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

No annotations were supplied, so the description carries the full burden. It does disclose two useful traits — the dataset is a static ISO 3166 reference and the operation is 'pure CPU, instant', implying no network calls, auth or rate limits. It does not address the default/pagination behaviour of `limit` or what an empty result looks like.

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 short sentences, zero filler, with the returned-field inventory front-loaded and the performance note trailing. Nothing can be removed without losing information.

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 two-parameter lookup tool with no output schema, the description usefully spells out the shape of each returned record, which is the main thing the missing output schema would otherwise have covered. The remaining gap is minor: default/pagination semantics for `limit` and matching rules for `q`.

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%, so the schema already documents both `q` ('Filter by name or code') and `limit` ('Max results'). The description's 'filterable by query' merely restates `q` and adds nothing about matching semantics (prefix vs substring, case sensitivity) or the default for `limit`, so 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 and resource ('Return ISO 3166 countries') and enumerates the returned fields (alpha-2, alpha-3, numeric code, region), making the tool's scope immediately unambiguous. Against near-neighbour list tools such as language_list, currency_list or emoji_list, the resource noun alone disambiguates.

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?

There is no explicit when-to-use guidance, no when-not-to-use condition, and no named alternative (e.g. 'use language_list for locales'). 'Filterable by query' implies the intended call pattern but stops short of guidance, so the agent must infer the use case from the name.

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