diff --git a/clang-tools-extra/clangd/ClangdLSPServer.cpp b/clang-tools-extra/clangd/ClangdLSPServer.cpp
index 1fd0e95aef48..e0489eb15b8b 100644
--- a/clang-tools-extra/clangd/ClangdLSPServer.cpp
+++ b/clang-tools-extra/clangd/ClangdLSPServer.cpp
@@ -97,6 +97,9 @@ void ClangdLSPServer::LSPProtocolCallbacks::onShutdown(JSONOutput &Out) {
 
 void ClangdLSPServer::LSPProtocolCallbacks::onDocumentDidOpen(
     DidOpenTextDocumentParams Params, JSONOutput &Out) {
+  if (Params.metadata && !Params.metadata->extraFlags.empty())
+    LangServer.CDB.setExtraFlagsForFile(Params.textDocument.uri.file,
+                                        std::move(Params.metadata->extraFlags));
   LangServer.Server.addDocument(Params.textDocument.uri.file,
                                 Params.textDocument.text);
 }
diff --git a/clang-tools-extra/clangd/ClangdUnitStore.cpp b/clang-tools-extra/clangd/ClangdUnitStore.cpp
index 56e6f547a8de..c892f9950a83 100644
--- a/clang-tools-extra/clangd/ClangdUnitStore.cpp
+++ b/clang-tools-extra/clangd/ClangdUnitStore.cpp
@@ -22,13 +22,12 @@ void ClangdUnitStore::removeUnitIfPresent(PathRef File) {
   OpenedFiles.erase(It);
 }
 
-std::vector<tooling::CompileCommand> ClangdUnitStore::getCompileCommands(GlobalCompilationDatabase &CDB, PathRef File) {
+std::vector<tooling::CompileCommand>
+ClangdUnitStore::getCompileCommands(GlobalCompilationDatabase &CDB,
+                                    PathRef File) {
   std::vector<tooling::CompileCommand> Commands = CDB.getCompileCommands(File);
-  if (Commands.empty()) {
+  if (Commands.empty())
     // Add a fake command line if we know nothing.
-    Commands.push_back(tooling::CompileCommand(
-        llvm::sys::path::parent_path(File), llvm::sys::path::filename(File),
-        {"clang", "-fsyntax-only", File.str()}, ""));
-  }
+    Commands.push_back(getDefaultCompileCommand(File));
   return Commands;
 }
diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
index 91d77026293c..9cf8572c257a 100644
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
@@ -12,17 +12,53 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 
-using namespace clang::clangd;
-using namespace clang;
+namespace clang {
+namespace clangd {
+
+static void addExtraFlags(tooling::CompileCommand &Command,
+                          const std::vector<std::string> &ExtraFlags) {
+  if (ExtraFlags.empty())
+    return;
+  assert(Command.CommandLine.size() >= 2 &&
+         "Expected a command line containing at least 2 arguments, the "
+         "compiler binary and the output file");
+  // The last argument of CommandLine is the name of the input file.
+  // Add ExtraFlags before it.
+  auto It = Command.CommandLine.end();
+  --It;
+  Command.CommandLine.insert(It, ExtraFlags.begin(), ExtraFlags.end());
+}
+
+tooling::CompileCommand getDefaultCompileCommand(PathRef File) {
+  std::vector<std::string> CommandLine{"clang", "-fsyntax-only", File.str()};
+  return tooling::CompileCommand(llvm::sys::path::parent_path(File),
+                                 llvm::sys::path::filename(File), CommandLine,
+                                 /*Output=*/"");
+}
 
 std::vector<tooling::CompileCommand>
 DirectoryBasedGlobalCompilationDatabase::getCompileCommands(PathRef File) {
   std::vector<tooling::CompileCommand> Commands;
 
   auto CDB = getCompilationDatabase(File);
-  if (!CDB)
-    return {};
-  return CDB->getCompileCommands(File);
+  if (CDB)
+    Commands = CDB->getCompileCommands(File);
+  if (Commands.empty())
+    Commands.push_back(getDefaultCompileCommand(File));
+
+  auto It = ExtraFlagsForFile.find(File);
+  if (It != ExtraFlagsForFile.end()) {
+    // Append the user-specified flags to the compile commands.
+    for (tooling::CompileCommand &Command : Commands)
+      addExtraFlags(Command, It->second);
+  }
+
+  return Commands;
+}
+
+void DirectoryBasedGlobalCompilationDatabase::setExtraFlagsForFile(
+    PathRef File, std::vector<std::string> ExtraFlags) {
+  ExtraFlagsForFile[File] = std::move(ExtraFlags);
 }
 
 tooling::CompilationDatabase *
@@ -63,3 +99,6 @@ DirectoryBasedGlobalCompilationDatabase::getCompilationDatabase(PathRef File) {
   // "\n");
   return nullptr;
 }
+
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.h b/clang-tools-extra/clangd/GlobalCompilationDatabase.h
index 933e566545b4..36c130c3512a 100644
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.h
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.h
@@ -25,6 +25,9 @@ struct CompileCommand;
 
 namespace clangd {
 
+/// Returns a default compile command to use for \p File.
+tooling::CompileCommand getDefaultCompileCommand(PathRef File);
+
 /// Provides compilation arguments used for building ClangdUnit.
 class GlobalCompilationDatabase {
 public:
@@ -45,6 +48,8 @@ public:
   std::vector<tooling::CompileCommand>
   getCompileCommands(PathRef File) override;
 
+  void setExtraFlagsForFile(PathRef File, std::vector<std::string> ExtraFlags);
+
 private:
   tooling::CompilationDatabase *getCompilationDatabase(PathRef File);
 
