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

Beyond the annotations, the description adds rate limiting, API-key requirements, the heuristic provider inference, and the exact return contents (PTR hostname, FCrDNS pass/fail, provider guess). This gives the agent behavioral context that annotations alone do not establish.

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?

Every sentence earns its place: purpose, verification, provider inference, use cases, alternative tool, and operational constraints (read-only, no API key, rate-limited). The structure is front-loaded with the core action and flows logically to outcomes.

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 an output schema, the description covers purpose, use cases, alternatives, constraints, auth, rate limiting, and return values. Nothing an agent needs to select or invoke this tool correctly is 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 coverage is 100%, so the parameter description already fully documents the 'ip' parameter, including IPv4/IPv6 examples and the disallowance of hostnames. The tool description does not add any semantic detail beyond the schema, so baseline 3 is 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?

The description states a specific verb and resource: 'Resolve the PTR (reverse DNS) record for an IPv4 or IPv6 address' and adds the clarifying FCrDNS verification. It distinguishes itself from asn_lookup explicitly by routing network/BGP ownership queries away, leaving no ambiguity about its scope.

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 description gives clear usage context: 'validate mail-server rDNS or identify a single IP's host' and explicitly names an alternative for a different use case ('Use asn_lookup instead for network/BGP ownership of the IP'). It stops short of contrasting with the closely related dns_lookup sibling, so it is clear but not exhaustive in exclusions.

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