To activate development mode, set development: true.

Bun.serve({
  development: true, 
  fetch(req) {
    throw new Error("woops!");
  },
});

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

error callback

To handle server-side errors, implement an error handler. This function should return a Response to serve to the client when an error occurs. This response will supersede Bun's default error page in development mode.

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