with
+  // the header file missing.
+  ASSERT_TRUE(DoUpdate(SourceContents, /*ForceRebuild*/false));
+
+  // Add the header file.
+  Files[Header] = "int a;";
+  Timestamps[Header] = time_t(1);
+
+  // The addition of the missing header file shouldn't trigger a rebuild since
+  // we don't track missing files.
+  ASSERT_FALSE(DoUpdate(SourceContents, /*ForceRebuild*/false)); 
+
+  // Forcing the reload should should cause a rebuild, though.
+  ASSERT_TRUE(DoUpdate(SourceContents, /*ForceRebuild*/true)); 
+}
 TEST_F(TUSchedulerTests, NoChangeDiags) {
   TUScheduler S(CDB, optsForTest(), captureDiags());
 
@@ -721,7 +761,8 @@
   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();
                   });
@@ -745,7 +786,8 @@
   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();
                   });