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

The description discloses that an unrecognised country returns an error rather than an empty list, and mentions the inclusion of the hub URL. While annotations already indicate read-only and idempotent behavior, this extra error-handling detail adds valuable context 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?

Two sentences, front-loaded with the core function, then usage guidance. Every sentence earns its place with no redundancy or 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 description provides enough context for a simple one-parameter read-only tool: what is returned (routes, hub URL), sorting order, error behavior, and alternatives. No output schema exists, but the description sufficiently covers the return content and edge cases.

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 input schema already fully describes the single 'country' parameter with examples and details (accent-insensitive, partial names). The description adds no new parameter-level semantics, but the schema coverage is 100%, so the baseline of 3 is appropriate.

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 clearly states the tool lists all atlas routes crossing one country, sorted by renown, and explicitly distinguishes itself from siblings by mentioning search_routes and night_trains. The verb+resource is specific and 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?

It provides direct usage guidance: use for "trains in X" questions, and explicitly names alternative tools (search_routes for combined filters, night_trains for sleepers), making it easy to decide when to use this tool over others.

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