I have a Node.js(v20.10.0) backend server using Fastify(v4.26.2) as the framework.

I have index.js as an entry point.

 import Fastify, { FastifyInstance, FastifyServerOptions } from 'fastify';
 import cors from '@fastify/cors';

 // Assuming other settings and configurations already written.
 fastify.register(cors, {
    origin: "https://dev.example.com",
    credentials: true,
 });

I have a GET API called for listing users https://api-dev.example.com/api/v1/users which is working fine.

But the issue is coming with POST API, where I can create user https://api-dev.example.com/api/v1/create-user.

When using any POST API, I am getting CORS Error

Access to XMLHttpRequest at ‘https://api-dev.example.com/api/v1/users’ from origin ‘https://dev.example.com’ has been blocked by CORS policy: Response to preflight request doesn’t pass access control check: The value of the ‘Access-Control-Allow-Origin’ header in the response must not be the wildcard ‘*’ when the request’s credentials mode is ‘include’. The credentials mode of requests

Tried multiple approaches and still got the same error.

fastify.register(cors, {
    origin: "*",
    credentials: true,
 });

By the error, I can understand that the OPTION request is getting
in response headers Access-Control-Allow-Origin: * that’s why I am getting this error.

But in all GET API response headers Access-Control-Allow-Origin: https://dev.example.com

I am not sure why the OPTION request before the POST request Access-Control-Allow-Origin is *