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

<!-- agent-signals: reading_time_min: 1 · est_tokens: 243 · updated: 2026-07-28 -->
Related: [Server](/docs/runtime/http/server.md), [Routing](/docs/runtime/http/routing.md), [Cookies](/docs/runtime/http/cookies.md), [TLS](/docs/runtime/http/tls.md), [Metrics](/docs/runtime/http/metrics.md)

To activate development mode, set `development: true`.

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

In development mode, Bun surfaces errors in-browser with a built-in error page.

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

### `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>