Paid API Pre-flight Check

Check an API's reachability, HTTPS use and HTTP 402 payment response for L402, x402 or MPP. Returns payment details and a proceed, caution or avoid result.

Pay by wallet (x402) paid-api-preflight

Example

Example answer

{
  "url": "https://api.example.com/v1/data",
  "issues": [],
  "server": "nginx",
  "ssl_valid": true,
  "status_code": 402,
  "is_reachable": true,
  "recommendation": "proceed",
  "payment_details": {
    "amount": "1000000",
    "scheme": "exact",
    "network": "base-sepolia",
    "has_pay_to": true,
    "has_accepts": true
  },
  "payment_protocol": "x402",
  "response_time_ms": 340,
  "facilitator_reachable": true,
  "payment_handshake_valid": true
}

Inputs

Field	Type	Required	What it is
`url`	string	Yes	The paid API endpoint URL to check before paying

Output fields

How reliably each field appears in a successful answer.

Field	Type	Appears
`url`	string	Always (present in every successful answer)
`issues`	array	Always (present in every successful answer)
`server`	string or null	Usually (present for most inputs)
`ssl_valid`	boolean	Always (present in every successful answer)
`status_code`	integer	Always (present in every successful answer)
`is_reachable`	boolean	Always (present in every successful answer)
`recommendation`	one of proceed, caution, avoid	Always (present in every successful answer)
`payment_details`	object	Always (present in every successful answer)
`payment_protocol`	one of L402, x402, MPP, unknown	Always (present in every successful answer)
`response_time_ms`	integer	Always (present in every successful answer)
`facilitator_reachable`	boolean or null	Usually (present for most inputs)
`payment_handshake_valid`	boolean	Always (present in every successful answer)

How to call it

The same tool through each route. MCP clients reach it through Strale’s server at https://api.strale.io/mcp; the call below is what the agent sends.

# Needs an API key. max_price_cents caps what this call may cost.
curl -X POST https://api.strale.io/v1/do \
  -H "Authorization: Bearer $STRALE_API_KEY" \
  -H 'Content-Type: application/json' \
  -d '{
    "capability_slug": "paid-api-preflight",
    "inputs": {
      "url": "https://httpbin.org/get"
    },
    "max_price_cents": 2
  }'

{
  "tool": "strale_execute",
  "arguments": {
    "slug": "paid-api-preflight",
    "inputs": {
      "url": "https://httpbin.org/get"
    },
    "max_price_cents": 2
  }
}

import { Strale } from "straleio";

const strale = new Strale({ apiKey: process.env.STRALE_API_KEY! });
const result = await strale.do({
  capability_slug: "paid-api-preflight",
  inputs: {
    "url": "https://httpbin.org/get"
  },
  max_price_cents: 2,
});

import os
from straleio import Strale, DoRequest

strale = Strale(api_key=os.environ["STRALE_API_KEY"])
result = strale.do(DoRequest(
    capability_slug="paid-api-preflight",
    inputs={
        "url": "https://httpbin.org/get",
    },
    max_price_cents=2,
))

# No account. The first request answers 402 with the price in USDC;
# an x402 client pays it from your wallet and repeats the request.
curl -i 'https://api.strale.io/x402/v2/paid-api-preflight?url=https%3A%2F%2Fhttpbin.org%2Fget'

Limitations

Only L402, x402 and MPP are detected
Parses L402 (Lightning), x402 (Base/Coinbase), and MPP (Stripe/Tempo) protocols only. Endpoints using proprietary or non-standard payment headers will return payment_protocol: "unknown" with a non-402 or unparsed 402 response.
What to do instead: For unknown protocols, inspect the raw response headers manually via the payment_details field which includes the raw header value.
Endpoint must return HTTP 402 to validate payment handshake
If the endpoint requires specific request parameters (POST body, auth token, etc.) to trigger the paywall, a plain GET request may return 200 or 401 instead of 402. The handshake check will be skipped and recommendation will be "proceed" based on reachability only.
What to do instead: Pre-flight is most accurate for endpoints that gate on HTTP 402 without requiring a specific payload. For parameterized paywalls, verify the protocol docs directly.
Facilitator reachability check uses HEAD with 2s timeout
The x402 facilitator reachability check sends a HEAD request with a 2-second timeout. Slow facilitators may appear unreachable even if functional.
What to do instead: Treat facilitator_reachable: false as a caution signal, not a hard block.