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

Annotations already declare read-only, idempotent, non-destructive and open-world, and the description goes further by disclosing the verdict taxonomy (avoid/caution/ok/unknown), the statistical payload with its 95% interval, and the presence of evidence links. Since no output schema exists, this return-value disclosure is doing real work an agent needs.

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?

It is a single front-loaded sentence that opens with the usage trigger, which is the right priority. The return-value list makes it long and slightly run-on, but every clause carries information.

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 one-parameter read-only lookup, the description covers the trigger, the parameter shape via schema, the safety profile via annotations, and the full return contents. Nothing an agent needs to decide whether and how to call it 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?

There is a single required parameter with 100% schema coverage, and the schema itself explains that it accepts either a full x402 resource URL or a bare seller domain. The description adds nothing beyond that, so the baseline 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 pairs a specific verb ('check') with a precise resource ('x402 seller') and enumerates what is returned (verdict, paid-call count, failure rate, price, max overcharge ratio, evidence links). Scope is unambiguous, but it never contrasts itself with the sibling list_reliable_sellers or recent_changes, so the agent must infer the split.

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?

Three concrete triggers are given: before any x402 payment, after an HTTP 402 response on retry, and when the user is about to buy an API call with USDC on Base or names a per-call API. That is unusually clear context, though no when-not condition or explicit alternative is named.

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