Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

5.6.1

Plugin version

No response

Node.js version

22.14.0

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

doesn't matter

Description

When constructing a Fastify server, one can pass a defaultRoute callback in routerOptions. The callback is defined the same way as a route handler, with FastifyRequest and FastifyReply parameters. However, at runtime, instead of FastifyReply we get a ServerResponse object, so it doesn't have any of the extension methods that Fastify is supposed to add, and some method signatures might just be different from what they really are at runtime.

The following snippet compiles just fine, but crashes with an unhandled rejection when you send a request to an unknown route.

  const app = fastify({
    logger: false,
    routerOptions: {
      defaultRoute: (req, res) => {
        res.statusCode = 404;
        // The following line should not compile, because `res` does not have a `send` method.
        res.send({
          error: {
            code: "ResourceNotFound",
            message: "The requested resource was not found.",
          },
        });
      },
    },
  });

Link to code that reproduces the bug

snippet above

Expected Behavior

I should either get a FastifyReply as the type suggests, or the type should be fixed to indicate that I'm getting a ServerResponse.