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

Annotations already establish readOnly/idempotent/non-destructive/openWorld, so the safety profile is covered. The description adds genuine value beyond that: it is a handoff only, dates/passengers must be confirmed on the partner site, the card merely preserves the requested date, and no live fares, availability, or tickets are provided. That is real behavioral disclosure an agent needs.

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 the core purpose, then routes, then caveats — a sensible order with no filler. The Omio parenthetical (Madrid-Barcelona vs. Tokyo-Kyoto) is slightly verbose and the Tokyo-Kyoto exception is a touch confusing against the Asian/12Go rule, costing a point but not the overall structure.

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 8-parameter handoff tool with no output schema, the description covers what the return is (a partner-page handoff), what regions produce nothing, and what the tool does not provide. Nothing critical is missing, though the exact card payload shape is only loosely described.

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 schema already documents all 8 parameters, including the currency fallback chain and the trip_id defaulting behavior. The description only gestures at date/passenger handling ('Confirm dates and passengers on the partner site'), adding little parameter-level meaning beyond the schema. Baseline 3 is appropriate.

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?

Opens with a specific verb and resource ('Return a train or bus search handoff between two cities, routed by region'), which an agent can immediately distinguish from the sibling flight_booking_link, car_rental_booking_links, and ground_transfer_booking_links. It also states the scope (two cities, no tickets) so the tool's boundary is clear 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 concrete when-to-use context via region routing: Asian routes go to 12Go, European routes to Omio, and other regions such as the Americas return no handoff — effectively a when-not-to-use rule. It does not name a sibling alternative explicitly, so it falls short of a 5, but the regional exclusions are actionable.

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