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How to pay for Recall Kitchen MCP tools with x402

#mcp #ai #api #x402

Recall Kitchen's MCP server lets AI agents look up government-published product, food, vehicle and drug recalls. Most lookups are free; past the free quota (and for photo search) an agent can pay per call with x402 in USDC on Base. Here's exactly how.

(Disclosure: I build Recall Kitchen.)

Docs: https://recallkitchen.com/docs?utm_source=devto&utm_campaign=x402_howto

Paying per call with x402 (no account)

You don't need an account to keep using the server. A few anonymous searches per IP are free (5 an hour, 15 a day), and photo search is never in that free quota. After the quota, an x402-capable client can pay per call: $0.025 in USDC on Base. When a call needs payment, the server returns a normal JSON-RPC result whose content is an x402 "Payment Required" challenge (HTTP 200, isError: true), not a bare HTTP 402. Your client pays it and retries. The price and pay-to address are published at https://app.recallkitchen.com/.well-known/x402. x402 covers search and photo search only. Account tools like watches and inventory need a free API key instead. The full rules are in the docs and the API Terms.

Endpoint

Which tools need payment

Tool Free anonymous quota x402 / API key
search_product_recalls (and other non-photo search tools) Yes — a few calls per IP (docs/API terms: 5/hour, 15/day) After the IP quota, pay with x402 or send an API key
search_product_recalls_from_image (photo, beta) No — never in the free anonymous quota Always needs an API key or x402

Account tools (watches, inventory, notifications, key management) do not accept x402; use a free API key from signup / Integrations.

What an unpaid call returns

For MCP tool calls that require payment, the server answers HTTP 200 with a JSON-RPC tools/call result whose content is an x402 Payment Required payload (isError: true). That HTTP 200 + JSON-RPC payment challenge is correct for this MCP integration — clients should not expect a bare HTTP 402 on the MCP POST for tool payment.

Live example (image search, unpaid), truncated:

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "isError": true,
    "structuredContent": {
      "error": "Payment Required",
      "x402Version": 2,
      "accepts": [{
        "scheme": "exact",
        "network": "eip155:8453",
        "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
        "amount": "25000",
        "payTo": "0x8E2d8e2850Baf623A8A886DE7c80CDfB83a5cfa3",
        "maxTimeoutSeconds": 60,
        "extra": { "name": "USD Coin", "version": "2" }
      }],
      "resource": {
        "url": "mcp://tool/search_product_recalls_from_image",
        "description": "Photo (beta): identify products in an image URL, data URI, or MCP image content",
        "mimeType": "application/json"
      }
    }
  }
}
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amount is USDC atomic units (6 decimals): "25000" = $0.025. The well-known document publishes the same price as decimal "0.025".

Network, asset, and pay-to wallet

From /.well-known/x402 and the live challenge (they match):

Field Value
Network Base mainnet — eip155:8453
Asset USDC on Base — 0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913
Currency USDC
Price $0.025 per call (unless a live challenge states otherwise)
Pay to 0x8E2d8e2850Baf623A8A886DE7c80CDfB83a5cfa3
Scheme exact
x402 version 2

Do not invent other prices; always prefer the live accepts[] on the challenge.

Steps for a client with an x402 wallet

  1. Connect to https://app.recallkitchen.com/mcp (streamable HTTP / JSON-RPC). Prefer a free API key (X-API-Key: rk_... for Grok, or Authorization: Bearer rk_... for Claude/Cursor) when you have one — that skips x402 for search tools.
  2. Call the tool (search_product_recalls after the free IP quota, or search_product_recalls_from_image any time without a key).
  3. Read the JSON-RPC result. If it is a Payment Required challenge, take accepts[0] (network, asset, amount, payTo).
  4. With a Base USDC wallet / x402 client (rk-mcp Go client uses X402_EVM_PRIVATE_KEY and registers scheme eip155:8453), sign and settle the exact USDC payment to payTo for amount atomic units on Base.
  5. Retry the same tool call with the x402 payment proof attached the way your x402+MCP client library expects (rk-mcp wraps the session with auto-payment via the x402 MCP helper).
  6. On success you get the normal tool result. Crypto micropayments are final (see API Terms).

rk-mcp Go client (reference) 

export X402_EVM_PRIVATE_KEY=0x...   # Base wallet funded with USDC (+ gas)
cd examples/go
go run ./search_product_recalls -query spinach -location Iowa
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Or set RK_API_KEY / RECALL_KITCHEN_API_KEY instead of paying per call.

Sources

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