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

Beyond the readOnlyHint/openWorldHint annotations, it discloses that results are cached recent data rather than live fares, lists what is returned, and spells out the cost model ($0.002 USDC per successful call via x402 on Base, Solana or Arc, failures not charged). The payment and recency semantics are exactly the kind of behavior annotations cannot convey.

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?

The core purpose is front-loaded and the dense parentheticals are informative rather than filler. The related-tools sentence is compressed into a slightly awkward fragment ("...: lowest;") that reads like schema notes, but no sentence is wasted.

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?

For a two-parameter read-only lookup with an output schema, the description covers source, data recency, inputs, cost, and alternatives – everything an agent needs to decide and invoke correctly.

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% and both parameters already document the momondo URL-style city format with examples, so the description's "City names as in momondo URLs (new-york-city, london)" largely restates the schema. Baseline 3 is appropriate since the schema carries the parameter burden.

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 names a specific source and resource (momondo public route page) plus the exact outputs (lowest recent price, up to 20 round-trip fares, deal dates, route facts). It also distances itself from sibling tools by naming travel_skyscanner and travel_kayak, so an agent can select it without reading either schema.

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 states an explicit exclusion ("Not a live date search") and gives conditional routing to two alternatives ("travel_skyscanner when you need Skyscanner flight prices...", "travel_kayak when you need KAYAK flight route overview"). The when-to-use boundary is unambiguous.

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