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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so the safety profile is covered; the description adds real value beyond that with failure semantics (candidates on ambiguity, non-retryable train_not_on_board) and a reliability caveat that predictions are unofficial and station screens are authoritative. The 'Read only' sentence merely restates the annotation and earns nothing.

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?

Purpose is front-loaded in the first clause and the subsequent sentences each carry distinct information about identification and failure modes. The only wasted line is 'Read only,' which repeats the annotation, and the reliability caveat is dense but worth its space.

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, the description carries the return-value burden and does so at a high level (platform status, including departed and withdrawn trains) plus explicit error outcomes and non-retryability. It omits what fields the status payload contains and the role of station, which are the remaining gaps for a 4-parameter 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 75% and the schema already documents headcode as an alternative to train_id used with time, so the description's headcode+time explanation largely duplicates structured data. The description adds the multi-match candidate behavior, but never mentions the station parameter or its EUS default, so it does not compensate for the one undocumented parameter.

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 clause states a specific verb and resource: 'Full platform status for one train id,' and immediately scopes it to include departed, due_now and platform_withdrawn trains. It does not name the sibling find_train, so the agent must infer the boundary, but the resource and return scope are unambiguous.

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 clearly explains the two identification paths (train_id, or headcode plus time 'HH:MM today') and the conditions under which each fails: several matches return candidates, a headcode alone returns train_needs_time, and an off-board train returns train_not_on_board which is explicitly non-retryable. It gives no explicit guidance on when to prefer this over find_train, which keeps it short of a 5.

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