Preflight an x402 Endpoint

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

Annotations provide only basic hints (readOnlyHint=false, openWorldHint=true, idempotentHint=false, destructiveHint=false), leaving the description to carry the burden. The description discloses that it is free, rate limited, and a single probe that yields evidence, not a guarantee. It also explains the output (reached_level, checks vector) and explicitly states limitations. No contradiction with annotations.

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 relatively long but well-structured and front-loaded with the core purpose. Every sentence adds value—purpose, use cases, output, limitations, and alternative—though it could be tightened slightly without losing meaning. It is not wasteful, but it is dense.

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?

Given the tool's moderate complexity (one parameter, output schema present), the description covers all necessary aspects: what it does, when to use, what it returns, its limitations, rate limiting, and an alternative. It even notes what the probe cannot tell you. Nothing critical is missing for correct invocation.

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?

The schema has 100% description coverage for the single 'url' parameter, and the schema description ('The https endpoint a buyer would GET expecting a 402 challenge') is already clear. The tool description does not add additional parameter-specific detail beyond what the schema provides, so the 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?

The description clearly states the tool's purpose: to preflight an x402 endpoint by sending an unpaid probe to check if it serves a well-formed x402 v2 payment challenge. It specifies the verb ('Check'), the resource ('any x402 endpoint's door'), and the outcome (verdict with reached_level). It distinguishes itself from siblings by noting it is a shape check, not an uptime claim, and points to buy_observation as an alternative for a signed version.

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 when to use it: for a buyer about to pay a door it has not paid before, and for a seller checking their own. It also states what not to use it for ('NEVER an uptime or delivery claim') and names an alternative ('buy_observation with item_id service_audit'). This is clear, actionable guidance.

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