Index: clangd/ClangdLSPServer.h
===================================================================
--- clangd/ClangdLSPServer.h
+++ clangd/ClangdLSPServer.h
@@ -42,7 +42,8 @@
   ///
   /// \return Wether we received a 'shutdown' request before an 'exit' request
   bool run(std::istream &In,
-           JSONStreamStyle InputStyle = JSONStreamStyle::Standard);
+           JSONStreamStyle InputStyle = JSONStreamStyle::Standard,
+           bool LogLspToStderr = false);
 
 private:
   // Implement DiagnosticsConsumer.
Index: clangd/ClangdLSPServer.cpp
===================================================================
--- clangd/ClangdLSPServer.cpp
+++ clangd/ClangdLSPServer.cpp
@@ -402,7 +402,8 @@
     : Out(Out), CDB(std::move(CompileCommandsDir)), CCOpts(CCOpts),
       Server(CDB, FSProvider, /*DiagConsumer=*/*this, Opts) {}
 
-bool ClangdLSPServer::run(std::istream &In, JSONStreamStyle InputStyle) {
+bool ClangdLSPServer::run(std::istream &In, JSONStreamStyle InputStyle,
+                          bool LogLspToStderr) {
   assert(!IsDone && "Run was called before");
 
   // Set up JSONRPCDispatcher.
@@ -412,7 +413,8 @@
   registerCallbackHandlers(Dispatcher, /*Callbacks=*/*this);
 
   // Run the Language Server loop.
-  runLanguageServerLoop(In, Out, InputStyle, Dispatcher, IsDone);
+  runLanguageServerLoop(In, Out, InputStyle, Dispatcher, LogLspToStderr,
+                        IsDone);
 
   // Make sure IsDone is set to true after this method exits to ensure assertion
   // at the start of the method fires if it's ever executed again.
Index: clangd/JSONRPCDispatcher.h
===================================================================
--- clangd/JSONRPCDispatcher.h
+++ clangd/JSONRPCDispatcher.h
@@ -30,8 +30,10 @@
   // JSONOutput now that we pass Context everywhere.
 public:
   JSONOutput(llvm::raw_ostream &Outs, llvm::raw_ostream &Logs,
-             llvm::raw_ostream *InputMirror = nullptr, bool Pretty = false)
-      : Pretty(Pretty), Outs(Outs), Logs(Logs), InputMirror(InputMirror) {}
+             llvm::raw_ostream *InputMirror = nullptr,
+             bool LogLspToStderr = false, bool Pretty = false)
+      : LogLspToStderr(LogLspToStderr), Pretty(Pretty), Outs(Outs), Logs(Logs),
+        InputMirror(InputMirror) {}
 
   /// Emit a JSONRPC message.
   void writeMessage(const json::Expr &Result);
@@ -44,6 +46,9 @@
   /// Unlike other methods of JSONOutput, mirrorInput is not thread-safe.
   void mirrorInput(const Twine &Message);
 
+  /// Whether to output all LSP communication to stderr.
+  const bool LogLspToStderr;
+
   // Whether output should be pretty-printed.
   const bool Pretty;
 
@@ -104,7 +109,8 @@
 /// replacements of \r\n with \n.
 void runLanguageServerLoop(std::istream &In, JSONOutput &Out,
                            JSONStreamStyle InputStyle,
-                           JSONRPCDispatcher &Dispatcher, bool &IsDone);
+                           JSONRPCDispatcher &Dispatcher, bool LogLspToStderr,
+                           bool &IsDone);
 
 } // namespace clangd
 } // namespace clang
Index: clangd/JSONRPCDispatcher.cpp
===================================================================
--- clangd/JSONRPCDispatcher.cpp
+++ clangd/JSONRPCDispatcher.cpp
@@ -66,7 +66,9 @@
     Outs << "Content-Length: " << S.size() << "\r\n\r\n" << S;
     Outs.flush();
   }
-  log(llvm::Twine("--> ") + S);
+
+  if (LogLspToStderr)
+    log(llvm::Twine("--> ") + S);
 }
 
 void JSONOutput::log(const Twine &Message) {
@@ -299,14 +301,17 @@
 void clangd::runLanguageServerLoop(std::istream &In, JSONOutput &Out,
                                    JSONStreamStyle InputStyle,
                                    JSONRPCDispatcher &Dispatcher,
-                                   bool &IsDone) {
+                                   bool LogLspToStderr, bool &IsDone) {
   auto &ReadMessage =
       (InputStyle == Delimited) ? readDelimitedMessage : readStandardMessage;
   while (In.good()) {
     if (auto JSON = ReadMessage(In, Out)) {
       if (auto Doc = json::parse(*JSON)) {
-        // Log the formatted message.
-        log(llvm::formatv(Out.Pretty ? "<-- {0:2}\n" : "<-- {0}\n", *Doc));
+        if (LogLspToStderr) {
+          // Log the formatted message.
+          log(llvm::formatv(Out.Pretty ? "<-- {0:2}\n" : "<-- {0}\n", *Doc));
+        }
+
         // Finally, execute the action for this JSON message.
         if (!Dispatcher.call(*Doc, Out))
           log("JSON dispatch failed!\n");
Index: clangd/tool/ClangdMain.cpp
===================================================================
--- clangd/tool/ClangdMain.cpp
+++ clangd/tool/ClangdMain.cpp
@@ -77,6 +77,11 @@
     llvm::cl::init(JSONStreamStyle::Standard));
 
 static llvm::cl::opt<bool>
+    LogLspToStderr("log-lsp-to-stderr",
+                   llvm::cl::desc("Output LSP communication on stderr"),
+                   llvm::cl::init(false));
+
+static llvm::cl::opt<bool>
     PrettyPrint("pretty", llvm::cl::desc("Pretty-print JSON output"),
                 llvm::cl::init(false));
 
@@ -190,7 +195,7 @@
 
   JSONOutput Out(llvm::outs(), llvm::errs(),
                  InputMirrorStream ? InputMirrorStream.getPointer() : nullptr,
-                 PrettyPrint);
+                 LogLspToStderr, PrettyPrint);
 
   clangd::LoggingSession LoggingSession(Out);
 
@@ -238,5 +243,7 @@
   llvm::set_thread_name("clangd.main");
   // Change stdin to binary to not lose \r\n on windows.
   llvm::sys::ChangeStdinToBinary();
-  return LSPServer.run(std::cin, InputStyle) ? 0 : NoShutdownRequestErrorCode;
+  return LSPServer.run(std::cin, InputStyle, LogLspToStderr)
+             ? 0
+             : NoShutdownRequestErrorCode;
 }