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

Annotations already declare read-only, idempotent, non-destructive, open-world behavior, so the safety profile is covered. The description adds genuinely useful context beyond that: it discloses the data source (Valhalla over OpenStreetMap) and the return shape (one polygon per contour, each with a geodesic area), which helps the agent interpret results.

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, purpose front-loaded as a question, then the return contract in a parenthetical. No filler; every clause carries information the agent needs.

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?

An output schema exists, so return-value details need not be exhaustively explained, and the description covers modes, time budget, and output shape. Minor gaps remain — it doesn't mention the max-4 contour limit, edge behavior for unreachable/water areas, or expected latency — but the essentials for correct invocation are present.

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 75% and already documents latitude/longitude and the minutes range, so the baseline is 3. The description adds value by linking the minutes array to output semantics — 'one polygon per contour' clarifies that each minute value produces a separate reachable-area ring, which the schema alone does not convey.

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 states a concrete question the tool answers (reachable area from a point within N minutes) and names all three supported modes, so the resource and output (polygons) are clear. It doesn't explicitly differentiate itself from siblings like route or travel_time_matrix, but the 'reachable area' framing is distinct enough to infer.

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?

Usage is implied by the question format — you call it when you want an area reachable within a time budget rather than a point-to-point path. However, there is no explicit when-not guidance or named alternative (e.g., use route for A-to-B, travel_time_matrix for many origins), so the agent must infer the boundary.

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