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

Annotations already declare readOnlyHint=true, openWorldHint=true, and destructiveHint=false, so the safety profile is covered. The description adds genuine behavioral value by disclosing coverage breadth and input tolerance for spaces/dashes, but it also advertises a two-code distance capability the single-code schema cannot support, which muddies rather than clarifies behavior.

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 first three clauses ('Place of a postal / ZIP code', 'Look up a postal code', 'Postal code lookup...') restate the same idea before any distinguishing detail arrives. The useful content (coverage, returned fields, input tolerance) is back-loaded, so the opening is not front-loaded with value.

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 only two required parameters, no output schema, and read-only annotations, the description does enough to convey return fields (place name, region, province, coordinates). However, the unsupported 'distance between two postal codes' and 'validation' claims leave the agent unsure whether those are real modes of this tool or separate siblings.

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 adds format tolerance ('accepts codes with or without spaces and dashes') beyond the schema's terse examples for 'code'. It adds nothing about the 'country' parameter's ISO format, but the schema already handles that.

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 gives a concrete verb+resource (look up a postal/ZIP code) and enumerates the returned fields plus coverage (121 countries, 1.8M codes), so the agent knows exactly what this tool produces. It does not, however, differentiate itself from plausible siblings such as reverse_geocode, city_search, or distance_between_places, and its mention of 'distance between two postal codes' blurs that boundary further.

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?

There is no explicit when-to-use guidance and no named alternative among the ~45 siblings. The claim that it computes 'distance between two postal codes' actually invites confusion, since the schema accepts only a single code and country, so an agent gets no signal about when to prefer this tool over distance_between_places.

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