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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint=false, so the safety profile is covered. The description adds genuinely useful context beyond that: the Vincenty-with-haversine-fallback algorithm, supported output units, and a concrete pricing/auth model ($0.05 per successful call via x402 USDC on Base).

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?

Front-loaded with the core operation and resource, then operations, then results and pricing. Dense but every clause carries information; the pricing sentence is non-obvious and worth its space, though the first sentence is long enough to require re-reading.

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?

No output schema exists, so the description must explain returns, and it does specify the result units and the quantities produced (distance, bearings, midpoint, destination, cumulative path length). The only gap is that it never clarifies the zero-required-parameter design or how invalid combinations of op and inputs are rejected.

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%, so the baseline is 3; the description earns above that by tying parameters to outcomes (e.g., 'destination point reached from a start point on a given bearing and distance') and stating that results come back in metres, kilometres, nautical miles and miles. It adds cohesive meaning without duplicating schema syntax.

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 ('distance and direction between WGS-84 coordinates') and enumerates the five operations (distance, destination, midpoint, path) plus the algorithm (Vincenty with haversine fallback). This clearly differentiates it from siblings like point_in_polygon and geohash_encode_decode.

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?

Usage is implied by the operation list and the schema's op enum, so an agent can infer when each mode applies. However, there is no explicit when-to-use guidance, no mention of prerequisites, and no routing to alternative sibling tools for related geo tasks.

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