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

Annotations already declare readOnly, idempotent, non-destructive, and open-world behavior, so the safety profile is covered. The description adds valuable context annotations cannot: a free-sample quota and a paid x402 flow at $0.001 USDC per call, which tells the agent about cost and rate limiting before invoking.

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, zero filler, and the front-loaded sentence describes the return content while the second carries the payment caveat. Every sentence earns its place.

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 values need not be described, and the single parameter is fully documented in the schema. Purpose and payment behavior are covered; only explicit when-to-use/alternative routing 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%, with the single 'stop' parameter documented as a NaPTAN StopPoint id and an example ('940GZZLUOXC'). The description adds nothing beyond the schema, 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 names a specific resource (a station) and enumerates the fields it returns (lines, modes, facilities, child stops), which lets an agent distinguish it from stop-arrivals, stop-search, and stops-near. It is clear but phrased as a noun list rather than a verb, so the action ('fetch details') is only implied.

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?

There is no statement of when to use this tool versus alternatives like stop-search or stops-near, nor any prerequisites. The agent must infer usage purely from the returned-fields list.

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