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

Annotations already declare readOnlyHint, openWorldHint=false and destructiveHint=false, so safety is covered. The description adds genuinely non-obvious behavior: prices are cached observations 'as last seen on the date in `checked`', not live quotes, and the round-trip component only appears 'when we hold one' — both material to how an agent should present results.

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 what is returned before the filtering advice and the pricing caveat. The opening sentence is long and parenthetical-heavy, but every clause carries information and nothing is redundant with the schema.

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?

With an output schema present, the description is not obligated to detail return values, yet it still characterizes the shape of a result (one-way plus optional round trip). Combined with the staleness caveat and filter semantics, an agent has enough to call and interpret the tool; only sibling routing is left implicit.

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 baseline is 3, but the description goes further: it explains that `from` and `to` are interchangeable anchors ('give from, to or both'), that the month/max_price filters apply to the one-way leg only, and that the `return` companion is a 3-to-21-day stay. That clarifies cross-parameter interaction the schema does not express.

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 and resource: the cheapest cached fare on every route from an origin (or into a destination), with one-way first and a round-trip companion in `return`. The scope is unambiguous and clearly a list-over-routes tool rather than a single-route lookup, though it never names `route_fares` or `find_airports` to sharpen the contrast.

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 one useful directive: filter by max price and departure month via the parameters 'rather than by reading the list.' That is guidance about how to invoke, not about when to choose this tool over its siblings, and no prerequisite or exclusion conditions are stated.

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