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

With no annotations at all, the description carries the full behavioral burden. It does disclose non-obvious behavior: the date default is Taipei-local today, and times from 24:00–29:59 denote the early hours of the day after the operating day — a genuinely valuable quirk. It says nothing about read-only nature, rate limits, or result ordering/volume, which leaves meaningful gaps for a schedule-query tool.

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 compact sentences with the core action first, followed by the parameter conventions, plus a short English gloss. Every clause carries information — station format, date default, time format, and the 24:00–29:59 rule — with no filler.

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 four-parameter read tool with no annotations and no output schema, the description covers the inputs, the default behavior, and the unusual time notation, and '時刻與車種' hints at the returned content. The remaining gap is that the shape of the result (list of departures, per-train fields) is not described at all, which matters since no output schema exists.

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 only 25%, so the description must compensate, and it largely does: it clarifies that origin/destination accept either station names or codes, explains the date default, and restates the HH:mm plus extended-hour convention for time. All four parameters get some semantic treatment, though it does not say whether origin and destination are symmetric or how ambiguity in station names is resolved.

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 states a specific verb and resource: query TRA (臺鐵) timetable between an origin and destination station on a given date, returning times and train types. 'TRA OD timetable with train types' reinforces the scope and implicitly separates it from the sibling thsr_timetable (high-speed rail) and tra_live (real-time). It does not explicitly name those siblings, so differentiation is inferred rather than stated.

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?

Useful operating context is given: stations may be supplied as names or codes, and the date defaults to 'Taipei today' if omitted. However, there is no explicit when-to-use vs. when-not guidance, and no pointer telling the agent to prefer tra_live for real-time information or thsr_timetable for high-speed rail. The agent must infer the boundary from the tool name alone.

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