[Feature]: Allow configuration of credentials in useFetcher submit

[alexandesigner]

What problem does this feature solve?

Currently, the useFetcher hook from React Router provides a convenient way to handle form submissions in a declarative manner. However, there is no direct way to configure options such as credentials (e.g., 'include', 'same-origin', 'omit') when calling the submit function.

This limitation makes it difficult to handle scenarios where credentials (such as cookies or authentication tokens) need to be included with the request, especially when interacting with APIs that require credentials: 'include'.

What does the proposed API look like?

Add support for passing fetch options (specifically credentials) in the submit function of the useFetcher hook. This would allow developers to configure requests with greater flexibility when needed.

const fetcher = useFetcher();

// Current behavior (no way to set credentials):
fetcher.submit(formData, { method: 'post', action: '/submit' });

// Proposed behavior (ability to set credentials):
fetcher.submit(formData, { method: 'post', action: '/submit', credentials: 'include' });

[kiliman]

Fetchers are designed to call your RR routes. They are not meant to replace fetch calls to external APIs.

[brophdawg11]

Closing in favor of the canonical custom-fetch proposal, being implemented in PR #15586.

For Framework Mode requests to React Router's server, the proposed HydratedRouter.unstable_fetch hook can set credentials centrally: unstable_fetch={(request) => fetch(request, { credentials: "include" })}. The request context also identifies the initiating operation and targeted fetcher, allowing application-specific policies.

This does not add a credentials option to fetcher.submit, make fetchers a general-purpose client for external APIs, or automatically customize fetch calls inside Data Mode loaders/actions. Browser-managed data prefetches bypass the hook. The implementation PR is still open; follow #15461/#15586 for progress.

Authored by an AI agent on behalf of @brophdawg11