x402 payment notes

Paid tools are metered per call with x402 v2 on X Layer (eip155:196), settled in USD₮0 via the OKX facilitator. Assay needs no account or API key; the buyer still needs a compatible wallet to authorize a paid replay.

The flow

1. Negotiate for free — initialize, tools/list and notifications are POSTs and complete without a payment challenge. This lets the client establish MCP and choose a service before any billing begins. A GET to /mcp instead returns the standard 402 challenge: /mcp is the registered marketplace endpoint for the 0.05 USDT ATS scan, and a buyer's client validates an endpoint by probing it, so a 200 there would read as "not an x402 service" and refuse the purchase. Free machine-readable discovery lives at /.well-known/assay.json.
2. POST /mcp · tools/call — invoke a paid tool with plain JSON-RPC.
3. HTTP 402 — the PAYMENT-REQUIRED header carries a base64 JSON challenge: x402Version: 2, an accepts entry using scheme exact, network eip155:196, the USD₮0 asset 0x779ded0c9e1022225f8e0630b35a9b54be713736, atomic amount, and pay-to address.
4. Sign — the client signs an EIP-3009 exact-scheme authorization for the amount and retries the same request with the PAYMENT-SIG header (also accepted: X-PAYMENT, PAYMENT-SIGNATURE).
5. 200 + PAYMENT-RESPONSE — the facilitator verifies and settles on-chain; the response carries the settlement proof and your result.

Marketplace resources

The generic /mcp endpoint must keep negotiation free. Assay therefore publishes concrete resources such as /x402/asy_story_bank and /x402/asy_interview_prep for direct marketplace integration. An unpaid GET or POST to one of those paid resources returns its own 402 immediately; the paid replay returns the service result directly. Promotion and freelancer resources use /x402/asy_promotion_dossier and /x402/asy_freelancer_proof_pack, which server-bind the correct asy_create_dossier_job variant. Free job status, job result, and seal verification resources return 200 without x402.

The complete live route map is published under marketplaceResources at /.well-known/assay.json.

Intake: tolerant input, and no charge for a request we cannot serve

Two rules protect a buyer who is calling Assay for the first time.

Synonyms are accepted. The published schema is canonical, but the obvious guesses are mapped onto it before anything else looks at the request: resume / cv / documentText → resumeText, jobDescription / jobPosting / role → jd, a single claims string → the array the schema declares, a base64 data: URI → raw base64, type: "promotion review" → variant: "promotion". A canonical key you supply always wins over a synonym, and a chat-style message (“can you scan my resume?”) is never mistaken for the résumé itself. Marketplace service names work wherever a tool name or route slug is expected — /x402/ats-resume-scan and tools/call { name: "ATS Resume Scan" } both resolve.

Preflight runs before settlement. If a request cannot produce the advertised capability — an ATS scan with no résumé, a cover letter with no evidence, a dossier job with an attachment that cannot decode — Assay answers with 400 invalid_request naming the missing input, the keys that would satisfy it, and a runnable example. No payment is taken: the check runs ahead of the x402 gate, so charged: false is a fact about the rail, not a promise. An unpaid probe of a paid resource still receives the standard 402 first, so marketplace validators see the challenge they expect.

GET /x402/:service/schema (free, no payment) publishes one service's input contract — arguments, price, and a working example — and GET /x402 lists them all.

Guarantees

• PolicyGate before payment. A request Assay refuses on policy is refused politely — and never charged.
• A refusal never costs money. Assay refuses to write what it cannot trace; that refusal is the product, so it is delivered before the paywall, not after it.
• Body-bound idempotent recovery. A settled call is cached against its Idempotency-Key (or the payment signature hash). A retry with the same key and canonical arguments recovers the original result even if the payment header was lost. Reusing the key for different input returns 409 idempotency_conflict — never another charge and never another person’s result.
• Free means free. asy_verify, asy_job_status, and asy_job_result are never gated.
• No charge before tool invocation. GET, malformed transport requests, initialize, tools/list, and notifications never enter payment handling. Only tools/call for a paid tool can return a 402.
• Fixed prices. The table in pricing is the table the paywall enforces, published machine-readably at /.well-known/assay.json.

The rail is proven end-to-end on X Layer mainnet with real USD₮0 — settlement tx 0x4babf76c….