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

Annotations already declare readOnly/idempotent/non-destructive, so the safety profile is covered. The description adds useful behavior beyond annotations: no auth required, default direction/time behavior, and the data source (iRail real-time SNCB/NMBS Belgium data). It doesn't contradict 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?

Three compact sentences with the core action front-loaded, followed by return fields, then defaults/auth/source. No filler or redundant restatement of the title.

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 a 100%-documented schema, an output schema, and annotations covering safety, the description adds the remaining context an agent needs: scope (Belgium), defaults, auth status, and data source. Everything needed to invoke correctly is present.

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%, with each of the six parameters documented including patterns, enums, defaults, and examples. The description's mention of defaulting to departures from current time merely restates the arrdep/time schema defaults, so it adds little semantic value beyond the schema. Baseline 3 applies.

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 uses a specific verb ('Get'), names the resource ('real-time departures or arrivals for a Belgian railway station'), and enumerates the returned fields (train number, destination/origin, scheduled time, delay, platform, cancellation, occupancy). This clearly distinguishes it from iRail siblings like connections, vehicle, and disturbances without needing to open the schema.

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: use for real-time departures/arrivals at a Belgian station, defaults to departures from current time, and no auth required. However, it doesn't explicitly state when-not-to-use or name alternatives like irail.transit.connections or irail.transit.vehicle, so exclusion guidance is left to the sibling context.

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