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

The annotations already state that this is a read-only, idempotent, non-destructive open-world call. The description adds genuinely new behavior: three outcome modes (`record`, `live`, `matches`), the shape of what each returns, and the upstream sources with licensing (Amtraker, Fintraffic, Trainline/ODbL). It doesn't say how or whether the caller selects `record` versus `live`, which keeps this 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 sentences, with the main purpose front-loaded before the response modes and the data-source footnote. The long field enumeration in the `record` parenthetical is dense but every item tells the agent what it will get back.

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?

There is no output schema, so the description must carry the return-value burden, and it does so in detail. The main remaining gap is that it never explains how a caller chooses between `record` and `live` — presumably automatic, but left to inference.

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 100% and there is only one parameter, so the baseline is already high. The description still adds value by enumerating the input formats accepted (prefixed operators, bare numbers, station names) that the schema's short example does not convey.

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 gives a specific verb and resource ('A train by number ... or a European station by name') and supplies concrete input examples (Amtrak 5, VIA 1, IC 1, bare number). The domain differences from the sibling lookups (airplane, ship, satellite) are obvious enough that the agent can select this tool quickly, though the description never names a 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?

The description makes the input shape clear, but gives no explicit guidance on when to prefer this tool over the `nichedb_transit_station` or `nichedb_transit_city` siblings, which overlap in the transit domain. Use is therefore only implied by the accepted inputs rather than stated.

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