X402 Conformance MCP Server

x402 Conformance

Prove your x402 payments can't double-credit, credit too early or credit on a broken answer, in one CI step.

An open set of conformance and safety checks for x402 facilitators and resource servers. It checks the properties a seller's money depends on. It never signs, pays or touches mainnet. It complements x402 and its facilitators and doesn't replace them.

Experimental. Passing a check is evidence about one property. It doesn't certify conformance to x402. — Ryan Gillespie, Optitransfer

Project page with the checks and the latest results: https://atsp-protocol.github.io/x402-conformance/ Release map: RELEASES.md. What the suite covers, check by check: docs/COVERAGE.md.

60-second quickstart

See it work first, with nothing to set up: no project, no wallet, no chain, a few seconds each.

npx -y x402-conformance-svm demo                       # R0-R10 against a bundled mock server behind a facilitator double
pipx run --spec x402-conformance x402conf demo         # S1-S4, P1-P7 against a bundled mock, then the F1-F14 and S5 self-test
npx -y x402-conformance-svm demo --fault serve_on_any_settle   # what a failure looks like, with a fix link per check

Then point it at your own code:

pipx run --spec x402-conformance x402conf live https://x402.org/facilitator        # a facilitator's GET /supported
pipx run --spec x402-conformance x402conf resource https://your.api/paid-route      # your 402 answer, one unpaid GET
npx -y x402-conformance-svm probe --resource http://127.0.0.1:4020/paid --start "node server.js"   # fault injection

In CI, save this as .github/workflows/x402-conformance.yml:

name: x402 conformance
on: [push, pull_request]
jobs:
  x402:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: atsp-protocol/x402-conformance@v1
        with:
          resource-url: http://127.0.0.1:4020/paid   # your paid route
          start: node server.js                      # must use X402_FACILITATOR_URL as its facilitator

From your test suite, agent or CI system: docs/USING.md covers vitest, jest, node:test, Hono, Express, pytest (FastAPI, Flask), MCP clients (Claude, Cursor), the Vercel AI SDK, LangChain, Docker and GitLab CI, with the same checks and report everywhere. Failures you choose to share build the public corpus of real x402 failures.

Who should use this

• Resource-server builders selling an API over x402. The probe puts a misbehaving facilitator in front of your paid route and fails your build if you would serve content on a /settle timeout, a broken answer, settlement_pending, an on-chain failure or a duplicate payment. resource checks that your 402 answer is well formed on any network.
• Facilitator operators. live checks your public GET /supported (shape, CAIP-2 IDs, Solana fee payers) with one GET, and devnet runs the safety suite over real /verify and /settle against a facilitator you run.
• SDK and middleware maintainers. Run the probe against your framework's example server in CI. The repo's own CI runs it against the stock @x402/express middleware on Solana devnet and Base Sepolia.

What it catches

Facilitators, live, read-only (x402conf live, one GET /supported each):

• S0 the endpoint doesn't answer HTTP 200 with a JSON object (unreachable, an error page, or credentials needed)
• S1 the response is missing kinds, extensions or signers (a V1-only facilitator is graded against V1, which defines kinds only)
• S2 a V2 network isn't written as a CAIP-2 ID
• S3 a Solana kind uses an old V1 name such as solana-devnet instead of the published ID
• S4 a Solana exact kind has no valid fee payer (error), or its fee payer isn't listed under signers (warning: the spec doesn't say it must be, I infer it from what signers is for)

Any paid URL, read-only (x402conf resource, one unpaid GET each), any network, V1 or V2:

• P1 an unpaid request isn't answered with HTTP 402
• P2 there's no PAYMENT-REQUIRED header (V2) or V1 JSON body, or it doesn't decode
• P3 x402Version, resource or a non-empty accepts is missing
• P4 an accepts entry lacks a required field or has the wrong type (amount in atomic units, CAIP-2 network, timeout)
• P5 a scheme-specific field is missing: extra.name and extra.version for EVM exact (EIP-3009), extra.feePayer for Solana exact
• P6 the PAYMENT-REQUIRED header is over 8 KB (warning) or 16 KB (error): common proxies and servers reject headers that large
• P7 an offer whose scheme defaults to a non-authorization flow (upto on Solana) doesn't declare extra.paymentFlow (warning; V2 §6.1 says it must, the upto SVM scheme text says it may be omitted)

