bug: Dashboard playground chat fails with CORS error: /in/append preflight returns 400 on api.trigger.dev (agent run never receives message)

[quintenb]

Provide environment information

System:
OS: macOS 26.5.1
CPU: (12) arm64 Apple M2 Max
Memory: 293.34 MB / 32.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 22.17.1
Yarn: 1.22.19
npm: 10.9.2
pnpm: 10.26.2
bun: 1.3.14

Describe the bug

Testing a chat.agent() task via the Trigger.dev dashboard playground shows "Failed to fetch" in the UI. Browser DevTools shows the append request to api.trigger.dev fails with a CORS error, and the OPTIONS preflight returns 400.

The agent run starts successfully (preload → waiting for first message → suspended), but user messages never reach the worker (messages: [] in run payload).

SDK | @trigger.dev/sdk@4.5.0
CLI | bunx trigger.dev@latest dev → Trigger.dev 4.5.0

To reproduce

I have this agent:

import { chat } from "@trigger.dev/sdk/ai";
import { streamText, stepCountIs } from "ai";
export const demoAgent = chat.agent({
  id: "demo-agent",
  run: async ({ messages, signal }) => {
    return streamText({
      ...chat.toStreamTextOptions(),
      model: /* OpenRouter gpt-4o-mini */,
      system: "You are a friendly demo assistant...",
      messages,
      abortSignal: signal,
      stopWhen: stepCountIs(15),
    });
  },
});

Steps to reproduce

1. Run local dev worker:
2. cd apps/trigger-test && bun run trigger:dev
3. Confirm: Local worker ready on branch: default [node] -> 20260704.4
4. Open Trigger.dev dashboard → dev environment.
5. Open task demo-agent → use the chat/agent playground UI (not the regular JSON "Test" tab).
6. Wait for preload (run starts, agent suspends at "waiting for first message").
7. Type a message and send.
8. UI shows "Failed to fetch".

Additional information

No response