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

The description goes well beyond the readOnlyHint/idempotentHint annotations by detailing the exact error behavior: 'it returns {"error": "<what is wrong and how to fix it>"}' and never raises a protocol error. It also explicitly states that every call is read-only, idempotent, and safe to retry, which adds actionable behavioral context not present in the annotations alone.

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?

Every sentence earns its place. The description uses a clear opening statement, a concrete example, usage guidance, exclusion from road routing, and error behavior. It is dense but well-organized and does not waste words.

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 tool with an output schema, the description still covers all necessary invocation details: input shape, example, units, limit, error responses, and retry safety. An agent has everything needed to select and call this tool correctly without additional inference.

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 all parameters are documented structurally. The description adds meaning by providing a complete typical input/output example, clarifying the points format ({id, lat, lon}), the unit options, the 200-point limit, and the shape of the returned matrix. This is far beyond the schema's minimal descriptions.

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 opens with a precise statement: 'Straight-line (great-circle) distance matrix from coordinates,' which clearly identifies the computation and resource. It further distinguishes itself from road routing by stating 'Not road routing: real driving distances are longer.' This makes it unmistakable relative to sibling tools like route_plan.

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?

Explicit guidance is provided: 'Use when you have no road matrix and a straight-line approximation is acceptable, or to sanity-check one.' It also states what the tool is not for: 'Not road routing.' This gives an agent clear selection criteria among the sibling tools.

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