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

The annotation readOnlyHint=true already declares the tool read-only, so the description's main contribution is specifying the return data (position, deviation, phase, remaining stops) and noting that includeSchedule defaults to true. It also discloses that serviceDateMs is only needed for previous service days. These details add context beyond the annotation, helping the agent understand what to expect without contradicting it.

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 sentences with zero fluff. The first sentence immediately conveys the verb, resource, and output, while the second provides actionable usage guidance. Front-loaded with the core purpose, every word earns its place.

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 3 parameters (1 required), a rich output schema, and read-only annotation, the description is sufficiently complete. It explains what the tool returns and how to source the input. It could optionally mention that tripId is required, but that is already in the schema. The absence of pagination or error details is acceptable given the tool's scope and the presence of an output schema.

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%, with each parameter (tripId, serviceDateMs, includeSchedule) already documented. The description adds minimal extra meaning: it mentions where to obtain tripId (arrivals) but does not elaborate on parameters beyond what the schema provides. Per the guideline, a baseline of 3 is appropriate when the schema does the heavy lifting, and the description offers no further parameter insight.

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 purpose: 'Real-time status and stop sequence for a trip' and enumerates the specific outputs (vehicle position, schedule deviation, current phase, remaining stops). It also differentiates from siblings by focusing on a single trip and explicitly referencing onebusaway_get_arrivals as the source of tripId, making it distinct from the many search/list 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?

The description gives a concrete usage scenario: 'Use tripId from onebusaway_get_arrivals to look up a specific vehicle's progress.' This implies when to use the tool (when you have a tripId from arrivals) and clarifies the relationship with a sibling. However, it does not explicitly state when not to use it or compare with other trip-related tools (e.g., get_block, get_route), so a clear exclusion is missing.

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