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

Annotations already declare readOnlyHint/idempotentHint/no-destructive, so the safety profile is covered. The description adds real behavioral context beyond that: 'Free, no auth or payment header,' plus which products are U.S.-only vs global and where `countries: null` occurs, which an agent needs to interpret results.

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?

Front-loads the purpose and return format, then appends dense product-specific caveats. It is on the long side for a zero-param tool and some sentences verge on tangential endpoint routing, but every sentence carries distinct, decision-relevant 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?

With no parameters, an output schema present, and full annotations, the definition needs little more; the description still supplies the domain semantics (per-product scope, U.S.-only vs global, null list) that make the returned codes interpretable. Nothing an agent needs to call it correctly is 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?

This tool takes zero parameters, so the baseline is 4. The `country` reference in the description pertains to a different endpoint rather than this tool's inputs, but there are no parameters to clarify.

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?

States a specific verb+resource ('Get supported countries') and then immediately clarifies what that means concretely: 'Returns where each product can be used or sent, as ISO 3166-1 alpha-2 country codes.' No sibling does the same thing, so the agent can distinguish it without opening the schema.

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?

Gives clear context for the various product-specific country lists and routes the agent to the right alternative endpoints (e.g. `GET /order-gift-card` with a listed code, `GET /search-merchants` for the international card). It doesn't phrase explicit 'use this vs X' when/when-not rules, but the routing guidance covers the relevant cases.

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