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

Annotations already cover read-only, open-world and non-destructive safety. The description adds genuinely valuable behavior: an empty-cache case that triggers background pricing and returns only a 'pricing started' notice, with a retry-after-a-minute hint. That async/latency trait is the kind of thing an agent cannot get from the schema or 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?

Two sentences, front-loaded with the return contents, and the retry caveat comes last. The middle enumeration ('cheapest one-way ... and the cheapest round trips') is somewhat list-heavy but each clause is informative rather than filler.

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 values needn't be spelled out, and the definition still covers the important edge case (no cached data yet). Combined with the safety annotations, an agent has enough to call it correctly. The only gap is the lack of routing guidance among siblings.

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 from/to and month are already documented in the schema. The description adds a little framing ('by departure date and by month') but no syntax or format meaning beyond the schema's 'YYYY-MM' note. 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?

The description states concretely what the tool returns for one airport pair: cheapest one-way, cheapest 3-21 day round trip, cheapest one-way by departure date and by month. That is a specific, enumerable resource. It does not name or contrast against the siblings cheapest_routes or find_airports, so it stops short of 5.

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?

There's useful situational guidance: if nothing is cached the tool starts pricing and the caller should ask again in a minute. But there is no explicit when-to-use-vs-alternatives routing (e.g. when to prefer cheapest_routes), so the agent must infer the boundary from the siblings' names.

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