fix(sdk): keep late response parts on a skipped empty response

When the model returns no content the response is no longer accumulated,
but a non-transient data part written during onBeforeTurnComplete still
has to be persisted. The late-part drain now commits the previously
skipped response into the UI transcript (kept out of the model lane,
since the parts are UI-only) instead of clearing the parts without
saving them.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VG39FXXkFFU24U5EtJMwPi

Author: ericallam

packages/trigger-sdk/src/v3/ai.ts

@@ -9482,10 +9482,18 @@ function chatAgent<
                         parts: [...(msg.parts ?? []), ...lateParts],
                       } as TUIMessage;
                       capturedResponseMessage = accumulatedUIMessages[idx] as TUIMessage;
-                      capturedPartialResponse = capturedResponseMessage;
-                      turnCompleteEvent.responseMessage = capturedResponseMessage;
-                      turnCompleteEvent.uiMessages = accumulatedUIMessages;
+                    } else {
+                      capturedResponseMessage = {
+                        ...capturedResponseMessage,
+                        parts: [...(capturedResponseMessage.parts ?? []), ...lateParts],
+                      } as TUIMessage;
+                      accumulatedUIMessages.push(capturedResponseMessage);
+                      turnNewUIMessages.push(capturedResponseMessage);
                     }
+                    capturedPartialResponse = capturedResponseMessage;
+                    turnCompleteEvent.responseMessage = capturedResponseMessage;
+                    turnCompleteEvent.uiMessages = accumulatedUIMessages;
+                    locals.set(chatCurrentUIMessagesKey, accumulatedUIMessages);
                     locals.set(chatResponsePartsKey, []);
                   }
 

packages/trigger-sdk/test/transcript-changesets.test.ts

@@ -528,6 +528,35 @@ describe("chat.agent transcript changesets", () => {
       await harness.close();
     }
   });
+
+  it("persists a late response part written by onBeforeTurnComplete on an empty response", async () => {
+    const chatId = "changeset-late-part";
+    const model = new MockLanguageModelV3({
+      doStream: async () => ({
+        stream: simulateReadableStream({
+          chunks: [{ type: "finish", finishReason: { unified: "stop", raw: "stop" }, usage }],
+        }),
+      }),
+    });
+    const agent = chat.agent({
+      id: "changeset-late-part",
+      onBeforeTurnComplete: async ({ writer }) => {
+        writer.write({ type: "data-note", data: { text: "late note" } } as never);
+      },
+      run: async ({ messages, signal }) => streamText({ model, messages, abortSignal: signal }),
+    });
+    const harness = mockChatAgent(agent, { chatId });
+    try {
+      await harness.sendMessage(userMessage("hello", "u1"));
+      await waitFor(() => storage.changesets.length === 1, "turn save");
+      const entries = storage.transcript(chatId)!.entries;
+      expect(entries.map((e) => e.message.role)).toEqual(["user", "assistant"]);
+      const assistant = entries.find((e) => e.message.role === "assistant");
+      expect(JSON.stringify(assistant?.message)).toContain("late note");
+    } finally {
+      await harness.close();
+    }
+  });
 });
 
 function assistantMessage(id: string): UIMessage {