SpendPreflight

SpendPreflight

Call preflight_payment before signing an x402 payment. Get allow, hold or block, reasons and a signed decision receipt; enforce the decision within your approved wallet and budget. Preflight combines payee screening, spending rules and cached price/recipient evidence where available. ERC-8478 wallet binding remains a separate verify_payee_binding call.

About to pay? Start with Preflight. Selling via x402? Start with free Lint and monitoring.

Three runnable examples: a guarded x402 client, Claude Code / Cursor MCP setup, and seller CI. The examples run offline by default; no payment or live probe is needed to try them.

MCP core tools appear first: preflight_payment, check_payee, lint_endpoint, check_token, payment_receipt. Free get_service_info groups the complete enabled catalog into Buyers, Sellers, Books and Data APIs. All products and prices remain available in the catalog; solar and FillPoint have their own Data APIs page.

New web and MCP tools

• Draft ERC-8478 Payee Verification: remote MCP verify_payee_binding or POST /v1/payee-binding, 0.01 USDC with the existing shared trial. Signed JSON reports exactly one of nine outcomes at a pinned registry block; not-read is uncharged. Checks direct agent-wallet equality or EIP-712 delegation, including ERC-1271, rotation and expiry. An observation is not payment approval. Base first, plus public Robinhood reads; the Draft may change.

• x402 Security Check: free passive coverage in Lint; an optional full report from a current complete saved Lint snapshot costs 0.10 USDC, with no new probe or shared trial. Replay protection, paid caching and settlement behavior remain unknown without controlled evidence.
• Quote Check: compare a supplied exact quote with cached public evidence; 0.001 USDC for a usable comparison, unknown without charge, no shared trial. No target request or automatic payment approval.
• Seller Index: top three cached public matches free; extended results up to 12 cost 0.01 USDC only if additional matches exist. Missing ranking evidence remains unknown; no crawl or safety certification.

Read the announcement and tool-selection guide. These are operated remote-service tools; this metadata update does not publish a new npm package or enable automatic payments.

Historical Quote Check before signing

createQuoteCheckHook attaches to onBeforePaymentCreation. It compares the selected exact-scheme quote with a dated cached public unpaid quote, never requests the merchant, and aborts on above_expected, payTo_changed, unknown or an error. ok only means the supplied amount is not above the last comparable observation with the same recipient; it is not payment approval or loss prevention. Keep the local guard for allowlists, per-payment limits and its in-process daily budget.

import { createQuoteCheckHook, guard } from 'spendpreflight';

// comparisonFetch is a SEPARATE locally guarded x402 fetch configured by you. // It can pay the comparison API's $0.001 fee; no shared trial or auto-renewal. client.onBeforePaymentCreation(createQuoteCheckHook({ fetch: comparisonFetch })); guard(client, { remote: false, rules: { maxPerPaymentUsd: 0.10, dailyCapUsd: 1 } });

Without an explicitly supplied paid fetch, plain fetch stops at the 402 offer. The same client cannot pay its own hook fee: recursion is rejected before signing. Missing/stale/ambiguous history returns unknown without a charge. No automatic retry, target refresh, signature replay or application of a suggested cap occurs. The informational suggested_max_amount_per_payment uses atomic units for the stated asset/network and expires with the cached observation; it is not a personal budget. Use quoteCheck(input, options) for a standalone comparison, or signed GET /quote-check / MCP quote_check. URL and quote fields are sent transiently to the comparison API; do not use private URLs or query secrets.

SpendPreflight — trust and verification tools for stablecoin payments, built for AI agents and the people who run them.

Product catalog · Trust and privacy

Allow, hold or block every x402 payment before your agent pays.

Maintained by the operator of SpendPreflight, the optional paid screening API linked below.

Drop-in spend guard for the official x402 client (@x402/core, @x402/fetch, @x402/axios). It checks each payment *before anything is signed*:

• Local rules (free): per-payment max, hold threshold, daily cap, network and asset checks (USDC only by default), and domain or payTo allow/blocklists.
• Remote screening (optional): an OFAC sanctions screen of the payee wallet and a new-domain check via the SpendPreflight API, at $0.02 per check over x402, or free on the trial.

