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

Annotations already declare readOnlyHint and idempotentHint, so the bar is lower. The description adds meaningful context: the ordering is based on 'World Train Map's renown order,' and the output is 'compact facts with id and URL per route.' It also implies a bounded list (top-ranked) without full search capability, consistent with openWorldHint=false. It doesn't detail the exact structure but is adequate for a simple read-only list.

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?

The description is two sentences, front-loaded with the main purpose and ranking criteria, then provides clear usage guidance. Every phrase adds value—no filler or redundancy.

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 list tool with one optional parameter, strong annotations, and no output schema, the description is complete. It specifies the ranking, return fields (id and URL), and directs users to siblings for other needs. No critical information 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% for the single optional 'limit' parameter, which already includes a description ('How many top-ranked routes. Default 25, max 100'). The description does not add further parameter semantics, so the baseline of 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?

The description clearly states the tool returns 'the world's most famous train journeys' in a specific ranking order, with 'compact facts with id and URL per route.' It uses a specific verb and resource, and immediately distinguishes itself from siblings like search_routes and get_route by describing its unique scope and ordering.

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?

Explicit usage guidance is provided: 'Best first call for bucket-list and "greatest train trips" questions' and alternatives are named: 'use search_routes to find something specific, get_route for full detail on one route.' This fully addresses when to use the tool versus alternatives.

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