Add helpful error message if redirectTo path is bad

What is the improvement or update you wish to see?

It looks like the redirectTo path in the signIn method has to start with a forward slash. I accidentally passed a value without a slash (e.g. dashboard instead of /dashboard) which causes a fatal error in next.js.

The error messages shown to the developer are ambiguous. Either next.js reports an error in the LoginButton UI component which calls await signIn("microsoft-entra-id", { redirectTo }); or there is a "bad configuration" error page. There is no helpful stacktrace that could lead a developer to the issue.

A server error in the runtime that is caused by a string value is really hard to debug. Please add a check and a helpful error message for the signIn method.

Is there any context that might help us understand?

I spent way too long debugging a weird login issue that was caused by a relative path as a fallback value to redirectTo.

Does the docs page already exist? Please link to it.

I haven't found it, but the error should be shown in the Error.message and the error page.

Start at the signIn method and the LoginButton UI component call await signIn("microsoft-entra-id", { redirectTo });, then reproduce the failure with dashboard as the redirectTo value. Done means an invalid relative path produces a helpful Error.message identifying the bad redirectTo value instead of an ambiguous runtime or configuration error.