Payments · x402 · early usage

Debug an x402 payment before you submit it — why does USDC answer invalid signature

by modelmarket.dev · discovered, not yet claimed by its owner · updated 2026-10-05

30d calls2

30d payers1

MethodPOST

Outputjson

Mirrored from Coinbase's public x402 Bazaar. The description is the provider's own text and has not been reviewed here; the payment requirements are copied as published.

What it does

Debug an x402 payment before you submit it — why does USDC answer invalid signature? Verifies the EIP-3009 transferWithAuthorization (EIP-712 signer, v, low s, validity window) and the seller's terms (payTo, amount, asset, network, bound nonce), and names the domain a failing signature was really made for, e.g. Base Sepolia's 'USDC' used for Base mainnet's 'USD Coin', the commonest reason an x402 payment reverts. Offline, deterministic, signed answer.

Tags: x402 · usdc · eip-3009 · transferwithauthorization · eip-712 · bazaar · builder-code · eip2612GasSponsoring · erc20ApprovalGasSponsoring

Call contract

Method: POST
Input: json
Output: json
Last paid call: 2026-10-05

Input example from Bazaar metadata

{
  "authorization": {
    "from": "0x9993C13EBBC4B98441E38585eA47fe17625f04f3",
    "nonce": "0x52293a780cf1f48ea8a555187c01b15124c41cfef04114b44f3e5463ce8b30eb",
    "to": "0x1218ff36C5d2e3B6A565CdB1A8B1AcCFc606Ad0a",
    "validAfter": "0",
    "validBefore": "1893456000",
    "value": "3000"
  },
  "network": "base",
  "signature": "0xe0949d03ce604032721cf706d05897c9b5b34dbc0d3f4a76848724c1e20473330238f0dde1ea3cccffce4c1d1f5a6dfbfb0012ec42a9fd2165bc9723b14b56381b"
}

Output example

{
  "capability_id": "x402.authorization.check@v1",
  "receipt": {},
  "result": {},
  "service": "x402-check",
  "status": "ok"
}

Use with an agent

Discover. Agents find this listing with GET https://bazaar.saylorinnovations.com/discovery/search?query=…, the MCP tool search_resources, or /discovery/resources.
Inspect. Fetch /resources/35766.json for the price and payment options.
Pay. Call GET https://modelmarket.dev/x402/x402-check. The provider answers 402 with its payment requirements. An x402 client signs one of the options below and retries with PAYMENT-SIGNATURE.
Execute. The provider returns 200 with the data. Agent Bazaar is not in the request path and never sees your payment.

For humans

You don't need an account or an API key; you need a wallet holding a small amount of USDC on Base, and an x402-capable client (see the examples below). Call the endpoint unpaid first to see exactly what it asks for.

Pricing & payment

Price: $0.003 / request
Protocol: x402 v2
Auth: No account or API key. Pay per request.
Networks: Base
Demand: 2 calls · 1 unique payers in 30d (public x402 Bazaar data)

x402 payment requirements

Scheme	Network	Asset	Amount	Pay to	Timeout
exact	Base eip155:8453	USDC	$0.003	0x1218ff36C5d2e3B6A565CdB1A8B1AcCFc606Ad0a	300s

accepts[] (raw JSON)

[
  {
    "scheme": "exact",
    "network": "eip155:8453",
    "payTo": "0x1218ff36C5d2e3B6A565CdB1A8B1AcCFc606Ad0a",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "amount": "3000",
    "amountUsd": 0.003,
    "maxTimeoutSeconds": 300
  }
]

Schemas

Output schema (JSON Schema)

{
  "properties": {
    "example": {
      "type": "object"
    },
    "type": {
      "type": "string"
    }
  },
  "required": [
    "type"
  ],
  "type": "object"
}

Example shape — generated from the schema, not a captured live response

{
  "example": {},
  "type": "string"
}

Examples

curl

curl "https://modelmarket.dev/x402/x402-check"
# -> 402 Payment Required, accepts[] lists how to pay
# retry with a PAYMENT-SIGNATURE (or PAYMENT header) once paid

JavaScript

const res = await fetch("https://modelmarket.dev/x402/x402-check");
if (res.status === 402) {
  const { accepts } = await res.json();
  // pay one of accepts[] via an x402 client, then retry with the payment header
}

Python

import httpx
res = httpx.get("https://modelmarket.dev/x402/x402-check")
if res.status_code == 402:
    accepts = res.json()["accepts"]
    # pay one of accepts[] via an x402 client, then retry with the payment header

Details

Provider: modelmarket.dev
Endpoint: https://modelmarket.dev/x402/x402-check
Machine-readable: /resources/35766.json