Agent Zero — ERC-8004 Trust Filter

check_x402_endpoint

Find out if agents can actually pay you. Free check of an x402 seller endpoint (Seller Check). Sends only unpaid requests: confirms the route answers HTTP 402, reads the challenge (header and body, v1 or v2), checks every accepts entry (scheme, network, asset, amount, payTo, maxTimeoutSeconds, and extra.name/extra.version against the token's real EIP-712 domain), the named payment service's /supported, and /pricing, /openapi.json, /.well-known/x402 for consistency.

Args: url: https URL of the paid route (required). method: "GET" or "POST" (optional; default from the seller's OpenAPI file, else GET). body: optional JSON body for POST routes (max 8 KB). Returns checked_at, overall (pass|warn|fail) and checks[] with status, reason and a fix hint. Never pays. Limits: 10 checks per caller per hour, 3 per host per hour.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "url": "<url>"
    },
    "name": "check_x402_endpoint"
  }
}

Input Schema

Name	Required	Description	Default
`url`	Yes
`body`	No
`method`	No

Schema Changelog

Changes observed during successful MCP inspections.

AddedNov 1, 2026

TDQS

A4.1/5.0

Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations provided, the description carries the full behavioral burden and does so thoroughly: it clarifies that only unpaid requests are sent, that it never pays, lists rate limits (10 per caller per hour, 3 per host per hour), describes exactly what is checked, and even summarizes the return structure.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is front-loaded with the key benefit and remains informative throughout. It is a bit long, but every sentence—including the detailed accepts entry checks and the Args/Returns sections—earns its place.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given no annotations, no output schema, and 0% schema coverage, the description is complete enough for an agent to call it correctly: it covers purpose, safety, rate limits, parameter constraints, and return shape (checked_at, overall, checks[] with status, reason, fix hint).

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters5/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0%, so the description must compensate, and it does: it explains that url is a required HTTPS URL, method is optional with default behavior (from OpenAPI else GET), and body is optional JSON for POST routes with an 8 KB limit. This adds essential meaning beyond the bare schema.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a clear verb and resource: check an x402 seller endpoint to see if agents can pay. It is specific about what it does, but it does not differentiate from the sibling tool seller_check, and its parenthetical '(Seller Check)' may even add confusion.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description explains the purpose but gives no explicit guidance on when to use this tool versus alternatives like seller_check or list_payable_x402_endpoints. There are no exclusion conditions or comparative advice.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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