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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, and the description redundantly notes 'Side effects: none (read-only).' However, it goes beyond annotations by explaining the semantics of is_available=false (not enabled for purchase, not out of stock) and prescribing user-facing phrasing, which is valuable behavioral context.

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 tightly written sentences deliver purpose, a key clarification, and side effects with zero filler. The most important information is front-loaded, and the guidance about is_available is both concise and actionable.

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 simple, parameterless tool with an output schema, the description covers everything an agent needs: what the list contains, how to interpret the availability flag, and how to communicate results to users. The output schema handles return structure, so no additional details are missing.

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 has zero parameters, so the schema fully covers inputs. The baseline for zero parameters is 4. The description adds no parameter-specific detail, but none is needed; it instead clarifies the meaning of a field in the output, which is helpful but not required for input semantics.

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 opens with a specific verb-resource pair ('List countries') and a clear domain ('where you can purchase fax numbers'). It distinguishes itself from sibling list tools (list-numbers, list-areas, etc.) by focusing on countries, and the clarification about is_available resolves any ambiguity about what the list contains.

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 description makes the tool's context obvious—countries relevant to fax number purchase—and adds explicit guidance on how to interpret is_available and what to tell the user. It doesn't name alternative tools or state when-not-to-use, but given the tool's simple, unparameterized nature, the context is clear enough without exclusions.

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