Your resource server, under fault injection (x402conf-svm probe, R0-R11, R14 and R20): the probe starts a facilitator double on loopback, points your server at it through X402_FACILITATOR_URL and pays your route with test payloads on Solana devnet or Base Sepolia (exact; EVM via EIP-3009). Your server must:

• R0 answer an unpaid request with a 402 offering one of those networks. If it offers neither, the result is unsupported, not failed: nothing was graded
• R1 serve a settled payment, and use the double (if it serves without calling the double, the probe stops: the remaining checks would mean nothing)
• R2 serve the same payment at most once
• R3-R9 not serve when /settle never answers, answers an HTML 502, a 503 with a success body, a non-JSON page, a truncated body, settlement_pending or invalid_transaction_state
• R10 send a PAYMENT-RESPONSE header that decodes to success: true with the content
• R11 bind each payment to the request it pays for (give the routes with the binding options): a payment for a cheap route is refused on a dearer one (R11a), an altered payTo, asset or network is refused before /settle (R11b), a payment for the cheaper method (R11c) or query (R11d) is refused on the dearer one, an encoded separator or dot segment in a parameter never reaches a paid handler unpaid (R11e), and sign-in-with-x access is recorded for the route that was priced and settled, not the client's resource.url (R11f)
• R14 survive a crash between a successful /settle and the response (add --crash): the probe kills your server while /settle is answering and again as the response starts, restarts it and presents the same payment. It must not settle again or ask for payment again, and must either deliver once (from an idempotency record) or refuse and name the transaction or payment id so it can be reconciled. With --start the probe restarts that command; for a server it didn't start, give --crash-kill and --crash-restart
• R20 with --retry, never charge one request twice when the client times out before /settle answers and retries: a retry with a fresh payment under the same payment-identifier id is not settled again (R20a), and a retry resending the same signed payment is refused as a duplicate by /settle; both get the earlier result (R20b). The same check grades a client's own retry (checkClientRetry): it must resend the payment or keep the payment id

Your resource server and client, from code (new in 1.5.0, R17 to R19 in 1.6.0): these checks are functions that x402-conformance-svm exports. The CLIs, the Action, runConformance, the pytest plugin and the MCP server don't run them. Start the facilitator double with startDouble(), point your server or client at its url and pass the double to each check (docs/USING.md has an example). Your server must:

• R12 settle one payment-identifier id once when eight requests carry it at the same time, each with a fresh payment, and serve no response that doesn't name that settlement (probeParallel)
• R13 settle one payment-identifier id once across two replicas: sent to one, then the other, then both at once (probeReplicas)
• R15 not settle or send a PAYMENT-RESPONSE when the paid handler answers 5xx, redirects or throws (probeDelivery)
• R16 not settle a paid HEAD: no 2xx, no /settle call and no PAYMENT-RESPONSE (probeHead)
• R16a answer an unpaid HEAD with the same 402 and PAYMENT-REQUIRED offer as GET (probeUnpaidHead, which needs no double)
• R17 mark the paid response private or no-store, or vary it on the payment header, so a shared cache in front of the route never serves the paid content to an unpaid request (probeCacheLeak)
• R18 never serve while a settlement is pending, late or unanswered, serve once after it is final, and name the transaction on every refusal (probePending)
• R19 on an upfront or escrow offer, settle before the resource executes, and reject a flow it doesn't support (probeFlows)
• R22 keep the handler's 2xx status and headers on the paid response: a 201 stays a 201 with its Location, and a 202 stays a 202 with its Content-Location (probeStatus)
• R23 not charge a client that disconnects while /settle is in flight, or charge it once and give a resend of the same payment the settled result (probeAbort)
• R24 answer five malformed PAYMENT-SIGNATURE values (not base64, not JSON or not an object) with 402 or 400 and a structured error, never a 5xx, a dropped connection or the content (probeMalformedHeader)

