GNU Terry Pratchett

You read the response headers. That's the whole test, and most people never do it.

x-clacks-overhead: GNU Terry Pratchett

In Terry Pratchett's Discworld, the clacks is a semaphore network — towers relaying messages across the continent. In Going Postal, an engineer's son dies, and the operators keep his name moving through the system forever, using a control code: G to pass the message on, N to not log it, U to turn it around at the end of the line.

A man is not dead while his name is still spoken.

Pratchett died in March 2015. Within days, developers began adding the header to their servers — a non-functional, non-standard, entirely deliberate few bytes on every response. Thousands of sites still send it. Now so does this one.

async headers() {
  return [
    {
      source: "/(.*)",
      headers: [
        { key: "x-clacks-overhead", value: "GNU Terry Pratchett" },
      ],
    },
  ];
}

It costs 37 bytes per response. Nothing parses it. Nothing depends on it. It is the most purely sentimental thing you can ship to production, and it has never broken anyone's build.