@@ -53,6 +58,9 @@ private:
   /// directories).
   llvm::StringMap<std::unique_ptr<clang::tooling::CompilationDatabase>>
       CompilationDatabases;
+
+  /// Stores extra flags per file.
+  llvm::StringMap<std::vector<std::string>> ExtraFlagsForFile;
 };
 } // namespace clangd
 } // namespace clang
diff --git a/clang-tools-extra/clangd/Protocol.cpp b/clang-tools-extra/clangd/Protocol.cpp
index 17618f3a1c37..c370e0db0b4c 100644
--- a/clang-tools-extra/clangd/Protocol.cpp
+++ b/clang-tools-extra/clangd/Protocol.cpp
@@ -204,6 +204,33 @@ TextDocumentItem::parse(llvm::yaml::MappingNode *Params) {
   return Result;
 }
 
+llvm::Optional<Metadata> Metadata::parse(llvm::yaml::MappingNode *Params) {
+  Metadata Result;
+  for (auto &NextKeyValue : *Params) {
+    auto *KeyString = dyn_cast<llvm::yaml::ScalarNode>(NextKeyValue.getKey());
+    if (!KeyString)
+      return llvm::None;
+
+    llvm::SmallString<10> KeyStorage;
+    StringRef KeyValue = KeyString->getValue(KeyStorage);
+    auto *Value = NextKeyValue.getValue();
+
+    llvm::SmallString<10> Storage;
+    if (KeyValue == "extraFlags") {
+      auto *Seq = dyn_cast<llvm::yaml::SequenceNode>(Value);
+      if (!Seq)
+        return llvm::None;
+      for (auto &Item : *Seq) {
+        auto *Node = dyn_cast<llvm::yaml::ScalarNode>(&Item);
+        if (!Node)
+          return llvm::None;
+        Result.extraFlags.push_back(Node->getValue(Storage));
+      }
+    }
+  }
+  return Result;
+}
+
 llvm::Optional<TextEdit> TextEdit::parse(llvm::yaml::MappingNode *Params) {
   TextEdit Result;
   for (auto &NextKeyValue : *Params) {
@@ -265,6 +292,11 @@ DidOpenTextDocumentParams::parse(llvm::yaml::MappingNode *Params) {
       if (!Parsed)
         return llvm::None;
       Result.textDocument = std::move(*Parsed);
+    } else if (KeyValue == "metadata") {
+      auto Parsed = Metadata::parse(Value);
+      if (!Parsed)
+        return llvm::None;
+      Result.metadata = std::move(*Parsed);
     } else {
       return llvm::None;
     }
diff --git a/clang-tools-extra/clangd/Protocol.h b/clang-tools-extra/clangd/Protocol.h
index 2e0c33d6d909..8239d8b2774f 100644
--- a/clang-tools-extra/clangd/Protocol.h
+++ b/clang-tools-extra/clangd/Protocol.h
@@ -118,6 +118,12 @@ struct Location {
   static std::string unparse(const Location &P);
 };
 
+struct Metadata {
+  std::vector<std::string> extraFlags;
+
+  static llvm::Optional<Metadata> parse(llvm::yaml::MappingNode *Params);
+};
+
 struct TextEdit {
   /// The range of the text document to be manipulated. To insert
   /// text into a document create a range where start === end.
@@ -152,6 +158,9 @@ struct DidOpenTextDocumentParams {
   /// The document that was opened.
   TextDocumentItem textDocument;
 
+  /// Extension storing per-file metadata, such as compilation flags.
+  llvm::Optional<Metadata> metadata;
+
   static llvm::Optional<DidOpenTextDocumentParams>
   parse(llvm::yaml::MappingNode *Params);
 };
diff --git a/clang-tools-extra/test/clangd/extra-flags.test b/clang-tools-extra/test/clangd/extra-flags.test
new file mode 100644
index 000000000000..4e5fcc9a4df1
--- /dev/null
+++ b/clang-tools-extra/test/clangd/extra-flags.test
@@ -0,0 +1,22 @@
+# RUN: clangd -run-synchronously < %s | FileCheck %s
+# It is absolutely vital that this file has CRLF line endings.
+#
+Content-Length: 125
+
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
+#
+Content-Length: 205
+
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"file:///foo.c","languageId":"c","version":1,"text":"int main() { int i; return i; }"},"metadata":{"extraFlags":["-Wall"]}}}
+# CHECK: {"jsonrpc":"2.0","method":"textDocument/publishDiagnostics","params":{"uri":"file:///foo.c","diagnostics":[{"range":{"start": {"line": 0, "character": 28}, "end": {"line": 0, "character": 28}},"severity":2,"message":"variable 'i' is uninitialized when used here"},{"range":{"start": {"line": 0, "character": 19}, "end": {"line": 0, "character": 19}},"severity":3,"message":"initialize the variable 'i' to silence this warning"}]}}
+#
+Content-Length: 175
+
+{"jsonrpc":"2.0","method":"textDocument/didChange","params":{"textDocument":{"uri":"file:///foo.c","version":2},"contentChanges":[{"text":"int main() { int i; return i; }"}]}}
+# CHECK: {"jsonrpc":"2.0","method":"textDocument/publishDiagnostics","params":{"uri":"file:///foo.c","diagnostics":[{"range":{"start": {"line": 0, "character": 28}, "end": {"line": 0, "character": 28}},"severity":2,"message":"variable 'i' is uninitialized when used here"},{"range":{"start": {"line": 0, "character": 19}, "end": {"line": 0, "character": 19}},"severity":3,"message":"initialize the variable 'i' to silence this warning"}]}}
+#
+Content-Length: 44
+
+{"jsonrpc":"2.0","id":5,"method":"shutdown"}
+
+