plan_journey

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

Annotations already declare readOnly/idempotent/openWorld, so the bar is lower, and the description still adds real value: the geographic coverage boundary, the precise meaning of the 422 outside_coverage error, and the fact that live bus positions are included (implicitly real-time data). It doesn't discuss limits, result counts, or how live data ages, so it isn't exhaustive.

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?

Four tight sentences, front-loaded with the core purpose before the caveats. The parameter semantics are packed into clauses rather than prose padding; every sentence earns its place, though the density is high.

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 exists, so return format need not be described, and the description covers coverage limits, error semantics, and the two non-obvious parameters. The main remaining gap is the undocumented limit and include_steps parameters for an 8-parameter tool.

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 0% across 8 parameters, so the description must carry the load. It usefully documents datetime (ISO 8601, defaults to now) and arrive_by (interpret datetime as latest arrival), but says nothing about limit, include_steps, or the four required coordinate parameters, leaving half the surface undocumented.

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?

States a specific verb+resource (public transport journeys between two points) and enumerates what the result contains (walking, fares, live bus positions). This clearly separates it from siblings like live_buses, nearby_stops, or rail_departures, so an agent can route without opening the 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?

Gives a concrete when-not-usable condition: coverage is limited to South/West Yorkshire and West Sussex, and elsewhere returns 422 outside_coverage, explicitly clarifying that this means 'not covered' rather than 'no route'. It stops short of naming alternative sibling tools for uncovered areas or for adjacent needs.

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