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

With no annotations, the description carries the full burden. It usefully discloses cost ('FREE') and the Japan-only coverage constraint, which are genuine behavioral facts. It omits what happens for out-of-bounds coordinates (error vs null), precision of the municipality match, and any rate limits — meaningful gaps for a lookup tool with zero annotation coverage.

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?

Two short clauses, front-loaded with the cost signal and then the action, scope, and outputs. No filler; every element 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?

No output schema exists, so the description is the only place return values are described — it names the two outputs but not their shape, encoding, or fallback behavior when no municipality matches. Adequate for a simple 2-param lookup but leaves real gaps.

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 0%, so the description must compensate. It adds the important constraint that coordinates must be within Japan, but gives no datum, decimal-degree convention, or accepted range for lat/lon. The parameter names are largely self-explanatory, which keeps this at an adequate 3 rather than lower.

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 (reverse-geocode) and resource (lat/lon) and names the exact outputs (municipality code, local name) plus a geographic scope (Japan). It implicitly distinguishes itself from batch_reverse (single point) and reverse_with_elevation (no elevation), but never names those siblings, so an agent must infer the boundary.

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 'in Japan' qualifier and the singular lat/lon imply when this tool applies and that out-of-Japan coordinates are out of scope, but there is no explicit when-not clause or pointer to batch_reverse for multiple points. Usage is implied rather than stated.

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