# Error Handling (/runtime/http/error-handling)

<!-- agent-signals: reading_time_min: 1 · est_tokens: 397 · updated: 2026-09-23 -->
Related: [Server](/runtime/http/server.md), [Routing](/runtime/http/routing.md), [Cookies](/runtime/http/cookies.md), [TLS](/runtime/http/tls.md), [Metrics](/runtime/http/metrics.md)

`Bun.serve()` runs in development mode by default. It is turned off when `NODE_ENV=production` is set or when you pass `development: false`.

```ts title="server.ts" icon="/icons/typescript.svg"
Bun.serve({
  development: false, // [!code ++]
  fetch(req) {
    throw new Error("woops!");
  },
});
```

In development mode, when a request handler throws and no `error` handler returns a response, Bun responds with a built-in error page that includes the error message, stack trace, source code around each frame, and file paths. This is meant for debugging locally.

<Frame>
  ![Bun's built-in 500 page](/_assets/b5db26c01af0ee8edd117976a390f0ed095d7bb1da325b7fd33a5b3f75cb5cd9)
</Frame>

<Warning>
  The development error page sends source code and file paths to whoever made the request. Set `NODE_ENV=production` (or
  `development: false`) when deploying so uncaught errors return a plain `500` instead. The `--production` flag does not
  turn it off: `bun build` and `bun install` have that flag, and `bun run` ignores it.
</Warning>

### `error` callback [#error-callback]

To handle server-side errors, implement an `error` handler. Return a `Response` to serve to the client when an error occurs. In `development` mode, this response replaces Bun's default error page.

```ts
Bun.serve({
  fetch(req) {
    throw new Error("woops!");
  },
  error(error) {
    return new Response(`<pre>${error}\n${error.stack}</pre>`, {
      headers: {
        "Content-Type": "text/html",
      },
    });
  },
});
```

<Info>
  [Learn more about debugging in Bun](/runtime/debugger)
</Info>