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POST /api/sessions requires an Idempotency-Key header no longer than 200 characters.
  • reuse a key only with the exact same create body
  • a different body with the same key returns 409
  • while the idempotency record is retained, a retry of the same body returns the original create result
  • provider mutation idempotency remains a separate concern
For CI, derive the key from the provider run ID, attempt, job, matrix values, shard, logical test group, and a digest of the exact create body. Hash long identity fields instead of truncating away the part that makes jobs unique. The create key prevents duplicate Archal sandboxes. It does not make GitHub, Slack, Stripe, or another provider write idempotent.