check_x402_serviceFREE. Before paying an x402 (HTTP 402) API, check it: is it up right now, does its real price match its listing, and is there a cheaper service that does the same job? Covers ~25,000 paid agent APIs, re-checked continuously.
| Name | Required | Description | Default |
|---|---|---|---|
| url | Yes | The paid service's URL | |
| method | No | HTTP method the service uses | GET |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds useful behavioral context: the service is FREE, covers roughly 25,000 APIs, and is re-checked continuously, which informs the agent about cost and data freshness.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
The description is two sentences, front-loaded with the most important fact ('FREE. Before paying...'), and every phrase earns its place. It avoids repetition and unnecessary detail.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
For a two-parameter tool with no output schema, the description communicates what the tool checks, when to use it, and its coverage. The exact return format is not specified, but the three checks imply the result categories well enough for an agent to invoke and interpret the tool.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Schema description coverage is 100%, so both parameters (url and method) are already documented. The description adds no additional parameter-level meaning, which is acceptable given the complete schema. Baseline of 3 applies.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
Description names a specific verb ('check'), a resource ('x402 service'), and three concrete checks: uptime, price match, and cheaper alternatives. This clearly differentiates it from sibling 'find_cheapest_x402_service', which is about discovery rather than verification of a specific URL.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
The description explicitly states the usage trigger: 'Before paying an x402 API, check it.' This gives clear context for when to call the tool. It does not explicitly mention when not to use it or name alternatives, but the trigger is sufficiently clear.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Add one secure layer between your agents and this server.