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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety bar is low. The description adds real value beyond them by disclosing the exact fields returned and asserting authority/recency ('right now', 'AUTHORITATIVE'), which is the key behavioral trait for a live-position feed. It does not discuss staleness windows or rate limits, keeping it short of a 5.

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: the first front-loads what the tool returns, the second front-loads the authority claim and the routing rule. There is no filler or restatement of the name.

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 no output schema, the description usefully enumerates the returned fields, which is exactly what an agent needs to decide whether this answers the question. Only minor gaps remain (whether results are paginated or how fresh the positions are), and the agency/limit semantics are covered by the 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%, so the operator code, line filter, limit and API key are all already documented with examples. The description adds no syntax or format detail beyond the schema, so the baseline 3 applies; the phrase 'the line and trip' merely echoes the existing line parameter.

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?

Specific verb+resource framing ('Where one Bay Area operator's vehicles are right now') with the exact payload enumerated (position, bearing, line, trip, next stop). It explicitly sets itself apart from the sibling stop-level tool, so an agent can distinguish it without opening either 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?

States both sides of the routing decision: 'AUTHORITATIVE for live fleet position' versus 'use bay511_stop_predictions instead when the question is about a specific stop.' That names the alternative and the condition that selects it, leaving nothing to inference.

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