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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=false, so the safety profile is covered. The description adds valuable context: the tool works without sign-in, and it enumerates what data is returned in each mode (zone, price, delivery time). It does not explicitly state rate limits or freshness of the data, but for a simple read tool this is a solid addition beyond annotations.

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 tightly packed sentences, each earning its place: when to use, behavior with/without the parameter, authentication, and scope limitation. Zero filler.

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?

The tool is simple (one optional parameter, no output schema, strong annotations). The description covers usage intent, both operational modes, expected return content, auth requirements, and scope limitation. 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?

Schema coverage is 100% and the single parameter's description already states 'ISO code for details of one country'. The description repeats the conditional semantics (with/without country) but adds no syntax or format details beyond what the schema provides. Baseline 3 is appropriate when the schema carries full parameter documentation.

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 (list) and resource (destinations), and frames the user's intent ('whether a letter can be posted to a country, or which countries can be reached'). This is clearly distinct from siblings like get_price_quote or send_letter.

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?

Explicitly distinguishes the two modes: 'Without country: all destination countries... With country: that country's zone...'. Also states scope restriction 'Letters only, no parcels', which routes the agent away from parcel-related siblings like send_letter or get_price_quote.

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