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. The description adds genuinely useful behavioral detail beyond them: the shape and units of the response (monthly price expressed in US cents) and the availability signal ('whether new numbers can be bought now'), which an agent cannot derive from the annotations.

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?

One dense sentence that front-loads the core purpose before the qualifiers, with every clause carrying information about the returned fields. It is slightly long for a single sentence and could be split, but there is no 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?

A read-only, zero-required-parameter listing tool with an output schema and full annotation coverage needs little more. The description supplies scope, response fields, units, and the optional filter, exceeding the bar for this complexity level.

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?

With 100% schema description coverage and an enum fully enumerated in the schema, the schema already carries the parameter meaning. The description only restates the filter at a high level ('only countries with a given number type') without adding syntax or default behavior, so the 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 gives a specific verb (List) and resource (countries) and goes further by enumerating exactly what each country record contains: number types, area count, lowest monthly price in US cents, and purchasability. This distinguishes it cleanly from siblings like get_area_pricing and search_areas, which operate on narrower scopes.

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 closing clause 'Optionally only countries with a given number type' communicates the one available filter, which is useful. However, it never states when this tool should be chosen over the pricing siblings (get_area_pricing, get_calling_rate) or that it is the discovery/entry point for those tools, leaving the routing decision to inference.

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