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

Annotations already cover safety and idempotency (readOnly, idempotent, openWorld, non-destructive), so the description's job is to add operational context, and it does: an explicit $0.001 cost, a no-charge guarantee when no route is found, and the coordinate-vs-address input constraint. It does not describe the response shape (no output schema exists), which leaves the agent guessing what fields come back.

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 sentences, front-loaded with what the tool does, then cost, then the geocoding prerequisite. Every sentence carries distinct information with no repetition or 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?

For a five-parameter, nested-object tool with no output schema, the definition covers cost, failure charging, input type, and the address workaround. The notable remaining gap is the return value: with no output schema, the description should say what a result contains (distance value, duration, units echo) for full completeness.

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 60% and the schema already documents units, vehicle (including the direct behavior), and the nested lat/long bounds. The description reinforces the vehicle=direct semantics but adds nothing about 'priority' (fast vs short) or the required origin/destination pairing beyond what the schema states. Baseline 3 fits given the schema does most of the work.

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 verb and resource: road distance and travel time between two latitude/longitude points, plus the straight-line variant when vehicle=direct. It is immediately clear what the call returns conceptually. It does not explicitly contrast itself with the sibling 'route' tool, which is the main nearby alternative, so it stops short of a 5.

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 gives a concrete prerequisite: 'To start from addresses, geocode them first', which routes the agent to the correct sibling for address input. It also implies the direct-vs-road choice. It does not state when to prefer this over 'route' or when not to call it, so it lacks explicit exclusions.

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