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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety and offline profile is partly covered. The description adds real value beyond them: it discloses that it consults only preloaded lists with no DNS lookup, returns the harvester observation time, and enumerates the classification categories. It stops short of stating rate limits or return structure, so 4 rather than 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 sentences, front-loaded with the classification purpose and then the key limitation, with no filler. It is dense and information-rich rather than padded, though the enumeration of categories makes it slightly long.

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?

There is no output schema, so the description carries the burden of describing the return: it conveys the classification outcome and the observation timestamp. That covers what an agent needs to interpret results, though it does not describe the exact response shape or what a negative result looks like.

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 single parameter is fully documented in the schema, including the maxLength and the domain-or-email duality. The description's 'Accepts a domain or an email address' merely restates what the schema already says, adding no syntax or format detail. Baseline 3 is correct.

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 (says whether / look up) and resource (domain), and enumerates the exact categories it classifies into: disposable burner list, relay service, free subdomain mail service, or real mailbox provider. It explicitly distinguishes itself from check_email, so 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?

Names the alternative (check_email) and the precise condition that selects it: a brand-new burner domain on a burner mail server is caught only by check_email because this tool uses loaded lists with no DNS lookup. That is an explicit when/when-not statement tied to a sibling.

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