diff --git a/.changeset/social-mirrors-juggle.md b/.changeset/social-mirrors-juggle.md
new file mode 100644
index 00000000..3c331202
--- /dev/null
+++ b/.changeset/social-mirrors-juggle.md
@@ -0,0 +1,5 @@
+---
+'@openai/agents-core': patch
+---
+
+ensure getTransferMessage returns valid JSON
diff --git a/packages/agents-core/src/handoff.ts b/packages/agents-core/src/handoff.ts
index 40b9d18a..9e97ea44 100644
--- a/packages/agents-core/src/handoff.ts
+++ b/packages/agents-core/src/handoff.ts
@@ -47,7 +47,7 @@ export type HandoffInputFilter = (input: HandoffInputData) => HandoffInputData;
 export function getTransferMessage<TContext, TOutput extends AgentOutputType>(
   agent: Agent<TContext, TOutput>,
 ) {
-  return `{'assistant': '${agent.name}'}`;
+  return JSON.stringify({ assistant: agent.name });
 }
 
 /**
diff --git a/packages/agents-core/test/handoffs.test.ts b/packages/agents-core/test/handoffs.test.ts
index 38132dbd..96e22fae 100644
--- a/packages/agents-core/test/handoffs.test.ts
+++ b/packages/agents-core/test/handoffs.test.ts
@@ -69,7 +69,17 @@ describe('Agent + handoffs', () => {
         outputType: z.object({ a: z.string() }),
       }),
     );
-    expect(result).toBe("{'assistant': 'Agent A'}");
+    expect(result).toBe('{"assistant":"Agent A"}');
+  });
+
+  it('getTransferMessage produces valid JSON', () => {
+    const result = getTransferMessage(
+      new Agent({
+        name: 'Agent A',
+        outputType: z.object({ a: z.string() }),
+      }),
+    );
+    expect(JSON.parse(result)).toEqual({ assistant: 'Agent A' });
   });
 
   it('Handoff#getHandoffAsFunctionTool', async () => {