A blocked payment is aborted before signing. A hold goes to your onHold callback, or is denied. Every decision is passed to onDecision so you can log it.

npm i spendpreflight @x402/fetch @x402/evm viem

import { x402Client, wrapFetchWithPayment } from "@x402/fetch";

import { ExactEvmScheme } from "@x402/evm/exact/client"; import { privateKeyToAccount } from "viem/accounts"; import { guard } from "spendpreflight";

const account = privateKeyToAccount(process.env.AGENT_KEY as 0x${string}); const client = new x402Client().register("eip155:8453", new ExactEvmScheme(account)); const pay = wrapFetchWithPayment(fetch, client);

guard(client, { rules: { maxPerPaymentUsd: 1, holdAboveUsd: 0.25, dailyCapUsd: 20 }, remote: false, // local rules; separate screening client shown below onHold: async v => askHuman(v), // return true to approve onDecision: v => console.log(v.decision, v.reasons, v.receiptId), });

const res = await pay("https://api.some-paid-service.com/data"); // guarded

Try it without paying

guard(client, { remote: { trial: true } }); // 3 free remote checks per day per IP

Or use local rules only, with no network call:

guard(client, { remote: false });

Framework examples

Copy-paste examples maintained by the operator of SpendPreflight:

• Vercel AI SDK tool: POST a merchant's 402 challenge and rules to /v1/preflight before paying ($0.02 screening fee).
• LangChain JS check_payee tool: screen a wallet/name/domain ($0.01 screening fee).
• OpenAI Agents SDK tool: a typed preflight tool with explicit error propagation ($0.02 screening fee).
• Plain x402 fetch + guard: enforce free local spending rules before signing, with no caller account or API key.

See setup and safety notes. The screening tools never pay the merchant under review. Enforce decisions in the actual payment client; a model selecting a tool alone is not a spending gate. Tests use local mocks and throwaway signers, with no paid calls.

Decisions

The remote check fails safe: if the API is unreachable, the decision is hold. Set remote.onError: "allow" to fail open instead.

Version0.2.0 requires Node20+ and a separate screening payment client, with its own local-only guard, exact payTo/asset/network allowlist and screening budget. A claimed resource hostname never bypasses the merchant guard. Reusing the merchant client recursively fails closed before signing.

Rules

Version 0.2.0 adds these three settings when remote is enabled. They use the public Endpoint Health Index: the price rule compares the current quote with a category median; liveness counts usable unpaid probes; payee changes are compared within the same network and asset. They do not run locally or trigger a crawl, and no merchant endpoint is paid to gather this data.

Missing, ambiguous or stale index data is explicitly reported in verdict.remote.index_evidence. It does not become an invented healthy result or a failed probe; the other spending and sanctions rules still apply. A failed remote request retains the existing fail-safe hold. Cart-only checkout has no endpoint history. The corresponding service rules are deployed. Index coverage is still being populated; inspect index_evidence instead of assuming full-catalog coverage.

The API directly

Any agent or language can call the same checks over HTTP with x402. No account, no API key:

• POST https://api.spendpreflight.com/v1/preflight: send the 402 body plus your rules; get allow, hold or block with a receipt. $0.02.
• POST https://api.spendpreflight.com/v1/check: JSON body with address, domain or name; payee sanctions and domain risk. $0.01.
• The full OpenAPI spec is at https://api.spendpreflight.com/openapi.json, with an llms.txt alongside it.

October22 migration: all paid API execution uses POST JSON. GET/HEAD return discovery challenges only and never settle, even with query parameters or payment headers. Build the POST request and obtain its challenge before signing; do not sign a GET quote for execution. ?trial=1 remains an optional URL flag on POST. The 0.2.1 Scout helpers and current LangChain source examples use this method; Scout production availability is still gated. The core remote preflight guard already used POST. Version0.2.0 is not republished.

Trust

Don't take our word for any of this; each point can be checked:

• The package never signs or moves funds. It only tells your x402 client to abort. Read src/index.ts; it's about 250 lines.
• The remote check fails safe. If the API is unreachable, the decision is hold, not allow.
• The API is read-only and never charges for its own errors. 4xx and 503 responses are not settled. Live status: https://api.spendpreflight.com/status
• Payments are public. They go to a published wallet on Base, so you can audit them on Basescan.
• Minimal data. No request bodies are stored. See https://api.spendpreflight.com/privacy and https://api.spendpreflight.com/terms
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