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

Beyond the annotations (readOnly, idempotent, non-destructive, closed-world), it discloses real behavioral traits: 158-country coverage, handling of no-postal-code countries, a pattern-only scope, and the meaning of high/medium/low data_confidence tiers. Some return-field enumeration overlaps with the output schema, but the confidence semantics and explicit non-guarantees add genuine value.

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 core purpose is front-loaded in the first sentence, and every part of the paragraph carries useful information. The response-field listing (estimated_items[], sources[], data_version) is somewhat dense and could have been shortened by referring to the output schema, but it does not derail the overall efficiency.

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 annotations, a full input schema and an output schema, the description supplies the missing behavioral context: coverage, confidence tiers, limitations and a concrete example. It is complete for an agent to call correctly, though the response-field detail is somewhat redundant given the output schema.

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 100%, and both parameters are fully documented in the input schema. The description adds only an illustrative example ('11564', 'SA'), which slightly reinforces the intended format but does not provide syntax or constraint details beyond what the schema already states.

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 names a specific verb (validate/check) and resource (postal/ZIP code format) and scopes it to 158 countries, which clearly separates it from the IBAN, BIC, phone and generic identifier validators in the sibling list. However, it never explicitly names an alternative tool or routing rule, so it stops short of the 5-level distinction.

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?

It gives a clear usage boundary with 'Pattern check only' and the disclaimer that it is 'never proof that an identifier is issued or belongs to someone', which tells the agent when not to rely on it. It also notes behavior for countries without postal codes, though it does not explicitly direct the agent to sibling tools for other identifier types.

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