FluentValidationSetup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc()
        .SetCompatibilityVersion(CompatibilityVersion.Version_2_2)
        .AddFluentValidation(config =>
            {
                config.RegisterValidatorsFromAssemblyContaining<FooValidator>();
                config.RunDefaultMvcValidationAfterFluentValidationExecutes = false;
            })
        .ConfigureApiBehaviorOptions(options =>
        {
            options.InvalidModelStateResponseFactory = context => new BadRequestObjectResult(context.ModelState.SelectMany(
                modelState => modelState.Value.Errors.Select(error =>
                    new ClientError
                    {
                        Item = modelState.Key,
                        Message = error.ErrorMessage
                    })));
        });
}