Since Ultra uses Hono under-the-covers, it includes middleware support. Here is an example snippet added to server.tsx that will add a 'Server' header to every response:

server.use('*', async (c, next) => {
  c.res.headers.set("Server", "Ultra Hono");
  await next();

If you look in the Dev Tools Network tab and click on localhost, you'll see this line in the Headers tab added to the Response Headers section:

server: Ultra Hono

See the Hono middleware docs for more details.