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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context beyond this: results are sorted nearest-first, each includes distance_km, and distances are measured to the locality centroid from source data, not to a boundary. This clarifies an important semantic that would otherwise be unknown.

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?

The description is three sentences with no wasted words. The core action is front-loaded, followed directly by example use cases and a single important caveat about centroid-based distances. Every sentence earns its place.

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 read-only lookup tool with no output schema, the description covers the essential usage scenarios, result ordering, and a key distance-calculation caveat. It does not describe the full response shape beyond the presence of distance_km, but with readOnly/idempotent annotations and clear examples, an agent has enough context to invoke the tool correctly.

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 60%, so the schema already describes `limit`, `country`, and `radius_km`. The description adds meaning for `lat` and `lon` as the center point of the search, and hints at `radius_km` via the 'within 10 km' example. However, it does not clarify parameter names or ranges beyond what the schema provides, nor does it explain how `country` or `limit` interact with the result ordering.

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 states a clear, specific action: 'List localities around a latitude/longitude, nearest first, each with `distance_km`.' It also gives example intents like 'postal codes within 10 km of...' and resolving coordinates to the closest postal code. It does not explicitly name a sibling tool as an alternative, but the action and expected output are unambiguous enough to distinguish it from the provided sibling tools.

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 description provides explicit use-case examples ('what ZIP codes are near...', 'postal codes within 10 km of...', 'resolve coordinates to the closest postal code') that tell an agent when to invoke this tool. It lacks explicit when-not-to-use guidance or named alternatives, but the examples are sufficient to select this tool over the siblings in most relevant scenarios.

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