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

Annotations already cover readOnly/idempotent/destructive/openWorld, yet the description adds real context beyond them: per-route errors and source provenance are preserved, partial results must not be treated as complete, and there is no booking, live-transport or suitability guarantee. These caveats materially shape how an agent should interpret and act on the output.

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?

Purpose is front-loaded in the first clause, and follow-on sentences carry routing and caveats rather than filler. The section enumeration overlaps with the schema enum, but it still reads efficiently rather than padded.

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 an output schema present, return-shape detail is not required, and the description still covers scope, input constraints, sibling routing, and interpretation caveats including partial results. 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 description coverage is only 33%, but the description compensates by naming the sections (equipment, huts, access, notices, daylight) and flagging that daylight is 'dated' (the date parameter), plus the six-route cap. It does not explain the slug/routeId pair format, which the nested schema does cover.

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 (compare) and resource (up to six routes), and enumerates the packet contents — grade, timing conflicts, equipment, huts, access, notices, daylight. It explicitly distinguishes itself from get_route (full details) and get_weather (forecasts), so an agent can route without inspecting schemas.

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?

Names both alternatives and the condition that selects them: batch comparison lives here, full detail in get_route, forecasts in get_weather. The 'up to six known routes' framing plus the schema note about slug/routeId pairs from search_routes tells the agent exactly what input is acceptable.

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