How to make Fluent API configuration work with MVC client side validation?

No. Fluent API is just mapping - correct. Data annotations are both mapping and validation - wrong. Data annotations are one of the worst features of EF code first because when used this way they couple persistence with presentation and validation logic.

Advice: don't use EF entities for presentation. Use special view models with data annotations and let your controller prepare view models from entities and vice-versa. Soon or later you will find situations where your validation is not 1:1 with your mapping or where your view needs more or less data than provided in entity type. Use view models and these situations will be handled by them.

I struggled with this for a while today, and this is not strictly client validation as it requires a round trip, but it does allow you to benefit from the Validation summary and messages helpers in the standard template. Within your controller action method you simply wrap your SaveChanges() call in a try - catch and add the resulting Errors to the ModelState as follows:

try {    //This does not pick up fluent validation failures    if (ModelState.IsValid) {        db.Entity.Add(entity);        db.SaveChanges();        //Users want to create loads of my entities without seeing the index...        return RedirectToAction("Create");    }} catch (DbEntityValidationException e) {    //Log errors    foreach (var result in e.EntityValidationErrors) {        foreach(var error in result.ValidationErrors){            ModelState.AddModelError(error.PropertyName, error.ErrorMessage);        }    }}//return to view with current model + validation errors return View(entity)

This would of course require a bit more work if you are saving multiple entities here.

Of course using a View Model objects as Ladislav suggests would be the correct approach, however I have used this to support a test UI requested for downstream systems integration testing ahead of schedule...