diff --git a/llvm/include/llvm/Support/JSON.h b/llvm/include/llvm/Support/JSON.h
--- a/llvm/include/llvm/Support/JSON.h
+++ b/llvm/include/llvm/Support/JSON.h
@@ -818,7 +818,13 @@
   void attributeBegin(llvm::StringRef Key);
   void attributeEnd();
 
- private:
+  /// Inform the writer that something external is about to write a value to OS.
+  void rawValueBegin();
+  /// Inform the writer that something external has finished writing a value to
+  /// OS.
+  void rawValueEnd();
+
+private:
   void attributeImpl(llvm::StringRef Key, Block Contents) {
     attributeBegin(Key);
     Contents();
@@ -832,6 +838,8 @@
     Singleton, // Top level, or object attribute.
     Array,
     Object,
+    /// Something outside this object is writing to OS such as a 3rd-party API.
+    ExternalStreamWriter,
   };
   struct State {
     Context Ctx = Singleton;
diff --git a/llvm/lib/Support/JSON.cpp b/llvm/lib/Support/JSON.cpp
--- a/llvm/lib/Support/JSON.cpp
+++ b/llvm/lib/Support/JSON.cpp
@@ -636,6 +636,18 @@
   Stack.back().HasValue = true;
 }
 
+void llvm::json::OStream::rawValueBegin() {
+  valueBegin();
+  Stack.emplace_back();
+  Stack.back().Ctx = ExternalStreamWriter;
+}
+
+void llvm::json::OStream::rawValueEnd() {
+  assert(Stack.back().Ctx == ExternalStreamWriter &&
+         "Expected matching rawValueBegin");
+  Stack.pop_back();
+}
+
 void llvm::json::OStream::newline() {
   if (IndentSize) {
     OS.write('\n');