Inertia - what to return to the form helper?

I have a large form.

I have split it into several smaller forms which are shown and hidden inside a page depending on what step a user is on.

When one form is completed the user clicks next and the form is validated using a form request, if successful, the step is increased showing the next form.

I have defined these validation endpoints like this:

Route::post('/products/validate-step-one', fn StepOneRequest $request) => ?????);

With the idea being, they are hit, and then the from request does the validation and kicks back errors, but if the form request passes, I do not know what to return.

I do not want a redirect, as I want to stay on the same page.

Route::post('/products/validate-step-one', fn StepOneRequest $request) => false);

The above works, but feels wrong.