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

With no annotations, the description carries the full burden and does disclose two useful behaviors: unsupported country codes produce a caller error (400), and the endpoint is metered (5 free trial calls per signed wallet). It says nothing about the success response shape (boolean vs parsed/normalized code), case sensitivity, or what happens when the value is malformed but the country is valid.

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?

A single compact sentence, front-loaded with the verb and resource, followed by the supported-country list and the error rule. The trailing promo fragment about free trial calls is marketing noise rather than tool semantics, which keeps it from a 5.

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?

There is no output schema, so the description should explain what a successful validation returns, and it does not. For a two-parameter validation endpoint with zero annotation and zero schema-description coverage, the definition covers input country scope and one failure mode but leaves the result contract and value formatting unspecified.

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 partially does by enumerating the accepted country values (us, uk, ca, fr, de, jp) and the 400 failure for unknown countries, which is meaningful semantic content. However, the opaque parameter name 'value' is only obliquely explained ('a postal code'), and no format, casing, or length expectations are given for either parameter.

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 ('Validate') and resource ('Postal Code') and narrows scope to six countries (us, uk, ca, fr, de, jp), which lets an agent tell it apart from generic validators like x402-address-validate. It does not distinguish itself from the closely named sibling x402-postal-code, leaving ambiguity about which of the two to pick.

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: 'Check a postal code against the pattern for country X' suggests calling it when you have a code plus a country. There is no explicit when-to-use/when-not, no steering toward sibling tools such as x402-zip, x402-postal-code, or x402-address-validate, and no prerequisites stated.

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