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

Annotations already cover readOnly/idempotent/openWorld/destructive, so the bar is lower; the description adds real value by disclosing the self-verification behavior and the deliberately different 'unavailable' vs 'clean' outcome for lists that refuse public resolvers. It also quantifies scope (~18 lists). It does not describe return shape or per-list status format, keeping it short of a 5.

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?

Two tight sentences: what it checks first, the verification caveat second. Lists are parenthetical examples rather than enumeration for its own sake, and nothing is repeated from the schema.

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 tool with annotations and no output schema, the description covers scope, target types, and the key caveat about unavailable lists. Only the shape of results (per-list status/response) is left implicit, which is a minor gap.

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?

With 100% schema coverage and a single parameter, the baseline is 3, but the description adds meaning by explaining that the target type selects which list family is queried (IP blacklists vs domain blacklists). That is genuine semantics beyond the schema's bare 'IPv4 address or domain'.

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?

Names a specific verb (check) and two concrete resources (IPv4 address / domain) against named blacklists, with examples (SpamCop, Barracuda, UCEPROTECT, PSBL, Mailspike). An agent can distinguish it from siblings like check_domain or check_record, which do not involve blacklist reputation.

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?

Usage is implied by the target-type split (IP vs domain) rather than stated: the description tells you what it checks but never says when to prefer it over check_domain or check_record. No explicit exclusions or routing to alternatives.

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