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

Beyond the readOnlyHint annotation, the description discloses that route filtering is client-side, meaning all agency vehicles are fetched before filtering, which is a performance-relevant behavior. It also enumerates returned fields (GPS coordinates, heading, schedule deviation, current trip), adding value beyond the bare annotation.

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?

Two concise sentences front-load the primary function and optional filter, then add a practical usage example. Every sentence contributes, and there is no filler or 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?

For a read-only tool with two parameters (one required), both fully documented in the schema, and an existing output schema, the description covers the essential usage context. It includes the critical client-side behavior and points to agency discovery, leaving no meaningful gaps.

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% — both agencyId and routeId already have detailed descriptions, including the client-side filtering caveat. The tool description adds no new parameter semantics beyond what the schema captures; it only echoes the filter behavior and the agency-listing pointer. This meets the baseline for full schema coverage.

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's core function: returning real-time positions of active vehicles for an agency, with an optional route filter. It is specific about the resource (vehicles) and the output (GPS coordinates, heading, schedule deviation, current trip), and it is easily distinguishable from sibling tools like get_arrivals or get_trip.

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 a concrete use case ('where are all the buses on route X right now?') and directs the agent to onebusaway_list_agencies for agency IDs. It does not explicitly enumerate when not to use this tool or name alternative tools, but the use-case framing gives clear context. Sibling names like get_arrivals imply alternatives without being called out.

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