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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so safety is covered; 'Read-only' is largely redundant. The description still adds real value by warning 'do not dial' and by pointing to 'Same data and limits as GET /v1/example/{country}', hinting at rate/usage limits.

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?

Front-loads the core purpose in one sentence with the safety caveat immediately after; no filler sentences. The inline parentheticals make it slightly dense but everything earns its place.

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?

An output schema exists, so return values need not be explained, and the description still notes that all formats are returned. Required parameter, safety profile and an endpoint equivalence are all covered; only sibling routing and limit specifics are absent.

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 description coverage is 100% (the single country param is fully documented with pattern and example codes), so the baseline is 3. The description adds no additional parameter meaning beyond what the schema provides.

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?

States a specific verb and resource ('Return the libphonenumber example (mobile) number for a region, in all formats') with a clear scope. It does not name or contrast with the sibling parse_phone_number, so an agent gets the purpose but no explicit differentiation.

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?

Gives a clear use context ('for format testing only') plus an explicit exclusion ('do not dial'), which tells the agent when this is appropriate. It stops short of naming alternatives or preconditions for choosing it over parse_phone_number.

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