Prochains RER vers Paris

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

With no annotations available, the description carries the full transparency burden and performs well: it discloses real-time behavior ('Temps réel'), delay reporting, exact output fields, and the RER B/no-shuttle-bus scope. It also explains that default stations are configuration-driven. For a read-only schedule lookup, there are no hidden side effects or auth requirements left undisclosed.

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 definition is a compact set of sentences that front-loads the primary use case and then efficiently lists returned fields, scope restrictions, and default-station guidance. Every sentence contributes operational information with no redundancy or 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 tool with four optional parameters, no output schema, and no annotations, the description provides the essential information: what is returned, real-time behavior, exclusions, and default handling. Count and when semantics are covered by the schema, so nothing critical an agent needs to select or invoke the tool correctly 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?

The input schema already documents all four optional parameters, so the baseline is 3. The description adds valuable guidance beyond the schema by stating that the default stations come from configuration and should only be overridden when the user mentions another station, which directly informs how the agent should populate from/to.

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 exactly what the tool does: for each next train from La Hacquinière to Denfert-Rochereau it gives real departure time and delay, arrival time, duration, mission code, terminus, and disruptions. It also marks itself as the tool for the specific question 'quand est le prochain RER pour Paris et j'arrive à quelle heure?' and limits scope to RER B, distinguishing it from board or journey tools.

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?

It explicitly identifies when to use the tool via the quoted user question and gives exclusions: RER B only and no shuttle buses. It also instructs that default stations come from configuration and should only be specified when the user mentions another station. However, it does not explicitly name sibling tools such as departures_board or plan_journey as alternatives, so some disambiguation remains implicit.

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