Trigger.dev Stream Stringify Bug Reproduction

This is a minimal reproduction of an issue where stream.append() returns [Object object] when passing an object directly, despite the TypeScript types suggesting this is valid.

The Bug

In apps/trigger/src/emit-messages.ts, the following code produces [Object object] in the stream:

const payload = { message: "one" };
await messageStream.append(payload); // Results in "[Object object]"

Expected: The stream should contain {"message":"one"}

Actual: The stream contains "[Object object]"

Likely Root Cause

stream.append() expects a BodyInit (string, Blob, etc.) at runtime, but the TypeScript types incorrectly suggest it accepts plain objects. When you pass an object, JavaScript calls .toString() on it, resulting in "[Object object]".

Workaround

In the streaming job, stringify the json directly:

await messageStream.append(
  JSON.stringify(payload) as unknown as { message: string }
);

Then parse it in the messageStream.read() call.

Steps to Reproduce

1. Clone and install dependencies:

   pnpm install

2. Start the server Find the TRIGGER_SECRET_KEY for your project, then:

   TRIGGER_SECRET_KEY=... pnpm dev

3. Open http://localhost:3000 and click "Start Stream"

You should see the stream error, because the messages are not correctly being serialized or deserialized.

4. Make the following changes:

// Bug demonstration: typescript thinks chunk is an object, but it's actually
// a string. Specifically not commenting in the next line, to demonstrate the bug.
// const data = JSON.parse(chunk as unknown as string);
// console.log("Parsed chunk:", data, "Type:", typeof data);
// res.write(JSON.stringify(data) + "\n");
res.write(JSON.stringify(chunk) + "\n");

To:

// Bug demonstration: typescript thinks chunk is an object, but it's actually
// a string. Specifically not commenting in the next line, to demonstrate the bug.
const data = JSON.parse(chunk as unknown as string);
console.log("Parsed chunk:", data, "Type:", typeof data);
res.write(JSON.stringify(data) + "\n");
// res.write(JSON.stringify(chunk) + "\n");

and:

// BUG DEMONSTRATION:
// The TypeScript types suggest we can pass an object directly to append(),
// but at runtime this results in "[Object object]" because append() expects
// a BodyInit (string, Blob, etc.) and JavaScript calls .toString() on objects.
//
// To fix this, you need to use JSON.stringify():
//   await messageStream.append(JSON.stringify(payload) as unknown as { message: string });
//
// But we're NOT doing that here to demonstrate the bug:
// await messageStream.append(payload);
await messageStream.append(
  JSON.stringify(payload) as unknown as { message: string }
);

to:

// BUG DEMONSTRATION:
// The TypeScript types suggest we can pass an object directly to append(),
// but at runtime this results in "[Object object]" because append() expects
// a BodyInit (string, Blob, etc.) and JavaScript calls .toString() on objects.
//
// To fix this, you need to use JSON.stringify():
//   await messageStream.append(JSON.stringify(payload) as unknown as { message: string });
//
// But we're NOT doing that here to demonstrate the bug:
await messageStream.append(payload);
// await messageStream.append(
//   JSON.stringify(payload) as unknown as { message: string }
// );

5. Reload the localhost:3000 page and observe the stream works as expected and displays the messages.

Project Structure

├── apps/
│   ├── frontend/          # React + Vite (port 3000)
│   ├── backend/           # Express server (port 4000)
│   └── trigger/           # Trigger.dev tasks
├── package.json
├── pnpm-workspace.yaml
└── turbo.json

Package Versions

• @trigger.dev/sdk: ^4.3.1
• @trigger.dev/build: ^4.3.1
• trigger.dev: ^4.3.1