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

Annotations declare readOnly/openWorld=false/idempotent, and the description adds substantial non-redundant behavior: no fetching of the caller's URL, no payment to the endpoint, no charge for unlisted/unavailable/stale data, $0.005 per cached lookup, a shared 3-call/day free trial, and the exact payment continuation protocol with 'stop and get approval' semantics. It also pre-empts a likely misinterpretation by explaining that an unknown score means missing evidence, not endpoint failure.

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 core purpose and sibling routing are front-loaded and effective, but the body is heavily padded: disclaimers repeat ('not a guarantee', 'illustrative output excerpt, not a live result...', 'freshness/data/availability checks still apply'), and the multi-sentence payment walkthrough dominates the tool's description. Useful information is present but buried in boilerplate that could be trimmed.

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 cached lookup with an output schema and full annotation coverage, the description covers everything an agent needs: input exactness, pricing and free-tier limits, unknown-score semantics, scope exclusions, and safe payment handling. Nothing required to call it correctly or interpret a null result is missing.

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 coverage is 100% and both parameters are documented, so the baseline is 3; the description slightly exceeds it by reinforcing that the URL must be exact and already public, and by supplying a concrete minimal input example. It does not add new format or syntax detail beyond what the schema already says about the method parameter.

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?

States a specific resource and scope: cached x402 endpoint observations returning a transparent score, its components, and seven-day unpaid probe history for an already-listed URL. It explicitly distinguishes itself from siblings by naming check_payee and preflight_payment and stating what each is for, so an agent can route without opening schemas.

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?

Explicit when-to-use ('endpoint history and price context before paying'), explicit alternatives with conditions ('check_payee for wallet/name screening or preflight_payment for an allow/hold/block decision'), and a parameter-level usage rule ('add method when the URL has multiple advertised methods'). Negative guidance is also present: it never fetches the caller's URL and is not a compliance guarantee.

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