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

The annotations already establish a safe, idempotent, open-world read, so the bar is lower. The description nonetheless adds genuinely useful context beyond them: the 'live' flag semantics (true when the TTC actually tracks the vehicle, otherwise the time derives from the timetable) and the shape of each arrival entry.

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 first and the route-filter caveat last; nothing is padded. Minor cost: 'the TTC' and 'as_of' are used without definition, which forces the reader to infer the operator and the reference timestamp.

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?

An output schema and rich annotations are supplied, so return values and safety profile need not be re-explained. The description covers the one non-obvious behavior (live vs timetable-derived times) and is adequate for a 4-parameter read tool; only the sibling differentiation and locale context are thin.

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%, so the schema already documents stop (id vs code), limit (1-30, default 10), route, and language. The description restates the route-filter behavior already present in the schema and introduces 'as_of' as an unexplained reference point that is not an input parameter, adding a small amount of confusion rather than meaning.

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 and resource ('Lists the next vehicles of the TTC at a stop') and enumerates the returned fields (route number, kind, headsign, minutes, expected instant). It never names or distinguishes itself from the closely related siblings transit_get_timetable and transit_plan_trip, so an agent must infer the boundary from the name alone.

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?

Usage is only implied: 'next vehicles' signals real-time arrival lookups, and the route clause explains behavior when a route is supplied. There is no explicit statement of when to prefer this over transit_get_timetable (scheduled departures) or transit_plan_trip, and no prerequisites or exclusions are given.

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