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

Annotations already cover readOnly, idempotent, and non-destructive traits. The description adds valuable non-annotation context: 'No auth required,' 'real-time SNCB/NMBS data,' a hard output ceiling of 10 journeys, and support for departure or arrival time selection. This is consistent with the annotations, so no contradiction.

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?

Three sentences with no filler. The first sentence states the action and enumerates the returned data, the second covers time-selection behavior, and the third provides auth and data-source context. Every sentence earns its place and the key verb phrase is front-loaded.

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 7-parameter tool with a fully described schema, an output schema, and safety annotations, the description finishes the picture: it specifies return fields, time-selection semantics, auth requirements, and the data source (iRail real-time SNCB/NMBS). Nothing needed to correctly invoke the tool is missing.

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 description coverage is 100%: every parameter (from, to, date, time, lang, results, timesel) has its own description with format, defaults, examples, and constraints. The description adds only a high-level mention of departure/arrival time selection, which maps to timesel, so it does not need to compensate for the schema.

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 opens with a specific verb and resource: 'Find train connections between two Belgian railway stations.' It enumerates the distinct output fields (departure/arrival times, duration, delays, vias, train numbers, platforms, cancellation flags), which clearly distinguishes it from sibling tools like irail.transit.liveboard (single-station departures) and irail.transit.vehicle (one train's details).

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 clear context for when to use this tool: when the agent needs point-to-point journeys between two Belgian stations, with optional departure/arrival time selection. However, it does not explicitly name alternatives such as irail.transit.liveboard for station departures or state when NOT to use this tool, so it stops short of full exhaustiveness.

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