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

The description adds useful context beyond the readOnly and idempotent annotations: examples are 'reviewed, source-linked', and it explicitly warns that live numbers are not verified. This is genuine behavioral disclosure, though it doesn't go into details like rate limits or response structure.

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 two sentences long and front-loaded with the core action. The optional examples feature is briefly mentioned, followed by the limitation. Every sentence earns its place with no redundancy or filler.

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?

The tool is a simple lookup with a full output schema and 100% schema coverage for parameters. The description adds the key limitation (no live number verification) and the nature of the examples. Given its simplicity and rich structured metadata, the description is complete enough for correct selection and invocation.

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 input schema provides 100% parameter coverage with detailed descriptions for all six parameters. The description maps high-level search methods (free text, ISO code, dial code, region) to corresponding parameters but doesn't add deeper syntax or format details beyond what the schema already states.

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 specifies exactly what the tool does: 'Look up international country calling codes' with supported search methods (free text, ISO code, dial code, region). This clearly distinguishes it from the sibling tool 'format_international_phone_number', which formats rather than looks up.

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 context: use this tool to look up country calling codes. It also notes an important exclusion: 'This tool does not verify live subscriber numbers.' However, it does not explicitly name the sibling tool as an alternative when formatting is needed, so it misses the 'alternatives' criterion for a 5.

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