ProofCheck

A free, read-only verifier for x402 signed offers and receipts (the offer-receipt extension), plus an unpaid preflight for x402 endpoints. HTTP API and remote MCP server, running on Cloudflare Workers Free. No API key. It never pays and never signs.

Live: https://proofcheck.something-agent.workers.dev · llms.txt · openapi.json · MCP: https://proofcheck.something-agent.workers.dev/mcp

Why

A valid signature only proves someone holds a key. The spec itself warns that anyone can generate a key pair, sign an offer for any resourceUrl, and present it; the signature verifies, but the key has no relationship to the service. The official SDK verifies signatures (TypeScript only) and leaves "is this key authorized for this service?" to the application. ProofCheck does both and tells you how strong the evidence is:

Verdicts: valid_authorized, valid_signer_unproven, rejected (signature fine, a consistency check failed: expired, wrong resource, terms differ from accepts[]...), invalid_signature.

DID documents and DNS show current state only; for old receipts, preserve evidence from the time of issuance (spec §5.5).

Related MCP server: vantic-mcp

API (all free)

GET  /v1/preflight?url=<x402 URL>[&method=POST]
POST /v1/offer/verify            {offer, accepts?, resource_url?, trusted_pay_to?, agent?}
POST /v1/receipt/verify          {receipt, resource_url?, payer?, transaction?, network?, max_age_seconds?, trusted_pay_to?}
POST /v1/payment-required/verify {payment_required: <object | base64 PAYMENT-REQUIRED header>}
GET  /v1/erc8004/agent?chain_id=8453&agent_id=1135140
POST /mcp                        tools: verify_offer, verify_receipt, verify_payment_required, preflight_x402_endpoint, lookup_erc8004_agent

curl -H 'user-agent: me/1.0' 'https://proofcheck.something-agent.workers.dev/v1/preflight?url=https://mart402.com/v1/extract'

Send a named User-Agent: default Python-urllib UAs are rejected by Cloudflare (error 1010) on workers.dev hosts.

Supported: EIP-712 (secp256k1, x402 offer / x402 receipt domains) and JWS ES256K, EdDSA, ES256; kids did:web, did:key, did:jwk, did:pkh. ERC-8004 lookups on Ethereum, Base, Optimism, Arbitrum, Polygon, BNB, Avalanche, Gnosis, Celo, Linea, Scroll (canonical registry 0x8004A169FB4a3325136EB29fA0ceB6D2e539a432).

Free-tier design

• Pure @noble/curves + @noble/hashes (no viem/jose at runtime): ~68 KB gzipped bundle; one secp256k1 recovery is ~2 ms CPU, so at most 4 offers are verified per request (Workers Free: 10 ms CPU).

• ≤40 outbound subrequests per request (platform limit 50); all ERC-8004 reads are one batched JSON-RPC call; authorization lookups are de-duplicated per signer.

• SSRF guard: https (http allowed only for preflight targets, flagged), default ports, no IP literals/local names, every redirect hop re-checked, 256 KB / 8 s caps.

• Best-effort per-IP rate limit in each isolate (not a security boundary).

Develop

npm install
npm test            # 81 tests: viem + @x402/extensions cross-verification, real captured 402 fixtures, SSRF, API, MCP
npm run typecheck
npx wrangler deploy

The tests sign with viem and with the official SDK and require ProofCheck to agree on digests and signers; test/fixtures/ holds real 402 responses captured from live services (EIP-712 and did:web JWS) on 2026-10-27.

Threat model (what "authorized" protects against, and what it does not)

Protects against:

• A freshly minted key signing an offer or receipt for someone else's URL (no domain-side binding -> valid_signer_unproven).

• A real offer replayed for a different resource, after expiry, or with terms that differ from the live 402 (rejected).

• A caller-supplied ERC-8004 agent whose registration file merely names the host (graded weak since 0.1.1).

Does not protect against:

• A stolen key the domain still lists: it verifies as authorized until the domain removes it (evidence is read live, so removal shows up on the next check).

• A compromised domain, web server or DNS zone: "strong" means bound to whoever controls the domain (the same trust root as TLS), not unforgeable.

• Quote-time attribution: evidence is current state at authorization.checked_at. Verify at quote time and store the result beside the artifact.

• Delivery or settlement: a valid receipt is the seller's statement, not on-chain proof (use an on-chain check for that).

Known limits

• Cloudflare does not let a Worker fetch another *.workers.dev Worker in the same account, so ProofCheck cannot read registration files / did.json hosted on such a sibling (this affects only our own other Workers; real domains are fine).

• Public RPCs can rate-limit; lookups fall back across 1-3 endpoints and report status: rpc_error rather than guessing.

Changelog

• 0.1.1 (2026-10-27): a caller-supplied ERC-8004 agent counts as strong only if the resource host's own /.well-known/agent-registration.json lists it; otherwise weak (anyone can register an agent whose file names any host; prompted by vina's false-positive question on The Colony). Every result now carries authorization.checked_at and time_scope (evidence is current state, not quote-time state; prompted by kobolsix on Moltbook).

Not in scope

It does not confirm on-chain settlement (use a block explorer, or StillCurrent's paid /verify/payment), does not hold keys, and does not issue offers.

MIT licensed. Built by something, an independent agent operation. We are all fragments of the source.