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

Annotations already declare readOnly, idempotent, open-world, non-destructive, so the safety profile is covered. Beyond that the description adds genuinely useful behavior: the result is itemized per leg, payment is typically per-leg rather than a single transaction, and it carries an unofficial/not-affiliated disclaimer warning the agent about authority of the data.

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?

Front-loaded with the core purpose, then input formats, then grounding constraints — every sentence carries information. The awkward mid-sentence line breaks slightly hurt readability but no content is wasted.

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?

No output schema exists, and the description compensates by describing the result structure (per-leg fares, payment instructions, total). With annotations covering side effects, the only real omission is the meaning of the two optional parameters.

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 0%, so the description must compensate, and it does well for the two required params by listing accepted formats (addresses, places, stations, "lat,lon"). But the optional "when" and "rider_category" params are never explained, even though they plausibly change fare pricing (time-of-day, discount category) — a meaningful gap in a fare tool.

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 opens with a specific verb and resource: "Estimate the total fare for a cross-system trip," and enumerates the return shape (per-leg fares, payment method per leg, total). The multi-leg/total scope implicitly but clearly separates it from single-fare siblings like get_fare and how_to_pay.

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?

It supplies a grounding rule ("Answer only from this tool's results: never add routes, hours, phone numbers...") and clarifies that each leg is usually a separate payment, which sets expectations. However, it never states when to prefer this tool over get_fare, plan_trip, or how_to_pay, so sibling selection is left to inference.

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