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fetchgate

by dev.fetchgate
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Check an x402 endpoint before paying it

check_x402_endpoint

Decide whether an unfamiliar pay-per-call (x402) endpoint is worth paying. Sends one unpaid GET to read its live payment challenge (format, price, network, signing domain), then adds its settled-call and unique-payer history, whether its live price matches its listed price, and its domain's reputation (template farm or not, share of its endpoints that answer). Returns a verdict — ok, caution, avoid or unknown — with the reasons. Nothing is paid. Use before an agent sends money to an endpoint it has not used before. Wraps GET /v1/x402/check. Same free daily tier as read_url; $0.01/call via x402 after that.

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

NameRequiredDescriptionDefault
urlYesThe x402 endpoint URL to check.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Added

TDQS

A4.4/5.0
Behavior5/5

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

With no annotations, the description carries the full burden and does so well: it discloses the mechanism (one unpaid GET to read the live payment challenge), the safety guarantee ('Nothing is paid'), the specific signals returned, and the cost model (free daily tier, $0.01/call via x402 after). This is far beyond what structured fields provide.

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?

It is front-loaded with the decision purpose and keeps every sentence information-bearing, but the middle sentence is a long clause chain listing returned signals. Slightly dense, though nothing is wasted.

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?

For a single-parameter, no-annotation tool with no output schema, the description fully covers mechanism, safety, returned verdict values, cost, and the wrapped endpoint. Nothing an agent needs to call it correctly is missing.

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

Parameters3/5

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

There is only one parameter (url) with 100% schema description coverage, so the schema already documents it. The description adds the context that the URL must be an x402 endpoint, which is marginally useful but largely duplicates the schema baseline.

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

Purpose5/5

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

The description states a specific verb+resource ('Decide whether an unfamiliar pay-per-call (x402) endpoint is worth paying') and clearly separates this from siblings like search_x402_services or read_url by framing it as a pre-payment vetting check. An agent can identify the tool's job without opening the schema.

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

Usage Guidelines4/5

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

It gives explicit usage context ('Use before an agent sends money to an endpoint it has not used before') and implies the alternative is skipping the check. It does not name a specific sibling alternative to use instead, but the when-to-use trigger is unambiguous.

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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