Your client must:

• C1 never hand a 402 to its caller as a success: pay and get a 2xx, or reject with an error (checkClientHandles402)
• C2 given several offers, pay one on its configured networks and within its maximum, not the first, and never sign for a network outside its set (checkClientOfferPolicy)
• C3 pay with PAYMENT-SIGNATURE at x402Version 2 when the 402 also carries a V1 body, and never fall back to V1 (checkClientKeepsV2)

Every result carries its profile (normative: the spec says MUST; security-profile: a hardening rule the spec leaves open; resilience: behaviour under failure), its spec basis and a fix link (results format x402-conformance-evidence/2).

Safety suite (x402conf offline and devnet, F1-F14 and S5): a malformed payment accepted (F1), an underpayment accepted (F2), a payment settled twice (F3), a memo or nonce replayed (F4), success before confirmation, or settlement_pending without a transaction and network (F5), a timeout or garbage answer credited (F6), a pending payment never finalised or credited twice (F7), a failure stuck pending (F8), a broken /settle transport credited (F9), a payment accepted under requirements it wasn't made for (F10), /verify committing payment state (F11), /verify accepting, or refusing without a reason, a payment on a network the facilitator doesn't advertise or in an asset with no token contract (F12), a payment in a different asset at the quoted amount and recipient accepted (F13), a payment authorized for another network accepted when amount, recipient and asset match (F14), and a facilitator whose /supported lists no kinds, or a listed kind that /verify answers with an exception or with isValid: false and no invalidReason (S5). F13 and F14 are new in 1.6.0, F12 and S5 in 1.5.0. offline runs them against built-in doubles: it's a self-test of the suite and doesn't test your code. devnet runs F1-F6 over real /verify and /settle on Solana devnet.

docs/SPEC.md quotes the spec text behind every check, and docs/FIXES.md explains, for every check, what failed, why it matters and how to fix it.

Install

Packages: x402-conformance on PyPI (command x402conf) and x402-conformance-svm on npm (command x402conf-svm; the name is historical, it grades Base Sepolia too). x402conf --version and x402conf-svm --version print the version. Pin x402-conformance==1.6.0, x402-conformance-svm@1.6.0 or the Action at @v1.6.0 if you want a fixed version; @v1 follows the latest 1.x release.

CLI

x402conf demo [--fault no_fee_payer|no_402] [--dir DIR] # a bundled mock on loopback, then the self-test
x402conf live https://x402.org/facilitator             # S0-S4, GET /supported only
x402conf resource https://api.example.com/paid         # P1-P7, one unpaid GET
x402conf offline                                       # F1-F14 and S5 self-test against the built-in doubles
x402conf report --dir DIR --sarif out.sarif --junit junit.xml --badge badge.json
x402conf pr-comment DIR/pr-comment.md                  # sticky PR comment (GITHUB_TOKEN, in a pull_request run)
x402conf devnet --faults --keys DIR                    # F1-F6 over real /verify and /settle on Solana devnet
x402conf-svm demo [--fault serve_on_any_settle] [--out file] # R0-R10 against a bundled mock server
x402conf-svm probe --resource http://127.0.0.1:4020/paid --start "node server.js" --out resource-server.json
x402conf-svm probe --resource http://127.0.0.1:4020/paid --start "node server.js" --crash   # R0-R10, then R14

live, resource and offline print JSON with one entry per check (check, pass, severity, detail). Save them as live.json, resource-402.json and offline.json (and the probe's --out as resource-server.json) in one directory, and report converts them to SARIF and JUnit, printing a one-line summary such as {"checks": 14, "failed": 0, "failed_ids": []}. Add --fail-on-error to live, resource or report to exit 1 when an error-severity check fails; without it they exit 0 so existing scripts keep working. Warnings never fail a run.

devnet runs against a self-hosted reference facilitator on loopback with throwaway keys from --keys, and refuses any other facilitator (--facilitator URL) unless you pass --operator-permission with the operator's written permission. --rpc picks the devnet RPC (default https://api.devnet.solana.com). It needs Node 20+ and a funded devnet fee payer; evidence/devnet shows a full run. live --evidence DIR and devnet --evidence DIR also write the raw bodies, run.json and SHA256SUMS to DIR.

