Suppress redirect on API URLs in ASP.NET Core

For .net core 2.x here's a fix (based on Edward's answer) :

services.ConfigureApplicationCookie(options =>
        {
            options.Events = new CookieAuthenticationEvents
            {
                OnRedirectToAccessDenied = ReplaceRedirector(HttpStatusCode.Forbidden, context => options.Events.RedirectToAccessDenied(context)),
                OnRedirectToLogin = ReplaceRedirector(HttpStatusCode.Unauthorized, context => options.Events.RedirectToLogin(context))
            };
        });

where ReplaceRedirector:

Func<RedirectContext<CookieAuthenticationOptions>, Task> ReplaceRedirector(HttpStatusCode statusCode, Func<RedirectContext<CookieAuthenticationOptions>, Task> existingRedirector) =>
context =>
{
    if (context.Request.Path.StartsWithSegments("/api"))
    {
        context.Response.StatusCode = (int)statusCode;
        return Task.CompletedTask;
    }
    return existingRedirector(context);
};

Replace the redirect event handler with one that uses the default behavior only if the path is not an API. In Startup.ConfigureServices, add this:

services.ConfigureApplicationCookie(options => {
    options.Events.OnRedirectToAccessDenied = ReplaceRedirector(HttpStatusCode.Forbidden, options.Events.OnRedirectToAccessDenied);
    options.Events.OnRedirectToLogin = ReplaceRedirector(HttpStatusCode.Unauthorized, options.Events.OnRedirectToLogin);
});

Use this helper method to replace the redirect methods:

static Func<RedirectContext<CookieAuthenticationOptions>, Task> ReplaceRedirector(HttpStatusCode statusCode, Func<RedirectContext<CookieAuthenticationOptions>, Task> existingRedirector) =>
    context => {
        if (context.Request.Path.StartsWithSegments("/api")) {
            context.Response.StatusCode = (int)statusCode;
            return Task.CompletedTask;
        }
        return existingRedirector(context);
    };

With this in place, the API controller methods can call Unauthorized() and Forbid() without causing redirects.

Update: The above is for ASP.NET Core 2. The code for ASP.NET Core 1 is different.

Other simple way

 .AddCookie(options =>
            {
                options.AccessDeniedPath = "/Home/401";
                options.Events = new CookieAuthenticationEvents
                {
                    OnRedirectToAccessDenied = context => 
                    {
                        if (context.Request.Path.StartsWithSegments("/api"))
                        {
                            context.Response.StatusCode = (int)(HttpStatusCode.Unauthorized);
                        }
                        return Task.CompletedTask;
                    },
                };
            })