#TIL 24 - Workaround for tRPC Fastify adapter CORS policy

What

Fix CORS preflight (OPTIONS) requests in tRPC with Fastify adapter.

Why

tRPC Fastify adapter doesn't handle OPTIONS requests by default, causing browser CORS errors.

How

app.register(fp(fastifyTRPCPlugin), {
  prefix: "/trpc",
  trpcOptions: {
    router: appRouter,
    responseMeta({ errors }) {
      // Handle OPTIONS preflight
      if (errors?.[0]?.code === "METHOD_NOT_SUPPORTED") {
        return {
          status: 204,
          headers: {
            "Access-Control-Allow-Headers": "Content-Type",
            "access-control-allow-origin": "*",
            vary: "Origin",
          },
        };
      }
      // Regular CORS headers
      return {
        headers: {
          "Access-Control-Allow-Headers": "Content-Type",
          "access-control-allow-origin": "*",
          vary: "Origin",
        },
      };
    },
  },
});