Attest a hash

Last updated: 2026-10-10

POST /v1/attest signs a timestamped receipt over a SHA-256 you supply. Price: US$0.002 per call.

Use it to record that you held a given document or output at a given time, for example when you deliver work to another agent. We never see the document, only its hash.

Request

{
  "sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
  "note": "delivery of report v3 to order 77"
}

With an x402 payment you can add "idempotency_key": "order-77-delivery" so a retry gets the same receipt. Leave it out on L402 and trial calls: they have no paying address, so the key is refused there (400).

Response

From the free sample (GET /v1/sample/attest), shortened:

{
  "attestation": {
    "sha256": "e3b0c442…",
    "note_sha256": "…",
    "seq": 1,
    "issued_at": "2026-10-09T14:20:14.636Z",
    "idempotency_key": null,
    "idempotency_key_sha256": null,
    "first_seen": true
  },
  "receipt": "eyJhbGciOiJFZERTQSIs…",
  "tier": "paid"
}

• seq is our running count of attestations.
• idempotency_key and idempotency_key_sha256 are null when no key was sent, as in the sample.
• first_seen is false when the same key returns an earlier receipt.
• Reusing an idempotency_key with a different hash or note gets 409 idempotency_conflict, not charged. The payment is verified first, then released without being taken.
• We keep only the SHA-256 of your key. The receipt carries idempotency_key_sha256, and the live answer also echoes the key you sent.
• If the payment fails to settle (502 settle_failed), the receipt was already made and is kept. A retry with the same key and hash gets it (first_seen: false) and pays once. Its payment block names the payer and price but is not a record that a payment settled: the receipt attests the hash and the time, not the payment. To avoid paying twice when the failed payment did go through, retry with the same payment-identifier (see payments).
• Keys are scoped to the paying address, so two payers can use the same key without affecting each other. Trial calls and L402 calls have no paying address, so they cannot use idempotency_key (400, and an L402 token is not spent).

What it shows

The receipt shows that this SHA-256 was sent to us by the stated time. It does not show who made the document, what it contains, or that it was not made earlier.