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

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so the safety profile is fully covered. The description adds only that these codes serve Meta Ads Library filtering; it discloses nothing extra about caching, stability, or format of the returned codes.

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?

A single front-loaded sentence with no filler or repetition of the title. It is tight, though it is arguably too terse to add much beyond the name.

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 no explanation, and the zero-parameter schema is trivially complete. For a simple reference lookup, the description is sufficient, though a note on how the codes are consumed by ad-search tools would round it out.

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 tool takes zero parameters, so per the baseline there is nothing for the description to disambiguate. No parameter meaning is missing.

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?

Specific verb+resource ('Get list of supported country codes') scoped to Meta Ads Library filtering, which clearly separates it from search/archive siblings like facebook_ads_search. It does not name a sibling explicitly, but the lookup nature of the tool is unambiguous.

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 purpose sentence implies usage ('for Meta Ads Library filtering'), so an agent can infer this is a reference-data call that feeds other ad queries. However, there is no explicit statement of when to call it versus facebook_ads_search or what to do with the returned codes.

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