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

With no annotations, the description adds useful behavior: it explains the two matching sources and states that withheld phones are excluded. However, it omits result cardinality, no-match behavior, and output shape, leaving meaningful gaps for a tool with no annotation or output-schema coverage.

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 short and front-loaded with the use case, with no filler. The second sentence is grammatically awkward ('Providers whose registry practice phone... is this number'), slightly reducing clarity but not length.

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 simple one-param lookup, the description covers the use case, matching sources, phone format, and a suppression caveat. It still lacks return-value and no-match details, and there is no output schema to compensate, so it is adequate but incomplete.

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?

Only one parameter exists, and the schema already documents it as 'A US phone number in any format' with 100% coverage. The description repeats 'Any US format' but adds no syntax or validation detail beyond the schema, so 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?

The description opens with the selecting question ('who a phone number belongs to') and the matching basis (registry practice phone or claimed-page phone), making the reverse-lookup purpose clear. It does not name sibling tools, so it falls short of a 5.

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?

It explicitly says 'Use when the question is who a phone number belongs to,' giving a clear triggering condition. It offers no when-not guidance or alternatives such as lookup_provider or search_providers, so it is not a 5.

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