The EnakGame client runs at its game_url, another origin than the API, so the browser asks before POST /customer/enakgame/sessions. Idempotency-Key was not in Access-Control-Allow-Headers, so the preflight refused it and a play could not start. X-Idempotency-Key, the older name the backend also reads, is allowed too. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>