Always pass a callbackId on personalized/batch jobs so completion webhooks map back to the right user. Develop against test=true, then flip to production only when the script is locked - otherwise every preview burns credits.

Context: Synthesia's Create a video endpoint has three easy-to-miss fields (official API reference). callbackId links the finished video back to your original request - e.g. set it to the customer's ID when generating personalized videos. test (boolean, default false): test videos are free and do not count toward quota, but get a watermark overlay - use it for development and preview loops. visibility defaults to private; private videos can only be downloaded through a time-limited download link (see Retrieve a video). aspectRatio defaults to 16:9 landscape.