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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint, signaling a safe read-only operation. The description adds value beyond this by disclosing no-auth requirement, a 100 req/min rate limit, the underlying data source (v6.bvg.transport.rest community HAFAS wrapper for BVG), and the specific result contents (lines, transfers, real-time delays, duration). This gives the agent a more accurate mental model of call behavior and expectations.

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?

The description is three sentences, each carrying distinct value: the core action and output, the input provenance, and operational constraints (data source, auth, rate limit). It is front-loaded with the most important information and contains no filler or repetition.

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 journey-planning tool with moderate complexity, the description covers the action, geographic scope, output contents, input sourcing, data backend, authentication, and rate limiting. Since an output schema exists, return-value details are presumably documented elsewhere, so nothing essential is missing for an agent to select and invoke the tool correctly.

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 baseline is 3. The description adds meaning by explicitly naming the tools that produce valid stop IDs for both required parameters ('transport-rest.transit.search_locations or transport-rest.transit.nearby_stops'), which is more precise than the schema's generic 'obtained from' phrasing. This extra guidance helps the agent construct valid input values.

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 opens with a specific verb and resource: 'Plan a public transit journey between two Berlin/Brandenburg stops'. It further details the output components (leg-by-leg routing, lines, transfers, real-time delays, duration) and the geographic scope, which clearly distinguishes it from sibling tools like departures or nearby_stops. There is no ambiguity about what this tool does.

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 clear usage context: it is for journey planning between two stops, and it explicitly names the prerequisite input tools ('Stop IDs come from transport-rest.transit.search_locations or transport-rest.transit.nearby_stops'). However, it does not explicitly state when not to use this tool in favor of alternatives like departures or nearby_stops, so it falls just short of a full when/when-not comparison.

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