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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), and the description adds real context beyond them: the data comes from previously observed passive DNS rather than live brute-forcing, and the output is subdomains with their IPs. It omits known limitations such as coverage gaps or rate limits, but is well above the annotation baseline.

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 query phrasings an agent would match against, followed by the mechanism and use cases. The quoted example queries are slightly verbose but earn their place by aiding intent matching; nothing is redundant.

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, no-output-schema tool, the description covers what it does, its data source, the shape of the result (subdomains + IPs), and when to reach for it. Only explicit alternatives and coverage caveats are 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?

Schema description coverage is 100%, so the 'registrable domain, not a subdomain' constraint is already documented in the schema. The description adds no additional syntax or format guidance for the target parameter, so the 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 and resource — passive subdomain/host enumeration of a domain — and names the data source (HackerTarget). It contrasts with active guessing ('rather than by guessing names'), which clarifies its method, but never names the closest siblings (dns_lookup, reverse_dns, find_shared_dns) so an agent must still infer which DNS tool to pick.

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 concrete when-to-use scenarios ('attack-surface inventory', 'find a forgotten staging host') plus intent-matching query phrasings. It does not state when NOT to use it or name an alternative tool, so it stops short of full routing guidance.

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