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

With no annotations supplied, the description carries the full burden and does useful work: it discloses that calls are free, that no API key is required, and that the response is either a boolean per country or a full list. Those are exactly the operational facts an agent needs before deciding to call. Missing only edge-case behavior, such as what happens with an unsupported or malformed country code.

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?

Two sentences, zero filler: the resource definition comes first, the calling convention second, and the cost/auth note last. The parenthetical explaining "current" earns its space because it is the one term an agent could misread.

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 single-optional-parameter read tool with no output schema and no annotations, the description covers purpose, optionality, and return shape, which is most of what an agent needs. The remaining gap is failure behavior for an invalid country code, a minor omission at 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?

Schema coverage is 100% and the country parameter already documents its two-letter ISO format and prefix-related purpose. The description adds the meaningful point that the parameter is optional and that omitting it returns the full list, which complements the schema's optionality without duplicating it. Baseline 3 is appropriate since the schema does the heavy lifting.

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 names the resource precisely (countries supporting mode "current") and even glosses the ambiguous term: the carrier currently serving a number after porting. That removes real ambiguity an agent would otherwise have to guess at. It does not reference any sibling tool (verify_phone, example_number), so differentiation is inferential rather than explicit.

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 gives clear invocation guidance for the parameter — pass a country for a yes/no check, pass nothing for the full list — which is genuine when-to-use information at the argument level. It stops short of saying when an agent should reach for this tool rather than verify_phone or example_number, so the tool-selection guidance is only implied.

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