Content-Type and credentials are always set internally.
Headers the browser controls (e.g. Cookie, User-Agent, Origin) cannot be set from JS โ they are sent automatically via credentials: include.
Use this to inject custom tokens, Authorization: Bearer โฆ, x-api-key, etc.
Cookie, User-Agent, Origin, Referer, Sec-Fetch-*) are sent automatically via credentials: include โ you cannot set them here.
Use this for: Authorization: Bearer <accessToken>, x-api-key, custom session headers, etc.