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

With no annotations present, the description carries the full burden, and it does disclose one real behavioral trait: both great-circle and rhumb-line routes are produced. However, it says nothing about output units, spheres vs. ellipsoids, validation failures on malformed coordinates, or the fact that this is a pure side-effect-free computation.

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 front-loaded sentence with the verb and the two distance models up front and zero filler. Nothing could be removed without losing meaning.

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 two-coordinate computation the description covers the essential scope, and the presence of an output schema means return values need not be enumerated. The remaining gaps, namely units and sibling routing, are modest for a stateless geometry helper.

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?

The reported top-level schema coverage is 0% and the description adds no parameter guidance at all, so by the stated rule it should compensate and does not. It is rescued from a lower score because the nested lat/lng properties carry unusually rich format documentation (signed decimal, hemisphere letters, DDM, DMS, and the sign-plus-hemisphere prohibition), leaving little for the description to add.

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?

Specific verb (calculate) plus a precise resource (great-circle and rhumb-line routes between two GPS coordinates), which tells the agent exactly what is computed and that two distance models are involved. It does not distinguish itself from siblings such as calculate_bearing or calculate_cross_track_error, 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?

The phrase 'between two GPS coordinates' implies the input shape but gives no when-to-use guidance, no prerequisites, and no routing to the four closely related sibling tools. The agent must infer that this is the tool for distance as opposed to bearing or cross-track error.

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