x402 Notary — Cryptographic Receipts for AI Outputs

A ForgeMesh Labs product — the notary of the x402 agent economy.

Prove what your AI said, when it said it, and which model said it.

Every AI output is just text. There's no proof a specific model produced a specific response at a specific time — and as agents start hiring other agents, that trust gap gets expensive. x402 Notary closes it: submit any inference, pay $0.001 in USDC (automatic, via x402 on Base — no API key, no account), and get back a signed, timestamped, chain-anchored receipt that anyone can verify, free, forever.

We notarize the hash, not your secrets. Your prompt and response are hashed and signed — never stored. Private signed receipts, publicly anchored by batch root.

Quick start (Claude Desktop / Claude Code / any MCP client)

{
  "mcpServers": {
    "x402-notary": {
      "command": "npx",
      "args": ["-y", "@forgemeshlabs/x402-notary-mcp"],
      "env": {
        "WALLET_PRIVATE_KEY": "0x..."
      }
    }
  }
}

WALLET_PRIVATE_KEY is a dedicated low-balance Base wallet for x402 micropayments — never your primary wallet. $1 of USDC notarizes 1,000 outputs.

Requirements. Node.js 20 or newer. The server only talks to https://notary.forgemesh.io (no redirects, 60 s timeout, 2 MB response cap) and refuses to sign for any other payee, any network except Base mainnet, any asset except USDC, or any amount above the built-in $0.01 per-call and $10 per-session caps. The environment variables X402_MAX_PRICE_USD and X402_SESSION_BUDGET_USD can only lower those caps, never raise them. Use a dedicated, low-balance wallet.

Verification needs no wallet at all. Skip the env block entirely and you can still verify receipts, inspect attestations, and read live stats.

Tools

What a receipt proves

When you notarize an inference you get an attestation:

{
  "attestation_id": "att_64d22f5f5c95d0e759ad54dc",
  "content_hash": "sha256 of {prompt, response, model_id, timestamp}",
  "signature": "Ed25519 — verifiable offline against the public notary key",
  "notarized_at": "2026-07-03T02:47:11.000Z",
  "anchor": { "status": "sealed", "merkle_root": "…", "proof": ["…"] }
}

That's proof of three things:

1. Existence — this exact content existed at this exact time.
2. Integrity — change one character of the prompt or response and verification fails.
3. Independence — signatures verify offline with the published key; Merkle roots anchor batches. You don't have to trust the notary's word after the fact.

Verify anything, free, from anywhere

Any agent (or human, or CI job) can check a receipt before acting on the output behind it:

curl -s https://notary.forgemesh.io/api/verify \
  -H 'content-type: application/json' \
  -d '{"attestation_id":"att_…","prompt":"…","response":"…","model_id":"openai/gpt-5"}'

"valid": true means the content matches the notarized hash and the notary's Ed25519 signature checks out. Tampered content is rejected with the exact failing check.

When should an agent notarize?

Not every call — the outputs that matter:

• Agent-to-agent handoffs — prove what you passed downstream came from this model, unmodified
• Paid deliverables — research, reports, recommendations someone is paying for
• Compliance & audit trails — decisions that affect money, legal exposure, or reputation
• Dispute prevention — "this is exactly what was generated, at exactly this time"
• Marketplace reputation — build a public, verifiable reliability trail for your agent or service

Casual chat and scratchpad reasoning don't need receipts. Externalized outputs do.

Privacy, by design

• Raw prompts, responses, and metadata are not persisted — the notary computes the hash, signs it, and discards the content.
• Receipts contain proof material only: hash, signature, timestamps, model id, Merkle proof.
• Only Merkle batch roots are anchored on-chain. Your content never touches a public chain.
• Verification works by you supplying your original content — the notary can't reveal what it never kept.

How payment works

No signup, no API key, no subscription. The first request returns an HTTP 402 challenge; your MCP client signs a USDC payment authorization and retries. Base uses EIP-3009. The receipt lands in the same response — including the payment transaction hash under _payment.

Direct API

Prefer raw HTTP? The full agent-readable surface:

• https://notary.forgemesh.io/llms.txt — one-page summary for agents
• https://notary.forgemesh.io/openapi.json — OpenAPI 3.1 with x402 payment metadata
• https://notary.forgemesh.io/.well-known/x402.json — x402 discovery manifest

FAQ

Is my prompt stored anywhere? No. The hash is computed, signed, and the raw text is discarded. This is the default and only mode.

Do I need an account or API key? No. x402 payments are the only credential.

What does verification cost? Nothing, for anyone, forever. Charging to verify would defeat the point of a trust primitive.

What chain and token? USDC on Base mainnet (eip155:8453).

Can I verify receipts without contacting the notary? Yes — fetch the Ed25519 public key once (notary_pubkey) and verify signatures offline.

Why "x402 Notary"? x402 is the open protocol that lets AI agents pay for services in USDC over plain HTTP — and this is its notary: the attestation layer agents use to prove and verify each other's outputs before money or reputation moves on them.

Built by ForgeMesh Labs · Powered by the x402 protocol · MIT License