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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuine value by disclosing that this is a deterministic pure calculation with no lookup, plus the output shape: a 12-digit v2 code and the 9-digit v1 code being its first 9 digits.

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 tightly written sentences, front-loaded with the transform, then the concrete example and the v1/v2 relationship, closing with the offline behavior. No filler or redundancy.

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?

With no output schema, the description steps in to describe the return values (12-digit v2 plus 9-digit v1 and their relationship), which is the key missing structured information. It is nearly complete; the only residual gap is how coordinates outside any covered region behave, though 'no lookup' suggests a code is always produced.

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% (both lat and lng documented with WGS84 decimal degrees and valid ranges), so the schema does the heavy lifting. The description adds no extra parameter semantics beyond the coordinate pair, making the baseline 3 appropriate.

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: latitude/longitude → WIA Pin Code, with a concrete example format (708-702-851-863). The direction of the transform makes it unambiguously distinct from the sibling pin_code_decode, and it notes the secondary 9-digit v1 output.

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 only implied by the tool name and the inverse sibling pin_code_decode; there is no explicit 'use this when you have coordinates and need a code' statement or exclusion guidance. The 'no lookup / pure calculation' note hints at when it is preferable (offline, no DB) but is framed as behavior rather than selection advice.

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