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

Annotations already cover safety (readOnly, idempotent, non-destructive, closed-world), so the bar is lower, and the description still adds real behavior: a hard cap of three departures, 'today' scoping, soonest-first ordering, the live-vs-scheduled distinction, and the extra payload fields (last bus, fare, downstream stops). It omits edge behavior such as what happens for an invalid place_id or when no service remains today.

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?

A single front-loaded sentence that leads with the core action and payload rather than burying it. It is dense but every clause carries information; the run-on structure makes it slightly harder to scan than a two-sentence split would be.

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 only two params, the description does the heavy lifting by enumerating exactly what each departure record contains (minutes until, last bus, fare, downstream stops) and how live positions alter times. What remains missing is failure/empty-result behavior, which matters for a lookup keyed on a place_id pattern.

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 100%, so both parameters (city with example values, place as a 16-char place_id pattern) are already fully documented in structured data. The description only echoes 'from a place' and says nothing about city semantics, so it adds no meaning beyond the schema — baseline 3.

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 ('Lists the next three departures today from a place') with the exact scope: per route and direction, soonest first. This is clearly a place-level departure board, distinguishable from route_details (single route) and plan_journey (point-to-point trip planning) without opening a schema.

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?

Usage is only implied: the schema says 'place' comes from find_places, so the agent can infer the find_places -> place_departures chaining. The description never states when to prefer this over route_details or plan_journey, nor any precondition or exclusion.

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