Screen a payment endpoint for phishing (free)

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

Annotations cover the safety profile (readOnly, idempotent, open-world), and the description adds substantive behavior beyond them: what is actually checked (hostname and parent domains vs. public phishing lists plus shape signals), the four-value return vocabulary with language-independent signal_keys, and the crucial caveat that 'clear' means no match was found, not that the endpoint is safe. It also discloses cost and no API key requirement.

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?

Front-loaded with the core action and the sibling distinction, and each sentence carries information — the x402 flow sentence justifies why endpoint-level screening is needed. It is somewhat dense and the threat narrative could be tightened, but nothing is pure filler.

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?

No output schema exists, so the description takes on the burden of explaining the return values, and it does so precisely: the four-level verdict, the meaning of signal_keys, and the critical semantics of 'clear'. Combined with the flow context and cost disclosure, an agent has everything needed to call and interpret this correctly.

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% and the single 'url' parameter is fully documented in the schema (full URL or bare domain, with examples). The description's 'URL or domain' phrasing merely restates the schema, adding no format, normalization, or edge-case guidance beyond it, so baseline 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?

States a specific verb (screen) and resource (payment ENDPOINT: URL or domain) and explicitly positions itself against the sibling screen_address — 'the half that address screening cannot answer.' An agent can route between the two without opening either schema.

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?

Gives clear timing ('before calling it') and the x402 discovery context in which the URL arrives, plus an explicit pairing instruction ('Use together with screen_address'). It stops short of stating when this tool should NOT be used or what to do with a 'caution' result, so it is clear context rather than full when/when-not guidance.

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