fix(sdk): fold queued response data parts into the managed error partial

The managed error path now folds data parts queued via chat.response /
writer.write() into the recovered partial before persisting, matching the
success path and the manual loop, so a data part queued just before a
source-stream failure isn't dropped.

Author: matt-aitken

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

@@ -7956,6 +7956,16 @@ function chatAgent<
             if (partialResponse && !partialResponse.id) {
               partialResponse = { ...partialResponse, id: generateMessageId() } as TUIMessage;
             }
+            if (partialResponse && !responseCommitted) {
+              const queuedParts = locals.get(chatResponsePartsKey);
+              if (queuedParts && queuedParts.length > 0) {
+                partialResponse = {
+                  ...partialResponse,
+                  parts: [...partialResponse.parts, ...(queuedParts as UIMessage["parts"])],
+                } as TUIMessage;
+                locals.set(chatResponsePartsKey, []);
+              }
+            }
             const includePartial = partialResponse != null && !responseCommitted;
             let erroredUIMessagesWithPartial: TUIMessage[] = !includePartial
               ? erroredUIMessages
@@ -7991,7 +8001,9 @@ function chatAgent<
                 }
                 if (erroredUIMessagesWithPartial !== accumulatedUIMessages) {
                   if (partialIdx === -1) {
-                    const appended = erroredUIMessagesWithPartial.slice(accumulatedUIMessages.length);
+                    const appended = erroredUIMessagesWithPartial.slice(
+                      accumulatedUIMessages.length
+                    );
                     accumulatedMessages.push(
                       ...(await toModelMessages(appended.map((m) => stripProviderMetadata(m))))
                     );

packages/trigger-sdk/test/chat-agent-source-stream-error.test.ts

@@ -368,6 +368,35 @@ describe("chat.agent managed loop — source-stream failure", () => {
       await harness.close();
     }
   });
+
+  it("folds queued response data parts into the recovered partial", async () => {
+    const turnCompletes: TurnCompleteEvent<unknown, UIMessage>[] = [];
+
+    const agent = chat.agent({
+      id: "chatAgent.error-queued-parts",
+      run: async () => {
+        chat.response.write({ type: "data-marker", data: { kept: true } } as never);
+        return erroringSource("UND_ERR_BODY_TIMEOUT") as never;
+      },
+      onTurnComplete: async (event) => {
+        turnCompletes.push(event);
+      },
+    });
+
+    const harness = mockChatAgent(agent, { chatId: "cae-error-queued-parts" });
+    try {
+      await harness.sendMessage(userMessage("hi", "u-1"));
+      await waitFor(() => turnCompletes.length >= 1);
+
+      const evt = turnCompletes[0]!;
+      expect(evt.responseMessage).toBeDefined();
+      const parts = evt.responseMessage!.parts as Array<{ type: string }>;
+      expect(extractText(evt.responseMessage)).toBe("partial answer");
+      expect(parts.some((p) => p.type === "data-marker")).toBe(true);
+    } finally {
+      await harness.close();
+    }
+  });
 });
 
 describe("chat.createSession turn.complete() — source-stream failure", () => {