agent-receipt-verify

Independent verification of payment- and outcome-proof receipts used by AI agents.

An agent economy has two kinds of receipt. One says who was allowed to act (delegation: UCAN, Authproof, Agent Passport). The other says what was paid for and whether the output matches (settlement and outcome: x402 offer-receipt, INAM countersigned receipts, and the proofs ERC-8004 consumers count).

receipt-commons/receipt-interop covers the first kind. This covers the second.

Use it from an agent: the MCP server

Both checks are also a Model Context Protocol server, so an agent can run them itself instead of a person running a command. It speaks MCP over stdio, protocol version 2025-11-25. The framing, initialize, tools/list and tools/call are implemented directly in src/mcp-server.mjs, with no MCP SDK and no required dependency at all.

Two tools, for the two moments where an agent actually needs an answer.

check_x402_endpoint, before paying. Sends one request and reports which levels of the ladder in CONFORMANCE.md the endpoint reaches. Levels that need a real settlement come back as not measured, never as failed.

link_receipt_to_settlement, after paying. Takes the receipt, the authorization salt and the asset that was paid, resolves the salted commitment against live chain state, and answers linked, not linked, or could not check. Those are three different answers and it never collapses them into two.

Add it to a client:

{
  "mcpServers": {
    "agent-receipt-verify": {
      "command": "npx",
      "args": ["-y", "github:holistis/agent-receipt-verify", "mcp"]
    }
  }
}

No account, no API key, nothing published to npm. If you would rather not run Node from a git ref, there is a Dockerfile:

docker build -t agent-receipt-verify . && docker run -i --rm agent-receipt-verify

The server opens no port and writes nothing to disk. Anything that is not a protocol frame goes to stderr, because stdout belongs to the client. npm test drives the server over real frames in 21 cases, including an unknown tool name, a notification that must not get a reply, and a network it does not know, which has to answer could not check rather than not linked.

ethers is an optional dependency, used for one signature check inside check_x402_endpoint. Without it that single check reports not measured instead of passing, and everything else works unchanged.

Related MCP server: x402-test-mcp

Why independent implementations matter

A receipt signed by a seller is a statement by an interested party. It only becomes evidence when someone who was not the signer can recompute it and reach the same conclusion. So every verifier here is written against the published specification, never against the reference implementation's source, and is tested against the vector files published by the protocol's own authors.

If two implementations that have not seen each other's code agree on the same vectors, that is interop. One implementation agreeing with itself proves nothing.

Status

npm test      # all of the above, no network, no keys

Node built-ins only, apart from keccak256. Only the settlement link below touches the network, and then only a public read-only RPC.

Confirmed from the other side, which matters more than the table: the author of the x402 specification ran their own implementation, which shares no code with this one, against all three live receipts below and reached the same three settlements, starting from the receipt, the salt and the asset as §5.6.4 requires. Their result and ours are in x402#3827.

Linking a receipt to its settlement

A receipt with an empty transaction field can be matched to a payment only by payer, payee, amount and time. Two payments with the same terms in the same block cannot be told apart at all. Section 5.6 of the x402 offer-receipt extension fixes that with a salted commitment to the ERC-3009 nonce: whoever holds the salt can recompute the link to exactly one settlement, and everyone else learns nothing.

Section 5.6.5 is the part that makes this matter in practice. A commitment that nobody has opened is still only a statement by the signer: they can issue two receipts with different salts for one settlement, or a commitment that opens to nothing at all. So a consumer that counts settlements, for a deduplicated or Sybil-resistant count, may count only the commitments that someone has opened.

Here are three payments with identical terms: same payer, same payee, same amount, same resource, minutes apart on Base Sepolia. Without a commitment those receipts could not be told apart. Each one still resolves to its own settlement:

npm run link-example

commitment  : 0x5c35a61a09a566a9c284fad18a70ac2ddfff34d01d2a3ce776ce94e8d5b38ff8
candidates  : 2 nonce(s) consumed by this payer in that window
LINKED      : yes, to exactly one settlement
  tx        : 0xed87b09bfb20a8c8b69908e163fcea6432d193d389640c0465afcb5561dfeb30

commitment  : 0xfecee93e1aec7d6858a218bf324f23e2a4eed07925d8d4542b9662c5026e3d22
candidates  : 3 nonce(s) consumed by this payer in that window
LINKED      : yes, to exactly one settlement
  tx        : 0x2898d0ceb97ddf60af98e19c94fc0adddbdf1e64f9032e62454085e37acfe36b

commitment  : 0xea8dd34b8beac90f021a4e8d893a689c25249f86947770c8e6647c1cde206008
candidates  : 2 nonce(s) consumed by this payer in that window
LINKED      : yes, to exactly one settlement
  tx        : 0x2a9e95d63a5af8210767baa69143b364dad51caffc29a152c77e03bc508d7199

Each window is centred on its own receipt's issuedAt, which is why the candidate counts differ: the middle receipt's window covers all three payments and it still picks out exactly one. That is the ambiguity section 5.6.1 describes, resolved against live state.

