Check Phone Number

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

The annotations already cover readOnly, idempotent, open-world, and non-destructive behavior explaining. The description adds useful behavioral context beyond those annotations by emphasizing that the lookup is 'public' and requires no API key, plus it implies the output includes carrier, line type, and spam risk. No contradictions exist.

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 entire description is one compact sentence that front-loads the core purpose and then adds key access context ('No API key required'). No filler or redundant restatement of the tool name or title.

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, single-parameter, low-complexity lookup tool with rich annotations, the description plus schema fully covers what an agent needs to select and invoke it correctly. Return-format details are not explicitly described, but the description already names the result dimensions (carrier, line type, spam risk), which is adequate without an output schema.

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% and the parameter description already explains E.164 format with an example. The description adds value by calling the number 'arbitrary,' clarifying that any public number can be checked regardless of account association, and by mentioning the specific categories the lookup covers.

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 clearly identifies a specific verb-resource pair: a public carrier and line-type lookup for an E.164 phone number. It also specifies the data categories returned (mobile/landline/VoIP, spam risk), which makes the tool's purpose unmistakable and distinct from the many sibling tools.

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 states that this is a public lookup requiring no API key, which signals when it can be used independently from account-bound tools. It does not explicitly name an alternative or exclusion, but no sibling appears to be a competing number-lookup tool, so the context is sufficient.

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