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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint false, so the safety profile is established. The description adds that the tool returns polygon(s) and supports public transport, but it does not disclose additional operational behaviors such as response format or mode-specific constraints.

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 fairly compact and front-loads the core purpose in the first sentence. The inline example is helpful for demonstrating a realistic call, though it partially duplicates the schema's examples, which keeps it from being maximally concise.

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?

This is a moderately complex tool with 8 parameters and no output schema, and the description does state the basic return type (reachable-area polygons) and shows a full example. However, it does not describe output structure, precision, or any transport-mode-specific behavior beyond the schema, leaving some context for the agent to infer.

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 covers 100% of parameters with detailed descriptions, including minutes, travel_time, transport modes, and departure/arrival times. The inline example reinforces correct usage but does not add meaning beyond what the schema already provides, 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 computes a reachable area within N minutes and returns reachable-area polygon(s), which is a specific resource and outcome. It also distinguishes itself from road-only routing by supporting public_transport, though it does not explicitly name the sibling traveltime_time_filter.

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?

The phrase 'Unlike road-only routing, this supports public_transport' provides a useful hint about when this tool is appropriate, especially for transit scenarios. However, it does not explicitly say when to use this tool versus alternatives, name sibling tools, or state exclusions.

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