Laravel Inertia and unauthenticated API routes

I'm banging my head against a problem that I don't really understand. I've got an out of the box Jetstream and Inertia setup running through Herd. I'm trying to implement chunked uploads for large files. My web.php has the route for the ui, returning an Inertia render for the upload page. Then I have a api.php route for the chunked-upload route.

If I wrap the api route in authentication (sanctum), it consistently says I'm not authenticated and I'm not even making it past the route to the controller.

What am I missing about API calls and authentication in Laravel with Inertia? Does anybody have any suggestions or help? I need authentication for the route, and I don't understand what I'm doing wrong.