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

Annotations already cover the safety profile (readOnly, idempotent, openWorld). The description adds real value beyond them: it discloses the passive nature of the lookup and warns that crt.sh is 'occasionally slow or unavailable', which sets latency/availability expectations 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?

Two tight sentences with the core purpose front-loaded and the caveat appended. No wasted words.

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 single-parameter read-only tool with full annotation coverage and no output schema, the description is nearly complete: it explains the source, scope, and reliability caveat. It stops short of describing the shape of the returned hostname list, but that 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?

Schema description coverage is 100% for the single 'domain' parameter, so the schema already documents it fully. The description adds no format or syntax detail beyond the schema, making the baseline 3 appropriate.

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+resource ('Passive subdomain discovery') and immediately names the data source (crt.sh / Certificate Transparency logs). Crucially it distinguishes itself from the sibling dns_lookup by clarifying it finds hosts from public TLS certificates, 'not every DNS record'.

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?

The scope statement ('hostnames that appeared in public TLS certificates, not every DNS record') effectively tells the agent when this tool is appropriate versus a general DNS tool. It does not explicitly name dns_lookup as the alternative, so it falls just short of a 5.

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