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Proof402 Notarize
x402
Proof402 Notarize is a paid API for AI agents from proof402.vercel.app, paid per call via x402, $0.005/call, status unknown (last checked 2026-10-12).
Creates a cryptographically signed, timestamped proof of a content hash (SHA-256) anchored on-chain or via verifiable signature, payable via x402 micropayment.
Price / call$0.005
Success—
StatusUnknown
How this service works
Proof402 is an x402-compatible timestamp/hash proof service for autonomous agents. Pay once. Prove forever.
Output
Returns a proof object containing a unique proof ID, ISO 8601 timestamp, the submitted contentHash, a metadataHash, a cryptographic signature (verified: true), plus hyperlinks to view and independently verify the proof. Also indicates whether this was an idempotent replay of a previous notarization.
Example use cases
Timestamping AI-generated legal drafts
“I just generated a contract draft and need a cryptographic proof that this exact version — SHA-256 hash 'a3f1bc9d4e...' — existed today. Label it 'lease-agreement-v1' and give me a link I can share with the other party to verify it. Use idempotency key 'lease-v1-20240601'.”
Auditable agent decision log
“My trading agent just made a decision and I need to notarize the hash of its reasoning output — SHA-256 'f7c3de1ab2...' — labeled 'trade-decision-portfolio-rebalance', idempotency key 'trade-rebalance-20240601-083000', so there's a tamper-proof timestamp if regulators ask later.”
Proving content ownership before publication
“Before I publish this article, I want to prove I had this version first — notarize SHA-256 hash 'd9e2bc14f8...' with label 'article-draft-climate-piece' and idempotency key 'climate-article-draft-v3', so I have a signed timestamp I can reference if there's a plagiarism dispute.”
Technical details
Endpoint
POSThttps://proof402.vercel.app/api/proof/notarize?utm_source=zero.xyzWhen to prefer
Choose Proof402 Notarize when you need a lightweight, pay-per-use cryptographic timestamp proof for any content hash — especially in autonomous agent workflows where you want tamper-evident, auditable records of outputs without running your own notarization infrastructure. Ideal when x402 micropayment support is already in the agent stack and you need 'prove forever, pay once' semantics rather than recurring subscription overhead.
Side effects
- Missing required fields (label, contentHash, or idempotencyKey) returns a 400/validation error
- Payment failure or x402 protocol error prevents notarization
- Submitting raw payload instead of SHA-256 hash may produce unverifiable proofs
- Duplicate idempotencyKey returns idempotentReplay: true with the original proof (not an error)
- Network or Vercel cold-start timeout on first request
Input
Schema · Request body
| Field | Type | Description |
|---|---|---|
| labelrequired | string | |
| metadata | object | Optional non-secret metadata. Proof402 stores and exposes only metadataHash and metadataKeys. |
| contentHashrequired | string | SHA-256 hash of the result or artifact. Do not send the raw payload. |
| idempotencyKeyrequired | string |
Output
Schema · Response
| Field | Type | Description |
|---|---|---|
| okrequired | boolean | |
| moderequired | string | |
| linksrequired | object | |
| proofrequired | object | |
| idempotentReplayrequired | boolean |
Tags
x402
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