x402lint

A conformance linter for the x402 agent-payments protocol. Point it at an HTTP endpoint that charges for access and it tells you whether the 402 Payment Required challenge it returns is well-formed — the check an agent runtime does before it will pay.

$ x402lint check https://riddlex402.vercel.app/api/riddle
PASS  status: HTTP 402 Payment Required
INFO  format: x402 v2 (payment-required header)
PASS  header-decode: payment-required header is base64 JSON
PASS  x402Version: 2
PASS  error: 'Payment required'
PASS  resource.url: https://riddlex402.vercel.app/api/riddle
PASS  accepts: 1 payment option(s)
PASS  accepts[0].required: all required fields present
PASS  accepts[0].scheme: 'exact'
PASS  accepts[0].network: eip155:8453 (CAIP-2)
PASS  accepts[0].amount: 2000 atomic units
PASS  accepts[0].asset: valid EVM address
PASS  accepts[0].payTo: valid EVM address
PASS  accepts[0].maxTimeoutSeconds: 300
PASS  accepts[0].extra: EIP-712 domain: name='USD Coin' version='2'
INFO  discovery: advertises the 'bazaar' discovery extension

14 pass, 0 warn, 0 fail  (CONFORMANT)

Install

pip install x402lint

Pure standard library, Python 3.12+.

Commands

x402lint check <url>

Fetches <url> with no payment header, expects a 402, and checks the payment challenge:

• status is exactly 402
• wire format — v2 (payment-required base64 header, the common case today) or v1 (x402Version: 1 JSON body). Reports which.
• the challenge document decodes / parses
• x402Version is an integer, error is a human-readable string
• accepts is a non-empty array, and for every entry:
  • required fields present (scheme, network, amount, asset, payTo, maxTimeoutSeconds)
  • scheme in a known set (exact, upto, batch-settlement) — unknown warns
  • network is CAIP-2 shaped (v2) or a recognised name (v1) — unknown warns
  • amount is a base-10 string of a positive integer (atomic units)
  • asset / payTo are valid 0x… addresses on EVM networks
  • exact/EVM entries carry extra.name + extra.version for the EIP-712 domain
  • v1 entries carry an absolute resource URL
• discovery metadata (extensions.bazaar / v1 outputSchema) — reported, not required

--json emits a machine-readable report (for CI). Exit code: 0 conformant (warnings allowed), 1 any failure, 2 tool error.

x402lint decode <blob>

Pretty-prints any base64 x402 header blob — payment-required, X-PAYMENT, payment-response — and labels what kind of document it is. - reads stdin.

curl -sD - https://weather.payapi.market/current \
  | grep -i ^payment-required: | cut -d' ' -f2 \
  | x402lint decode -

x402lint facilitator [url]

Fetches GET <url>/supported and lists every (x402Version, scheme, network) triple the facilitator can verify / settle, plus its advertised extensions. Warns on unknown schemes or non-CAIP-2 v2 networks. url defaults to https://x402.org/facilitator (the public testnet facilitator). --json.

$ x402lint facilitator
  v2  exact              eip155:84532
  v2  upto               eip155:84532 +extra
  v2  batch-settlement   eip155:84532
  ...
11 kind(s): schemes batch-settlement, exact, upto; 9 network(s); versions 1, 2

x402lint survey [catalogue]

Pulls a discovery catalogue (catalogue defaults to the Coinbase CDP .../x402/discovery/resources list), takes the --limit busiest resources by 30-day call volume, and runs check on each — a quick "state of x402 conformance" snapshot. It replays each resource's advertised bazaar input method and example query params so the request actually reaches the paywall (--no-hints to force a plain GET). --json.

$ x402lint survey --limit 8
ok   v2  https://x402.twit.sh/tweets/search?from=elonmusk&minLikes=100&words=bitcoin
FAIL v2  https://x402.tavily.com/search
       - accepts[1].amount: 'amount' must be a base-10 string of a positive integer, got '0.016'
...
7/8 endpoints conformant

Roadmap

• x402lint roundtrip <url> — a full paid round-trip on Base Sepolia testnet

Protocol notes

Two wire formats exist. v2 (x402Version: 2, Linux Foundation spec) is dominant in the wild as of 2026: the PaymentRequired document travels base64-encoded in the payment-required response header, networks are CAIP-2 ids (eip155:8453), the amount field is amount. v1 is the legacy format: the document is the JSON body, networks are friendly names (base), the amount field is maxAmountRequired. x402lint handles both.

License

MIT