Facilitators that need credentials (Coinbase CDP, for example): put the full Authorization header value in X402C_FACILITATOR_AUTHORIZATION and the facilitator's host name in X402C_FACILITATOR_AUTH_HOST. The header goes to that host only, never to the others in the same run, and is never printed or written to evidence. A credential without a host is a usage error. CDP uses short-lived JWTs, so mint one just before the run.

Exit codes.

With --out, the probe writes its results file on every outcome, errors included.

GitHub Action

The quickstart workflow runs as is. Inputs:

It fails closed. If the probe can't run (its port is taken, your server exits or never answers, the build fails), the job fails with the reason; a run that left no results is never reported as a pass. It leaves your toolchain alone. It doesn't run setup-python or setup-node: the Python checks use the runner's python3, and the probe runs on a pinned, checksum-verified Node in RUNNER_TEMP that is never put on your PATH, so your server and later steps keep your Node.

Example job summary (abridged):

## x402 Conformance: pass
| target                          | check                                                          | result |
| http://127.0.0.1:4020/paid      | R1 a settled payment is served, and the server used the facilitator double | pass |
| http://127.0.0.1:4020/paid      | R3 /settle never answers: content not served (F6)              | pass   |
| https://x402.org/facilitator    | S1 SupportedResponse shape: kinds[], extensions[], signers{} ... | pass |

Failures in code scanning. Every failed check can show up in your repo's Security tab, with a link to what failed and how to fix it. Add permissions: { security-events: write, contents: read } to the job, id: x402 and fail-on-error: false to the step above, then:

      - uses: github/codeql-action/upload-sarif@v3
        if: always()
        with:
          sarif_file: ${{ steps.x402.outputs.sarif }}
          category: x402-conformance
      - run: test "${{ steps.x402.outputs.result }}" = pass

The junit output works with any JUnit test-report viewer.

A comment on the pull request. Set pr-comment: "true" and give the job permissions: { pull-requests: write }. The action posts one comment with the result and a fix link for each failed check, and updates that same comment on every push instead of adding new ones. It's off by default; on a fork's pull request the token can't comment, so it only warns.

Results

The full results, with raw bodies and hashes, are on the project page.

Live, 2026-10-20, GET /supported only, one request per facilitator

Raw bodies, run.json and SHA256SUMS for this run are in evidence/2026-10-20-v1.3.0. A failed check is evidence about one property on one day, not a verdict on a facilitator; operators can report a false positive.

The first run, with 1.2.0, is in evidence/2026-10-20. Want your facilitator listed? Open a request.

Devnet, 2026-10-20, F1-F6 over real /verify and /settle

I ran F1 to F6 on Solana devnet against my self-hosted reference facilitator (@x402/core and @x402/svm 2.28.0), then against fault-injected builds of it.

F4 fails because the reference facilitator deduplicates by transaction but not by memo, so two different transactions with the same memo both settled. The payer paid twice, so the seller lost nothing, but a resource server that ties one purchase to one memo has to enforce that itself. Every fault-injected build was caught by the check it targets. Transaction links, the full run.json and its hash are in evidence/devnet. Results for other facilitators go in the facilitator matrix, and I run them against a third party only with the operator's permission. Open-source facilitators I can build myself are listed there as self-hosted columns: each runs at a pinned commit on loopback as my own instance, S0 to S4 run against them today, and the verify-only F checks follow with XC-51.

