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

Beyond the read-only and idempotent annotations, it discloses important runtime behavior: domain inputs are resolved to IPv4 A records, IPv6-only domains are unsupported, results are cached for 24 hours, and a daily quota of 100 calls applies. This is substantive context that neither the annotations nor the schema provide.

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?

The description is front-loaded with the core purpose and remains information-dense; quota, source, cache, and fallback instructions all earn their place. It is slightly repetitive by restating read-only/idempotent traits already present in annotations, but this is minor.

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?

With a single parameter, an output schema, and full annotation coverage, the description fills the remaining gaps: accepted input formats, unsupported IPv6-only cases, API source, caching, quota, and an explicit fallback to report_feedback. Nothing needed to invoke it 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?

The schema already documents the single parameter well, but the description adds the A-record resolution rule and the IPv6-only exclusion, which are not in the schema. That added nuance justifies a score above the coverage baseline, though the core parameter meaning is largely restated.

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 opening sentence states a specific verb and resource: 'Find domains co-hosted on the same IP address (reverse IP lookup).' This unambiguously identifies the tool's output and clearly distinguishes it from siblings like domain_fetch_dns_records and domain_fetch_subdomains.

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 concrete use cases ('identifying shared hosting risk and mapping corporate infrastructure') and clearly defines the accepted input forms. It does not explicitly name alternative tools or state when not to use this one, 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.