Two things in that output move and the verdict does not. The candidate count rises as that payer consumes more nonces, and the upper block number is capped at the current chain head until the head passes it. The transaction each receipt resolves to is fixed.

The three files in examples/ hold exactly what section 5.6.4 says a later verifier has: the receipt, its salt, and the asset that was paid. The nonces and the transaction hashes are deliberately kept out of them. Those come out of AuthorizationUsed(address,bytes32) logs on the token contract, and the tool accepts an answer only when exactly one candidate opens the commitment. Zero matches and more than one match are both reported as not linked, never as a best guess.

Point it at your own receipt with --receipt=<file>, and add --salt= and --asset= if they are not stored alongside it. All three are required: a receipt and a salt without the asset cannot link, which is section 5.6.4's own warning.

Those payments went to a live x402 seller that issues version 2 receipts, and it publishes these cases itself, with the settlement each one should resolve to, at https://api.tokenizen.nl/api/x402/conformance. Point your own implementation at that and compare, or call its paid route and link a receipt you obtained yourself. A fixture cannot tell you whether the salt arrives at the location §5.6.3 names, inside a real settlement envelope; only a paid call can. Buyer and seller are both us, so this shows the chain closes end to end. It does not show that anyone else is using it.

Honest limits

The format checks test the form of a receipt: signatures, content addressing, and whether the hashes match the bytes a counterparty observed. They do not tell you whether an operator is honest, or whether a registry holds a receipt as finalized.

The settlement link does read live chain state, and these are its real edges:

• It searches a window around the receipt's issuedAt, not the whole chain, because the public Base Sepolia RPC caps eth_getLogs at 200 blocks per call (measured, not read in documentation). The window it searched is always printed, so "not found" reads as "not in this window" and never as "does not exist". Widen it with --window=.

  A receipt may be signed before its settlement lands, since the seller holds the nonce first, so the window is centred rather than backward-looking. What that costs, measured against a live receipt, with the reach derived from the measured block time:

  --window	reach around issuedAt	RPC calls	wall clock
  50	1.7 min	1	0.7s
  200	6.7 min	3	0.8s
  400 (default)	13 min	5	1.3s
  1000	33 min	11	1.7s
  2000	67 min	21	2.9s

  Calls grow linearly with the window and not with the payer's history, which is the trade the window buys: an hour of tolerance for 21 requests. Searching a payer's full set of consumed nonces instead needs no issuedAt at all and cannot miss on skew, at a cost that grows with how much that payer has ever spent.

• A refused RPC call raises an error instead of returning an empty result. A search that could not run must never be reported as a search that found nothing.

• Only eip3009 is implemented, because that is the only method the specification currently defines.

• It confirms that a receipt commits to one settlement. It does not confirm that the amount or the payee were the agreed ones; read the matching transaction for that.

• Receipt version 2 is defined in a pull request that is still open, so its preimage can still change. The commit it was built against is pinned in the source and in the status table above.

Provenance and license

Apache-2.0, the same licence both verified projects use. Their published test material is redistributed unmodified with attribution; see NOTICE for the exact commits. Neither project has reviewed or endorsed this repository.

Check your own endpoint, in one command

No install, no account, nothing published to npm:

npx github:holistis/agent-receipt-verify https://your-endpoint.example/paid

It measures levels 0 to 2 of the ladder in CONFORMANCE.md without paying. Levels 3 to 5 need a real settlement and come back as not measured, never as failed. What it deliberately does not measure is listed in that same file.

A badge for an endpoint that was measured

If an edition covered your endpoint, there is a badge for it:

[![x402 receipt](https://api.tokenizen.nl/api/x402/badge?url=https%3A%2F%2Fyour-endpoint.example%2Fpaid)](https://github.com/holistis/agent-receipt-verify/blob/main/CONFORMANCE.md)

The same data as JSON is at /api/x402/badge.json?url=....

Two things about that badge, because both are deliberate. It never measures live: it answers from a dated measurement, so nobody can use that endpoint to probe arbitrary hosts, and the number does not change on every hiccup. And the measurement date is rendered inside the badge, so a badge that has gone stale is visibly old rather than quietly wrong.

An endpoint no edition has covered comes back as measured: false, not as level 0. Those are different claims: one says we looked and found nothing, the other says we never looked.