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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered; the description adds a genuinely valuable guarantee that it never calls the URL, dispelling the assumption that this probes the endpoint live. It also discloses the shape of the verdict (score, history, price, warnings such as a mismatched payment wallet). It omits what happens for an unvetted or unknown URL and any auth or rate-limit constraints.

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?

One front-loaded sentence opens with the imperative "Call this before paying," then packs the return contents and the non-invocation guarantee without waste. The documentation link sits at the end where it does not interrupt the decision-making content.

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?

With no output schema, the description carries the burden of describing return values and does so explicitly, which is the main gap it needed to fill. Minor omission: no statement of behavior when the URL has never been checked or is not in the registry.

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 the schema itself explains the url format ("exactly as you would call it"), so the single parameter is fully documented. The description's "you were given or found elsewhere" only adds the sourcing scenario, not new syntax.

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?

Specific verb (check) plus a specific resource (a given API URL), and it enumerates exactly what comes back: reliability score, recent check history, price per call, paying-user count, and warnings. That clearly separates it from the discovery siblings (find_best, most_popular, list_categories, compare_category), which operate on categories rather than a known 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?

"Call this before paying a specific API URL you were given or found elsewhere" supplies a concrete trigger condition and scopes the input to a known URL rather than a category. It never names a sibling as the alternative, so the routing to find_best/most_popular is left implicit.

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