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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so the safety profile is covered. The description adds genuinely useful context the annotations cannot: the free daily allowance and the $0.0005 USDC per-call x402 charge, plus that it normalises and splits outward/inward codes rather than only validating.

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 front-loaded sentence with the core function first, the output behavior second, and the cost model last. Every clause earns its place and nothing is padded.

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 an output schema present, return values need no explanation, and the description covers purpose, normalization behavior, and pricing. The only gap is the absence of any statement about invalid-input handling or format expectations, which is minor for a single-parameter validator.

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?

Only one parameter, and schema description coverage is 100%, so the schema already carries the parameter semantics. The description adds nothing about accepted postcode formats, casing, or whitespace handling, so the baseline 3 applies.

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 specific verbs (validate, normalise, split) on a specific resource (UK postcode format), which cleanly separates it from sibling validators like validators__uk-phone, validators__email, and validators__iban. An agent can identify the tool's function without opening the schema.

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 implied by the resource name — validate/normalise a UK postcode — but there is no explicit when-to-use guidance, no mention of how it differs from related validators, and no stated behavior for invalid input. The pricing sentence is a cost note, not usage guidance.

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