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

Beyond the readOnly/idempotent annotations, it discloses non-obvious behaviors: ambiguous names return candidates rather than a guess, via reduces output to one best journey, and results are planned timetable times with Swiss offsets with no live delay data — with the instruction to say so when it matters. It also explains the picture_url artifact and when to offer it. No contradiction with annotations.

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?

Front-loaded with purpose and usage, then inputs, then a single example — a logical order with no wasted sentences. It is on the long side and the picture_url sentence borders on promotional, but each paragraph carries information the agent needs.

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 an eight-parameter planning tool with an output schema, the description covers everything an agent must know to call it correctly: formats, time semantics, mutual exclusions, alternatives limit, and the absence of real-time data. Return-value detail is legitimately delegated to the 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 coverage is 100%, so baseline is 3, but the description adds real value beyond the schema: accepted input formats for from/to (station, address, named place, 'geo:lat,lng', 'stop:8503000'), the departure/arrival mutual exclusion, ISO 8601 Swiss local time with offset conversion, and the bare '08:00 means today' rule. Only transfer_time_factor is left to the schema alone.

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+resource ('Find the best connections by train, tram, bus, boat and mountain railway between two places in Switzerland') and names the data source (Spick Me's routing engine over the official Swiss timetable). This is clearly distinguishable from siblings like travel_time_matrix, reachable_area and plan_day_out, which it also explicitly routes away from.

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?

It gives concrete triggering phrases ('how do I get from A to B', 'I need to be in Bern by 9') and an explicit exclusion: to stop somewhere along the way, use plan_day_out instead. It also tells the agent how to obtain a place_id (find_place) and what an ambiguous name does (returns candidates, no guessing).

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