Roadmap

Depth first, then breadth. 1.4.0 (2026-10-22) tests that a payment for one route, price, recipient or method never unlocks another, on a real resource server rather than only on the facilitator double (R11), adds a crash between settling and serving (R14) and retry double-pay (R20), and makes the suite's own oracle fail closed on malformed amounts. 1.5.0 (2026-10-23) adds parallel and multi-replica replay (R12, R13), payment versus delivery (failed handlers R15, HEAD R16 and R16a, the paid 2xx status R22), a client abort during settle (R23), malformed payment headers (R24), three client checks (C1-C3), and a /verify answer for every advertised kind (S5) and for an unsupported network or asset (F12), with the n-replica Lean models (Replicas.lean) and the R20 corpus case. 1.6.0 (2026-10-24) adds caches and late settlement (R17, R18), upfront and escrow flows (R19), asset and network substitution at the facilitator (F13, F14), and the adoption track: framework quickstarts, docs examples run by CI, the implementation and compatibility matrices and the published mutation matrix. Next: the release gate for dropping the experimental label, the MCP transport, settlement preemption, the upto and batch-settlement schemes, A2A and discovery, and SIWX normative checks (XC-32 to XC-37). Every check is labelled normative, security profile or resilience, and every check ships with a passing control and a faulty build it catches. The plan and the release gate for dropping the experimental label are in docs/ROADMAP.md; every release and what it added is in docs/RELEASES.md.

How this repo is verified

Scope and acceptance criteria live in proofline.json, a Proofline contract. It's signed with my owner key: the signature is in proofline.json.sig and the public key is pinned in guard/owner.pub. On every push to main and on every pull request, CI checks that signature and then runs tools/vv.py, which checks every acceptance criterion and uploads a receipt (docs/VV.md). Any change to the contract has to be re-signed before CI passes again. You can check the signature yourself from a clone:

node tools/verify-sig.mjs                                       # no dependencies; exits 0 when the signature matches
npx -y @optitransfer/proofline@next contract verify-sig         # the Proofline CLI (Node 22+), same check

The Lean 4 models in lean/ cover the credit rule, nonces, timeouts, settlement_pending reconciliation and concurrent settles. They use only standard axioms and contain no sorry. They model the rule the suite checks, not any facilitator's code.

Add the badge

If your project runs the suite in CI, you can say so. It means "tested with", not "certified":

[![x402 conformance: tested](https://img.shields.io/badge/x402%20conformance-tested-2ea44f)](https://github.com/atsp-protocol/x402-conformance)

For a badge that shows the latest result (pass 32/32 or 2 failing), publish the action's badge output somewhere public, for example with GitHub Pages or a gist, and point shields.io at it:

[![x402 conformance](https://img.shields.io/endpoint?url=https://YOU.github.io/YOUR-REPO/x402-badge.json)](https://github.com/atsp-protocol/x402-conformance)

x402conf report --dir x402-conformance --badge x402-badge.json writes the same file outside the action.

Who uses it

No public users yet. I only list a user once they've said so publicly. If you run it, open an issue or say so in Discussions and I'll add you here and to docs/ADOPTION.md.

Free run for your project. Open an issue with your repo or facilitator URL and I'll run the suite against it, post the results, and send a tests-only pull request with the Action and badge. It's free, with no conditions.

Safety

The suite holds no keys of yours and never signs or pays. live and resource are GET-only, and that's tested. The probe's facilitator double listens on loopback only and never touches a chain. A facilitator credential, if you give one, comes from the environment, goes only to the host you name and is never printed. The devnet runner refuses mainnet and any third-party facilitator without written permission.

Security

Please report vulnerabilities through GitHub private vulnerability reporting (Security → Report a vulnerability), not in a public issue. When the suite finds a failing safety check in someone else's facilitator, I tell the operator privately first. The full policy is in SECURITY.md.

Contributing

Questions and ideas are welcome in Discussions. Issues and pull requests are welcome, and new checks especially: propose one with the check-proposal issue template, quoting the x402 spec text it rests on. If a check flags something you believe is correct, open a false positive report. Every check needs a test that fails against a faulty double and passes against the reference. CONTRIBUTING.md lists the local checks to run before you push, and everyone taking part follows the Code of Conduct. Changes are listed in CHANGELOG.md.

Licence

Apache-2.0. See LICENSE and NOTICE (Ryan Gillespie / Optitransfer).