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

Annotations already declare readOnlyHint=true and destructiveHint=false, establishing safety. The description adds behavioral detail: live delays, platforms per leg, and time interpretation in Belgian local time. This goes beyond annotations but does not discuss rate limits or authentication, though those are not critical for a read-only 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?

The description is three sentences with an example, no redundant words. The first sentence front-loads core purpose and outputs. Every sentence adds value: purpose, usage examples, parameter format hint, and a code example.

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?

Given 5 parameters (2 required) and no output schema, the description covers tool purpose, major outputs, parameter usage, and timezone. Missing details include default limit (4) and result ordering, but these are partially addressed in the schema. It is mostly complete for a straightforward read-only 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?

With 100% schema coverage, baseline is 3. The description adds meaning beyond schema by clarifying that depart_at/arrive_by accept ISO datetime or time formats and are interpreted in Belgian local time. It also provides a concrete example of required parameters, enhancing understanding of the from/to naming convention.

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 clearly states the tool plans a train journey between two Belgian stations via SNCB/NMBS, listing specific outputs like legs, transfers, delays, platforms. It includes example questions that make the purpose unmistakable, and the verb 'plan' combined with 'between two Belgian stations' distinguishes it from sibling tools like irail_liveboard (station departures) or irail_disturbances.

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 provides example queries showing when to use ('next train from Brussels to Antwerp'), but does not explicitly state when not to use this tool or mention alternatives like irail_liveboard for single-station departures. The context is clear but lacks exclusionary guidance.

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