fix: persist modelId in assistant message content.metadata (#12969)

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Tyler Barnes <tylerdbarnes@gmail.com>

Author: 3 people

.changeset/olive-bees-film.md

@@ -0,0 +1,7 @@
+---
+'@mastra/core': patch
+---
+
+Fixed assistant messages to persist `content.metadata.modelId` during streaming.
+This ensures stored and processed assistant messages keep the model identifier.
+Developers can now reliably read `content.metadata.modelId` from downstream storage adapters and processors.

packages/core/src/agent/__tests__/memory-metadata.test.ts

@@ -66,6 +66,40 @@ function memoryMetadataTests(version: 'v1' | 'v2') {
       }
     });
 
+    it.skipIf(version !== 'v2')(
+      'should persist modelId in assistant message content.metadata using stream',
+      async () => {
+        const mockMemory = new MockMemory();
+        const agent = new Agent({
+          id: 'test-agent',
+          name: 'Test Agent',
+          instructions: 'test',
+          model: dummyModel,
+          memory: mockMemory,
+        });
+
+        const res = await agent.stream('hello', {
+          memory: {
+            resource: 'user-1',
+            thread: { id: 'thread-model-stream' },
+          },
+        });
+
+        await res.consumeStream();
+
+        const { messages } = await mockMemory.recall({
+          threadId: 'thread-model-stream',
+          perPage: false,
+        });
+        const assistantMessages = messages.filter(m => m.role === 'assistant');
+        expect(assistantMessages.length).toBeGreaterThan(0);
+
+        for (const msg of assistantMessages) {
+          expect(msg.content.metadata?.modelId).toBe('mock-model-id');
+        }
+      },
+    );
+
     it('should create a new thread with metadata using generate', async () => {
       const mockMemory = new MockMemory();
       const agent = new Agent({

packages/core/src/loop/workflows/agentic-execution/llm-execution-step.ts

@@ -58,6 +58,11 @@ type ProcessOutputStreamOptions<OUTPUT = undefined> = {
   transportResolver?: () => StreamTransport | undefined;
 };
 
+function buildResponseModelMetadata(runState: AgenticRunState): { metadata: Record<string, unknown> } | undefined {
+  const modelId = runState.state.responseMetadata?.modelId;
+  return modelId ? { metadata: { modelId } } : undefined;
+}
+
 async function processOutputStream<OUTPUT = undefined>({
   tools,
   messageId,
@@ -131,6 +136,7 @@ async function processOutputStream<OUTPUT = undefined>({
                 ...(providerMetadata ? { providerMetadata } : {}),
               },
             ],
+            ...buildResponseModelMetadata(runState),
           },
           createdAt: new Date(),
         };
@@ -274,6 +280,7 @@ async function processOutputStream<OUTPUT = undefined>({
                   providerMetadata: chunk.payload.providerMetadata ?? runState.state.providerOptions,
                 },
               ],
+              ...buildResponseModelMetadata(runState),
             },
             createdAt: new Date(),
           };
@@ -313,6 +320,7 @@ async function processOutputStream<OUTPUT = undefined>({
                 providerMetadata: chunk.payload.providerMetadata ?? runState.state.providerOptions,
               },
             ],
+            ...buildResponseModelMetadata(runState),
           },
           createdAt: new Date(),
         };
@@ -346,6 +354,7 @@ async function processOutputStream<OUTPUT = undefined>({
                   mimeType: chunk.payload.mimeType,
                 },
               ],
+              ...buildResponseModelMetadata(runState),
             },
             createdAt: new Date(),
           };
@@ -373,6 +382,7 @@ async function processOutputStream<OUTPUT = undefined>({
                   },
                 },
               ],
+              ...buildResponseModelMetadata(runState),
             },
             createdAt: new Date(),
           };
@@ -1075,6 +1085,7 @@ export function createLLMExecutionStep<TOOLS extends ToolSet = ToolSet, OUTPUT =
                 providerExecuted: toolCall.providerExecuted,
               };
             }),
+            ...buildResponseModelMetadata(runState),
           },
           createdAt: new Date(),
         };