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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered; the description adds genuinely new context beyond that — fares are cached, quoted in GBP while the airline bills in its own currency, and Slashtel is not a seller. It also discloses the 12-vs-3-month coverage ceiling and that a citable source page is included. Freshness/staleness of the cache and failure behavior are the only notable omissions.

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?

Content is front-loaded: what is returned comes first, then input format, then the currency/seller caveat. The sentences are long and clause-heavy, but each carries distinct information (return fields, coverage window, currency caveat, citation source) 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?

With only two required parameters and no output schema, the description does the work of describing the response shape (price, airline, stops, cheapest month, source page) plus currency and coverage caveats, which is nearly everything needed to call and interpret the tool. Only error/no-route handling and result ordering are left unstated.

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% and both parameters are already documented as 'IATA airport or city code' with examples. The description reinforces that city codes are acceptable (LON, DEL, DXB, NYC, YTO, MNL), which is a marginal restatement rather than adding format rules, validation constraints, or case-sensitivity guidance. Baseline 3 applies.

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 ('Cheapest one-way fares by month for a route') and enumerates exactly what is returned: price, airline, stops per month plus the cheapest month. An agent can immediately tell this is a monthly fare-lookup tool, and nothing in the sibling list (eSIM, top-up, checkout, price alerts) overlaps with it.

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?

Usage is implied by the framing rather than stated: the description explains coverage depth differs between tracked routes (12 months) and any other route (3 months), which is a useful condition for interpreting results. However it never says when to prefer this over a price-alert or checkout flow, nor what to do if a route is untracked or codes are invalid, leaving prerequisites to inference.

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