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 LogToStderr = false,
+             bool Pretty = false)
+      : LogToStderr(LogToStderr), 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 LogToStderr;
+
   // Whether output should be pretty-printed.
   const bool Pretty;
 
Index: clangd/JSONRPCDispatcher.cpp
===================================================================
--- clangd/JSONRPCDispatcher.cpp
+++ clangd/JSONRPCDispatcher.cpp
@@ -70,8 +70,12 @@
 }
 
 void JSONOutput::log(const Twine &Message) {
-  llvm::sys::TimePoint<> Timestamp = std::chrono::system_clock::now();
   trace::log(Message);
+
+  if (!LogToStderr)
+    return;
+
+  llvm::sys::TimePoint<> Timestamp = std::chrono::system_clock::now();
   std::lock_guard<std::mutex> Guard(StreamMutex);
   Logs << llvm::formatv("[{0:%H:%M:%S.%L}] {1}\n", Timestamp, Message);
   Logs.flush();
Index: clangd/tool/ClangdMain.cpp
===================================================================
--- clangd/tool/ClangdMain.cpp
+++ clangd/tool/ClangdMain.cpp
@@ -77,15 +77,20 @@
     llvm::cl::init(JSONStreamStyle::Standard));
 
 static llvm::cl::opt<bool>
+    LogToStderr("log-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));
 
-static llvm::cl::opt<bool> Test(
-    "lit-test",
-    llvm::cl::desc(
-        "Abbreviation for -input-style=delimited -pretty -run-synchronously. "
-        "Intended to simplify lit tests."),
-    llvm::cl::init(false), llvm::cl::Hidden);
+static llvm::cl::opt<bool>
+    Test("lit-test",
+         llvm::cl::desc(
+             "Abbreviation for -input-style=delimited -pretty -log-to-stderr "
+             "-run-synchronously. Intended to simplify lit tests."),
+         llvm::cl::init(false), llvm::cl::Hidden);
 
 static llvm::cl::opt<PCHStorageFlag> PCHStorage(
     "pch-storage",
@@ -140,6 +145,7 @@
   if (Test) {
     RunSynchronously = true;
     InputStyle = JSONStreamStyle::Delimited;
+    LogToStderr = true;
     PrettyPrint = true;
   }
 
@@ -190,7 +196,7 @@
 
   JSONOutput Out(llvm::outs(), llvm::errs(),
                  InputMirrorStream ? InputMirrorStream.getPointer() : nullptr,
-                 PrettyPrint);
+                 LogToStderr, PrettyPrint);
 
   clangd::LoggingSession LoggingSession(Out);
 
Index: test/clangd/protocol.test
===================================================================
--- test/clangd/protocol.test
+++ test/clangd/protocol.test
@@ -1,5 +1,5 @@
-# RUN: not clangd -pretty -run-synchronously < %s | FileCheck -strict-whitespace %s
-# RUN: not clangd -pretty -run-synchronously < %s 2>&1 | FileCheck -check-prefix=STDERR %s
+# RUN: not clangd -pretty -log-to-stderr -run-synchronously < %s | FileCheck -strict-whitespace %s
+# RUN: not clangd -pretty -log-to-stderr -run-synchronously < %s 2>&1 | FileCheck -check-prefix=STDERR %s
 # vim: fileformat=dos
 # It is absolutely vital that this file has CRLF line endings.
 #
Index: test/clangd/too_large.test
===================================================================
--- test/clangd/too_large.test
+++ test/clangd/too_large.test
@@ -1,4 +1,4 @@
-# RUN: not clangd -run-synchronously < %s 2>&1 | FileCheck -check-prefix=STDERR %s
+# RUN: not clangd -run-synchronously -log-to-stderr < %s 2>&1 | FileCheck -check-prefix=STDERR %s
 # vim: fileformat=dos
 # It is absolutely vital that this file has CRLF line endings.
 #