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

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety is covered; the description adds real behavioral detail beyond that: it computes the route itself, only considers stations within roughly 3 km of it, and ranks by total cost including detour rather than sticker price. The 1,200 km ceiling and the returned links (TankoUp, station directions) are useful operational context, though it doesn't say what happens when a route exceeds the limit or no stations qualify.

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?

Three short lines, front-loaded with the core action, then the mechanics, then the constraint — nothing redundant. The line breaks are slightly choppy but the information ordering is sound and 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?

With an output schema present, the description needn't explain return values, and annotations cover the safety profile; the remaining essentials (Spain-only coverage, route-length ceiling, cost-based ranking, 3 km corridor) are all present. It could be more complete by stating failure behavior for over-length routes or unresolvable place names, but the gaps are minor for a 5-parameter read-only tool.

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%, with fuel enum values, limit bounds, and the place-or-lat/lng pattern all documented in the schema itself. The description reinforces 'for one fuel' and the route-length ceiling but adds no syntax or behavioral meaning for limit, open_now, or the location objects beyond what the schema already provides, so 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 ('Find the cheapest petrol stations along a driving route in Spain, for one fuel') and immediately scopes it to a route, which cleanly separates it from the sibling find_cheapest_fuel_nearby. The country restriction and single-fuel constraint are stated up front, so an agent can tell it apart from get_province_fuel_prices and get_station_details without reading the 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?

The phrase 'along a driving route' plus 'Routes up to 1,200 km' gives a clear condition for choosing this tool over the nearby variant. It does not explicitly name the alternative or state when-not to use it (e.g., what to do for a route over 1,200 km, or for a single point), leaving a small amount to inference.

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