How to set header for all requests in route group

You can create a middleware for that.

You'll have check and enforce the Accept header so Laravel will output json no matter what..

class WeWantJson
{
    /**
     * We only accept json
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $acceptHeader = $request->header('Accept');
        if ($acceptHeader != 'application/json') {
            return response()->json([], 400);
        }

        return $next($request);
    }
}

And in your App\Http\Kernel you can add the middleware to you api group. Then there's no need to manually add it in the routes/routegroups.

---

Edit:

You could also add a middleware to enforce json no matter what...

class EnforceJson
{
    /**
     * Enforce json
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $request->headers->set('Accept', 'application/json');

        return $next($request);
    }
}