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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds real value beyond that by disclosing the actual computation: arrival 2 hours before departure, plus drive time, plus 10 minutes spare, making the result predictable without invocation.

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?

One sentence with a front-loaded usage condition and a parenthetical that carries the entire algorithm. No filler, no restatement of the title or annotations.

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?

The description explains the core calculation but says nothing about what the tool returns (a lone time? a full itinerary?) or how terminal_id affects the drive component, and with no output schema the description carries that burden. For a 5-parameter tool it is adequate but leaves real gaps.

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 every parameter is already documented, and the baseline of 3 applies. The description restates the 2-hours-before-flight rule but does not explain how the optional at_airport_by override interacts with that default, even though the schema flags it as a user-supplied alternative.

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 gives a specific verb and computed output ('works out the pickup time') and scopes it to a narrow scenario ('a trip to Gatwick to catch a flight'), which distinguishes it from quote/booking siblings. It stops short of naming an alternative tool, but the scope restriction does the differentiation work.

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?

'For a trip to Gatwick to catch a flight' is an implicit when-to-use condition, so the intent is inferable. There is no explicit statement of when not to use it, no mention of prerequisites such as needing a pickup_id from find_place first, and no routing to get_quote or request_booking.

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