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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered; the description adds context the annotations cannot, namely that the tool is an unofficial, unaffiliated service and that answers must come only from its results rather than model memory. It still doesn't say how results are shaped or what happens when origin/destination are ambiguous or unroutable.

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 outcome first, then an Args block, then a grounding constraint — each sentence earns its place. Slightly long for a three-parameter tool, but nothing is redundant enough to cut.

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 no output schema and no annotation-level detail about results, the description still tells the agent what information the answer will contain (legs, hub, feasibility, fare, payment) and warns it is guidance rather than a booking. The main remaining gap is the response format and failure behavior for unroutable or ambiguous places.

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 carries the full burden, and it does: origin and destination accept address, place name, station, or "lat,lon", and `when` is a day value like "sunday" or "weekday" used to check feasibility. It adds real meaning beyond the bare string types, though it doesn't clarify what `when` values are actually accepted beyond those two examples.

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 and resource — advising on connections between two places "across systems" — and enumerates exactly what the answer contains: system per leg, transfer hub, days/times feasibility, fare and payment per leg. It implicitly separates itself from the single-agency sibling plan_trip via "across systems" and "Guidance, not a booked itinerary," but it never names plan_trip or any other sibling explicitly.

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?

Clears up the biggest ambiguity for a transit agent: this returns advice, not a booked itinerary, and the optional `when` exists to test day-level feasibility. There is no explicit when-not or named-alternative routing (e.g. use plan_trip for a single agency, estimate_trip_cost for pricing only), so an agent must infer when to prefer a sibling.

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