Synthesia callbackId vs Sume Idempotency-Key for per-recipient videos

Synthesia's create-video request accepts a callbackId string whose documented use is linking a video to your request, and the reference suggests a customer email for personalization. The Sume pages I checked have no field of that name on Format runs or video jobs (some jobs, such as music, take a caller metadata field; check the model's page). To tie a rendered video to one recipient on Sume, send a stable Idempotency-Key per recipient and keep the mapping from job or run id to recipient in your own table. Webhooks repeat request_id, which is what you dedupe on.

Synthesia details come from its create-video reference and API overview, read 2026-10-04.

What callbackId does

The field is an opaque string you choose. The reference positions it as the link between a video and the request that made it, so a completion notification can be matched to a row without a lookup by video id. The page does not describe length limits or uniqueness rules, so treat it as a plain label.

What Sume offers for the same job

Sume splits the work into three handles. The Idempotency-Key header on a create call makes a retry safe: replaying a spent key returns the original acceptance rather than a second paid render. The job or run id that comes back is the durable handle. A per-item webhook URL, on Format runs, delivers one signed terminal event for that item.

For batches, bulk runs take 1 to 100 items with a concurrency of 1 to 16. The queue object has no webhook of its own, so each item carries its own communication.webhook_url, and the batch itself takes one fresh idempotency key per batch, so the per-recipient mapping stays in your own table keyed by each child run id.

A recipient-keyed pattern

Derive the key from something stable that is not the raw email. A hash of campaign, version and recipient id works: the same triple always reuses the same key, and a new script version gets a new one. Write the intent row before you submit, so a crash between submit and save cannot orphan a paid job.

• Key = hash(campaign, version, recipient id).
• Row = key, job or run id, status, media URL.
• On webhook, look up by request_id and ignore a second delivery.
• Never put a plain email in a URL or key.

What to watch

Changing the body under an existing key is a conflict, not a silent overwrite, so change the key when the script changes. Also remember a webhook can arrive twice; the receiver must be idempotent. The run webhooks page lists the delivery rules and the cookbook has receivers in Node and Python.