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

With no annotations, the description carries the full behavioral burden and does so well: it discloses that prices are cached snapshots and never live quotes, dictates the correct phrasing ('last observed', never 'current price'), and instructs surfacing live_check_url to any user about to book. That is exactly the data-freshness and downstream-action context 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-loads the return payload, then usage triggers, then the directional/required-argument constraints, then the caching caveat. Every sentence carries distinct, non-redundant information with no 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 no annotations and no output schema, the description compensates by describing what comes back (cheapest fare, six-month low, observation date, live_check_url) and how to present it. Nothing needed to call or interpret the tool correctly is missing.

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 adds semantics the schema does not: the relationship is directional and per-pair (PAR to BKK is not BKK to PAR, and is not PAR to SIN), and both arguments are jointly required. That materially affects correct invocation.

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 (Returns), resource (cheapest observed one-way fare between two cities), and even enumerates the payload (six-month low and its observation date). It is trivially distinguishable from list_routes_from and resolve_city.

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 explicit trigger conditions: the user names an origin and destination and asks about price, cheapest date to fly, or whether a fare is a good deal. It also flags that both arguments are required and neither alone works. It stops short of naming alternatives or when-not-to-use cases in the description text (the resolve_city routing lives only in the schema).

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