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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, open-world), so the description is free to add data-provenance context — 'cached from Travelpayouts' tells the agent results may be stale, and 'cheapest known' warns the value may be absent for a given pair. It does not say what happens on a cache miss or how frequently the cache refreshes.

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?

A single front-loaded sentence that packs scope, unit, date window, and data source with no redundant phrasing. Nothing could be removed without losing information.

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 three required parameters, no output schema, and rich annotations, the description covers the inputs well but says nothing about the returned value — currency, whether a missing fare yields null, or how many results come back. That is a meaningful gap given the absence of an 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 the baseline is 3; the description earns above that by clarifying that the fare is return (round-trip) and per person, and that 'friday' anchors a Friday-to-Monday weekend rather than being an arbitrary date. It still adds no format or validation detail beyond the schema's YYYY-MM-DD note.

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 sentence names the resource (cheapest known return fare per person), the inputs (two IATA codes), and the temporal scope (Friday-to-Monday weekend), so the agent knows exactly what is returned. It doesn't explicitly name the sibling estimators (estimate_all, estimate_itinerary), though 'cheapest known' versus 'estimate' implicitly separates a cached fare lookup from a computed estimate.

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 only implied: the word 'cached' hints at when this is appropriate (a quick lookup rather than a fresh computation), and the Fri–Mon constraint signals when it is not applicable. There is no explicit statement of when to prefer estimate_all or estimate_itinerary, and no prerequisites are given.

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