Index: ClangdLSPServer.h
===================================================================
--- ClangdLSPServer.h
+++ ClangdLSPServer.h
@@ -43,7 +43,7 @@
   /// each instance of ClangdLSPServer.
   ///
   /// \return Whether we received a 'shutdown' request before an 'exit' request.
-  bool run(std::FILE *In,
+  bool run(std::FILE *In, bool TestMode,
            JSONStreamStyle InputStyle = JSONStreamStyle::Standard);
 
 private:
Index: ClangdLSPServer.cpp
===================================================================
--- ClangdLSPServer.cpp
+++ ClangdLSPServer.cpp
@@ -447,7 +447,8 @@
       CCOpts(CCOpts), SupportedSymbolKinds(defaultSymbolKinds()),
       Server(CDB.getCDB(), FSProvider, /*DiagConsumer=*/*this, Opts) {}
 
-bool ClangdLSPServer::run(std::FILE *In, JSONStreamStyle InputStyle) {
+bool ClangdLSPServer::run(std::FILE *In, bool TestMode,
+                          JSONStreamStyle InputStyle) {
   assert(!IsDone && "Run was called before");
 
   // Set up JSONRPCDispatcher.
@@ -457,7 +458,7 @@
   registerCallbackHandlers(Dispatcher, /*Callbacks=*/*this);
 
   // Run the Language Server loop.
-  runLanguageServerLoop(In, Out, InputStyle, Dispatcher, IsDone);
+  runLanguageServerLoop(In, Out, InputStyle, Dispatcher, IsDone, TestMode);
 
   // 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: JSONRPCDispatcher.h
===================================================================
--- JSONRPCDispatcher.h
+++ JSONRPCDispatcher.h
@@ -109,7 +109,8 @@
 /// with signals, which are sent by debuggers on some OSs.
 void runLanguageServerLoop(std::FILE *In, JSONOutput &Out,
                            JSONStreamStyle InputStyle,
-                           JSONRPCDispatcher &Dispatcher, bool &IsDone);
+                           JSONRPCDispatcher &Dispatcher, bool &IsDone,
+                           bool TestMode);
 
 } // namespace clangd
 } // namespace clang
Index: JSONRPCDispatcher.cpp
===================================================================
--- JSONRPCDispatcher.cpp
+++ JSONRPCDispatcher.cpp
@@ -342,8 +342,8 @@
 // The C APIs seem to be clearer in this respect.
 void clangd::runLanguageServerLoop(std::FILE *In, JSONOutput &Out,
                                    JSONStreamStyle InputStyle,
-                                   JSONRPCDispatcher &Dispatcher,
-                                   bool &IsDone) {
+                                   JSONRPCDispatcher &Dispatcher, bool &IsDone,
+                                   bool TestMode) {
   auto &ReadMessage =
       (InputStyle == Delimited) ? readDelimitedMessage : readStandardMessage;
   while (!IsDone && !feof(In)) {
@@ -362,6 +362,8 @@
         // Parse error. Log the raw message.
         vlog("<<< {0}\n", *JSON);
         elog("JSON parse error: {0}", llvm::toString(Doc.takeError()));
+        if (TestMode)
+          exit(EXIT_FAILURE);
       }
     }
   }
Index: tool/ClangdMain.cpp
===================================================================
--- tool/ClangdMain.cpp
+++ tool/ClangdMain.cpp
@@ -308,5 +308,6 @@
   llvm::set_thread_name("clangd.main");
   // Change stdin to binary to not lose \r\n on windows.
   llvm::sys::ChangeStdinToBinary();
-  return LSPServer.run(stdin, InputStyle) ? 0 : NoShutdownRequestErrorCode;
+  return LSPServer.run(stdin, Test, InputStyle) ? 0
+                                                : NoShutdownRequestErrorCode;
 }