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

Annotations already declare readOnly, idempotent and closed-world, so the safety bar is low. The description adds real context beyond them: it works without sign-in and covers letters only, not parcels — useful constraints for an agent. It does not describe response format or pagination, but no output schema exists so this is a minor gap.

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?

Four short, front-loaded sentences: usage condition first, then the two parameter modes, then behavioral constraints. No filler and every sentence adds usable 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 single-parameter read tool with no output schema, the description covers when to use it, both parameter modes, and the returned fields (zone, prices, delivery time). Return values are only sketched, but that is acceptable given the tool's simplicity.

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%, so the single optional country parameter and its ISO alpha-2 format are fully documented in the schema. The description restates the behavioral consequence of omitting or providing the code, which is helpful but largely mirrors what the schema already says; 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?

States a specific verb and resource (list destination countries) and distinguishes itself from siblings like get_price_quote by describing the postal-eligibility question it answers. An agent can tell what it returns and how it differs from letter-composition or pricing tools.

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 explicit when-to-use triggers (user asks whether a letter can be posted to a country, or which countries are reachable) and clarifies the with/without-country split. It does not name alternative tools for related questions, but the usage context is unambiguous.

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