Index: clangd/ClangdLSPServer.h
===================================================================
--- clangd/ClangdLSPServer.h
+++ clangd/ClangdLSPServer.h
@@ -52,6 +52,7 @@
 private:
   // Implement DiagnosticsConsumer.
   void onDiagnosticsReady(PathRef File, std::vector<Diag> Diagnostics) override;
+  void onFileUpdated(PathRef File, const TUStatus &Status) override;
 
   // LSP methods. Notifications have signature void(const Params&).
   // Calls have signature void(const Params&, Callback<Response>).
Index: clangd/ClangdLSPServer.cpp
===================================================================
--- clangd/ClangdLSPServer.cpp
+++ clangd/ClangdLSPServer.cpp
@@ -802,6 +802,10 @@
          });
 }
 
+void ClangdLSPServer::onFileUpdated(PathRef File, const TUStatus &Status) {
+  notify("textDocument/clangd.fileStatus", Status.render(File));
+}
+
 void ClangdLSPServer::reparseOpenedFiles() {
   for (const Path &FilePath : DraftMgr.getActiveFiles())
     Server->addDocument(FilePath, *DraftMgr.getDraft(FilePath),
Index: clangd/Protocol.h
===================================================================
--- clangd/Protocol.h
+++ clangd/Protocol.h
@@ -973,6 +973,33 @@
 };
 bool fromJSON(const llvm::json::Value &, ReferenceParams &);
 
+enum class MessageType {
+  Error = 1,
+  Warning = 2,
+  Info = 3,
+  Log = 4,
+};
+struct ShowMessageParams {
+  /// The message type.
+  MessageType type;
+  /// The actual message.
+  std::string message;
+};
+llvm::json::Value toJSON(const ShowMessageParams &);
+
+/// Clangd extension: indicates the current state of the file in clangd,
+/// sent from server via the `textDocument/fileStatus` notification.
+struct FileStatus {
+  /// The text document's URI.
+  URIForFile uri;
+  /// The human-readable string presents the current state of the file, can be
+  /// shown in the UI (e.g. status bar).
+  std::string state;
+  /// Details of the state that are worth sufacing to users.
+  std::vector<ShowMessageParams> details;
+};
+llvm::json::Value toJSON(const FileStatus &FStatus);
+
 } // namespace clangd
 } // namespace clang
 
Index: clangd/Protocol.cpp
===================================================================
--- clangd/Protocol.cpp
+++ clangd/Protocol.cpp
@@ -716,6 +716,20 @@
   };
 }
 
+llvm::json::Value toJSON(const ShowMessageParams &S) {
+  return json::Object{
+      {"type", static_cast<int>(S.type)}, {"message", S.message},
+  };
+}
+
+llvm::json::Value toJSON(const FileStatus &FStatus) {
+  return json::Object{
+      {"uri", FStatus.uri},
+      {"state", FStatus.state},
+      {"details", FStatus.details},
+  };
+}
+
 raw_ostream &operator<<(raw_ostream &O, const DocumentHighlight &V) {
   O << V.range;
   if (V.kind == DocumentHighlightKind::Read)
Index: clangd/TUScheduler.h
===================================================================
--- clangd/TUScheduler.h
+++ clangd/TUScheduler.h
@@ -77,6 +77,8 @@
     /// Indicates whether we reused the prebuilt AST.
     bool ReuseAST = false;
   };
+  /// Serialize this to an LSP file status item.
+  FileStatus render(PathRef File) const;
 
   TUAction Action;
   BuildDetails Details;
Index: clangd/TUScheduler.cpp
===================================================================
--- clangd/TUScheduler.cpp
+++ clangd/TUScheduler.cpp
@@ -729,6 +729,34 @@
   return wait(Lock, RequestsCV, Timeout, [&] { return Requests.empty(); });
 }
 
+// Render a TUAction to a user-facing string representation.
+// TUAction represents clangd-internal states, we don't intend to expose them
+// to users (say C++ programmers) directly to avoid confusion, we use terms that
+// are familiar by C++ programmers.
+std::string renderTUAction(const TUAction& Action) {
+  std::string Result;
+  raw_string_ostream OS(Result);
+  switch (Action.S) {
+  case TUAction::Queued:
+    OS << "Queued";
+    break;
+  case TUAction::RunningAction:
+    OS << "RunningAction"
+       << "(" << Action.Name << ")";
+    break;
+  case TUAction::BuildingPreamble:
+    OS << "Parsing includes";
+    break;
+  case TUAction::BuildingFile:
+    OS << "Parsing main file";
+    break;
+  case TUAction::Idle:
+    OS << "Idle";
+    break;
+  }
+  return OS.str();
+}
+
 } // namespace
 
 unsigned getDefaultAsyncThreadsCount() {
@@ -741,6 +769,17 @@
   return HardwareConcurrency;
 }
 
+FileStatus TUStatus::render(PathRef File) const {
+  FileStatus FStatus;
+  FStatus.uri = URIForFile::canonicalize(File, /*TUPath=*/File);
+  FStatus.state = renderTUAction(Action);
+  if (Details.BuildFailed)
+    FStatus.details.push_back({MessageType::Error, "failed to build the file"});
+  if (Details.ReuseAST)
+    FStatus.details.push_back({MessageType::Info, "reused the AST"});
+  return FStatus;
+}
+
 struct TUScheduler::FileData {
   /// Latest inputs, passed to TUScheduler::update().
   std::string Contents;
Index: test/clangd/filestatus.test
===================================================================
--- /dev/null
+++ test/clangd/filestatus.test
@@ -0,0 +1,14 @@
+# RUN: clangd -lit-test < %s | FileCheck -strict-whitespace %s
+{"jsonrpc":"2.0","id":0,"method":"initialize","params":{"processId":123,"rootPath":"clangd","capabilities":{},"trace":"off"}}
+---
+{"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"uri":"test:///main.cpp","languageId":"cpp","version":1,"text":"int x; int y = x;"}}}
+#      CHECK:  "method": "textDocument/clangd.fileStatus",
+# CHECK-NEXT:  "params": {
+# CHECK-NEXT:    "details": [],
+# CHECK-NEXT:    "state": "Parsing includes",
+# CHECK-NEXT:    "uri": "{{.*}}/main.cpp"
+# CHECK-NEXT:  }
+---
+{"jsonrpc":"2.0","id":3,"method":"shutdown"}
+---
+{"jsonrpc":"2.0","method":"exit"}