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

Annotations already provide readOnlyHint=true and idempotentHint=true, but the description adds significant behavioral details: it returns per-stop delay in minutes, platforms, current delay, and live position. It also specifies acceptable ID formats. No contradiction with annotations.

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 long, front-loaded with the core purpose, then detailed outputs, and ends with a concrete example. Every sentence adds value without redundancy.

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 the tool's simplicity (2 parameters, no output schema, but rich return data), the description covers all essential information: what it returns, how to call it with different ID formats, and the source of train numbers. No gaps remain.

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%, but the description adds meaning beyond the schema. It explains acceptable formats for the id parameter (e.g., 'IC 1832', 'IC1832', 'BE.NMBS.IC1832') and provides an example call. For the date parameter, it clarifies the default behavior (today Belgian time).

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 it tracks one Belgian train by number, providing detailed stop times, delays, platforms, and live position. It distinguishes from siblings by mentioning train numbers come from irail_liveboard or irail_journey, making the tool's specific purpose 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?

The description gives clear context for when to use: to check if a Belgian train is delayed. It explains that train numbers are obtained from irail_liveboard or irail_journey, guiding the agent to first use those tools. However, it does not explicitly state when not to use this tool or mention alternatives, though the sibling list provides context.

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