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

Annotations already declare readOnlyHint=true, so safety is covered; the description adds genuinely new context beyond that: it is free of cost/quota concerns, the delay value is live/current, and results are 'upcoming' (forward-looking ordering). It does not discuss pagination or what happens when limit is exhausted, which keeps it from a 5.

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 short sentences, front-loaded with the 'Free.' qualifier and the payload contents, then the chaining hint. No filler and nothing redundant with the schema.

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 only two parameters and no output schema, the description usefully takes on the burden of describing the return fields and the cost model. The only remaining gap is behavior of the limit parameter, which is minor for a simple read tool.

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 50%: 'station' is documented as a stop_id from search_stations, but 'limit' (default 10, max 30) has no schema description and the tool description never explains it either. The description clarifies that results are upcoming departures, which implies ordering, but the truncation parameter remains unexplained.

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 the resource (upcoming departures) and enumerates exactly what comes back: trip_id, line, scheduled/actual time, and current delay. It is clearly distinct from plan_transfer and search_stations, though the noun-phrase phrasing lacks an explicit verb and uses 'stop_id' while the parameter is named 'station'.

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 gives a concrete downstream rule of thumb: take trip_id and feed it to plan_transfer for the train the traveller is already on, which tells the agent how this fits into a larger itinerary flow. It does not state when to prefer search_stations first or any exclusions, but the schema's 'stop_id from search_stations' covers the prerequisite.

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