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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered. The description adds genuinely useful behavior beyond them: an explicit metered cost of $0.005 USDC per call via x402, plus a concrete list of what is returned (distance in km and miles, travel time, turn-by-turn steps, optional GeoJSON line).

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 dense sentence, front-loaded with the primary purpose and feature list, with the cost caveat appended last. No filler, though the length makes it slightly heavy for one sentence.

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 output schema, the description carries the return-value burden and does so: it names distance, travel time, turn-by-turn steps and optional GeoJSON, and discloses the per-call cost. Nothing an agent needs to invoke this 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 71% and the required from/to parameters carry no schema descriptions, so the description's clarification that they accept 'addresses, names or lat,lon' adds real value. It also corroborates via, mode, avoid, units and geometry semantics, though it adds no syntax or format detail for the stop separator beyond what the schema states.

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+resource ('Directions between places') and enumerates the computed outputs (distance, travel time, turn-by-turn steps) plus supported modes. It is highly specific, though it never names or distinguishes itself from the routing siblings route_matrix and isochrone.

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 mode and avoid lists imply when particular options apply, and the address/name/lat,lon notation tells the agent what input forms are accepted. However, there is no explicit when-to-use guidance and no pointer to route_matrix for many-to-many queries or geocode for resolving ambiguous names.

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