We’re currently facing an issue with authentication popups using custom FastAPI endpoints.

Our setup involves handling OAuth-based authentication through a popup window. The flow itself works correctly — the user is redirected to the provider’s auth page, signs in successfully, and the callback is supposed to hit our custom FastAPI endpoint to complete the authentication.

However, the problem occurs after the authentication step:

The internal redirect URL isn’t working as expected.
The authentication status remains stuck in a pending state.
It seems like the popup isn’t properly receiving or handling the final redirect back to our app once the token is generated.

We’ve already confirmed that the OAuth provider side is fine, and when we test the same redirect URL without the popup flow, it works correctly. The issue only appears when using the popup-based authentication with our custom FastAPI endpoints.

Has anyone run into a similar issue or found a reliable way to handle popup-based OAuth callbacks and redirects in FastAPI? Any suggestions or best practices for managing this kind of flow would be really helpful.

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