diff --git a/clang-tools-extra/clangd/JSONTransport.cpp b/clang-tools-extra/clangd/JSONTransport.cpp
--- a/clang-tools-extra/clangd/JSONTransport.cpp
+++ b/clang-tools-extra/clangd/JSONTransport.cpp
@@ -126,22 +126,31 @@
   bool handleMessage(llvm::json::Value Message, MessageHandler &Handler);
   // Writes outgoing message to Out stream.
   void sendMessage(llvm::json::Value Message) {
-    std::string S;
-    llvm::raw_string_ostream OS(S);
+    OutputBuffer.clear();
+    llvm::raw_svector_ostream OS(OutputBuffer);
     OS << llvm::formatv(Pretty ? "{0:2}" : "{0}", Message);
-    OS.flush();
-    Out << "Content-Length: " << S.size() << "\r\n\r\n" << S;
+    Out << "Content-Length: " << OutputBuffer.size() << "\r\n\r\n"
+        << OutputBuffer;
     Out.flush();
-    vlog(">>> {0}\n", S);
+    vlog(">>> {0}\n", OutputBuffer);
   }
 
   // Read raw string messages from input stream.
-  llvm::Optional<std::string> readRawMessage() {
+  // This stores the message in buffer owned by this class, as such once this
+  // method is called, any previous StringRefs held from this are invalidated.
+  llvm::Optional<StringRef> readRawMessage() {
     return Style == JSONStreamStyle::Delimited ? readDelimitedMessage()
                                                : readStandardMessage();
   }
-  llvm::Optional<std::string> readDelimitedMessage();
-  llvm::Optional<std::string> readStandardMessage();
+  llvm::Optional<StringRef> readDelimitedMessage();
+  llvm::Optional<StringRef> readStandardMessage();
+
+  // Accept that these buffers are going to be large enough that there is no
+  // point in inline storage. Using a SmallString also saves the need for
+  // unnecessary things null terminators.
+  llvm::SmallString<0> JsonContentBuffer;
+  llvm::SmallString<0> JsonLineBuffer;
+  llvm::SmallString<0> OutputBuffer;
 
   std::FILE *In;
   llvm::raw_ostream &Out;
@@ -190,7 +199,7 @@
 
 // Tries to read a line up to and including \n.
 // If failing, feof(), ferror(), or shutdownRequested() will be set.
-bool readLine(std::FILE *In, std::string &Out) {
+bool readLine(std::FILE *In, llvm::SmallVectorImpl<char> &Out) {
   static constexpr int BufSize = 1024;
   size_t Size = 0;
   Out.clear();
@@ -215,17 +224,16 @@
 // Returns None when:
 //  - ferror(), feof(), or shutdownRequested() are set.
 //  - Content-Length is missing or empty (protocol error)
-llvm::Optional<std::string> JSONTransport::readStandardMessage() {
+llvm::Optional<StringRef> JSONTransport::readStandardMessage() {
   // A Language Server Protocol message starts with a set of HTTP headers,
   // delimited  by \r\n, and terminated by an empty line (\r\n).
   unsigned long long ContentLength = 0;
-  std::string Line;
   while (true) {
-    if (feof(In) || ferror(In) || !readLine(In, Line))
+    if (feof(In) || ferror(In) || !readLine(In, JsonLineBuffer))
       return llvm::None;
-    InMirror << Line;
+    InMirror << JsonLineBuffer;
 
-    llvm::StringRef LineRef(Line);
+    llvm::StringRef LineRef(JsonLineBuffer);
 
     // We allow comments in headers. Technically this isn't part
 
@@ -264,23 +272,23 @@
     return llvm::None;
   }
 
-  std::string JSON(ContentLength, '\0');
+  JsonContentBuffer.resize(ContentLength);
   for (size_t Pos = 0, Read; Pos < ContentLength; Pos += Read) {
     // Handle EINTR which is sent when a debugger attaches on some platforms.
-    Read = retryAfterSignalUnlessShutdown(0, [&]{
-      return std::fread(&JSON[Pos], 1, ContentLength - Pos, In);
+    Read = retryAfterSignalUnlessShutdown(0, [&] {
+      return std::fread(&JsonContentBuffer[Pos], 1, ContentLength - Pos, In);
     });
     if (Read == 0) {
       elog("Input was aborted. Read only {0} bytes of expected {1}.", Pos,
            ContentLength);
       return llvm::None;
     }
-    InMirror << llvm::StringRef(&JSON[Pos], Read);
+    InMirror << llvm::StringRef(&JsonContentBuffer[Pos], Read);
     clearerr(In); // If we're done, the error was transient. If we're not done,
                   // either it was transient or we'll see it again on retry.
     Pos += Read;
   }
-  return std::move(JSON);
+  return StringRef(JsonContentBuffer);
 }
 
 // For lit tests we support a simplified syntax:
@@ -288,12 +296,11 @@
 // - lines starting with # are ignored.
 // This is a testing path, so favor simplicity over performance here.
 // When returning None, feof(), ferror(), or shutdownRequested() will be set.
-llvm::Optional<std::string> JSONTransport::readDelimitedMessage() {
-  std::string JSON;
-  std::string Line;
-  while (readLine(In, Line)) {
-    InMirror << Line;
-    auto LineRef = llvm::StringRef(Line).trim();
+llvm::Optional<StringRef> JSONTransport::readDelimitedMessage() {
+  JsonContentBuffer.clear();
+  while (readLine(In, JsonLineBuffer)) {
+    InMirror << JsonLineBuffer;
+    auto LineRef = llvm::StringRef(JsonLineBuffer).trim();
     if (LineRef.startswith("#")) // comment
       continue;
 
@@ -301,7 +308,7 @@
     if (LineRef.rtrim() == "---")
       break;
 
-    JSON += Line;
+    JsonContentBuffer += JsonLineBuffer;
   }
 
   if (shutdownRequested())
@@ -310,7 +317,7 @@
     elog("Input error while reading message!");
     return llvm::None;
   }
-  return std::move(JSON); // Including at EOF
+  return StringRef(JsonContentBuffer); // Including at EOF
 }
 
 } // namespace