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

Annotations already declare read-only/open-world/non-destructive, and the description adds non-redundant behavior: an empty board is not a no-information answer, `modes` and `direction` are hard filters, an invalid mode or stop name is an argument error, and all journey facts must be cited only from this tool's output. This is exactly the anti-fabrication and error-semantics context an agent needs beyond the annotations.

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?

The text is dense but front-loaded, leading with the one-stop purpose before moving to alternatives and invocation rules. Bold key phrases and the final anti-fabrication rule earn their place; it is longer than a minimal description, but for a 15-parameter tool with several traps, the length is justified. A more scannable list structure would have pushed it to 5.

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?

For a tool with no output schema, it covers selection, required/optional parameter usage, error cases, empty-board behavior, and delegation to `get_usage_guide` for real-time/cancellation/citation fields. It stops short of self-contained output details (which are referred to the guide), so an agent without access to that guide would be missing some return-field semantics.

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?

Although the schema already documents all 15 parameters, the description adds value by grouping them by intent: required resolved `id`, time/arrival switches, hard vs soft direction filters, the `time_range_seconds` + `last_of_window` combination for last-trip questions, and payload/verbosity controls for chat vs drawing applications. It also flags the mutual exclusion of `target_bearing` and `direction` and the tram/subway labeling trap.

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 opening sentence names the operation precisely: departures or arrivals for a single stop, delivered as data for downstream processing. It also distances the tool from route planning by steering A→B questions to `connections`, and distinguishes it from the human-facing `show_departures` sibling, so an agent can select it without opening the 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?

The description gives explicit when-to-use rules: single-stop departure questions come from this tool family, A→B questions from `connections`, and `show_departures` is the front end when present. It also mandates a pre-resolved stop `id`, names the `get_usage_guide` route for nuance, and tells the agent how to recover from an empty filtered board.

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