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

With no annotations, the description carries the full burden and does so: it discloses that validation is offline and deterministic with no network call, that coverage is bounded to check-digit algorithms 'where one exists', that the response distinguishes check-digit verification from format-only, and it discloses the $0.005 per-call x402/USDC cost. Those are exactly the traits an agent needs before committing a paid call.

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?

Front-loaded with the trigger condition, then scope, then cost. Every sentence carries distinct information - coverage limits, determinism, result semantics, pricing - with no restatement of the name or title.

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 must convey return semantics, and it does so at a high level by promising an explicit distinction between check-digit and format-only outcomes. It does not say what happens for a malformed or non-EU input (error vs. invalid verdict), which is the one piece an agent would still have to discover by calling.

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 the schema already covers it at 100% including the tolerance for missing country prefix and embedded spacing. The description adds no syntax detail beyond the schema, but with a single fully documented parameter the baseline of 4 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 a specific verb (validates) and resource (one EU VAT number) plus the exact scope of the check: country prefix, country format, and published check-digit algorithm. It is clearly distinguishable from sibling util_iban and the red_* lookup tools, which do network-based validation of other identifiers.

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?

Gives an explicit decision context ('when an agent must decide whether a company's tax number is usable before invoicing') and a usage pattern ('cleaning a list of companies one at a time'). It does not name an alternative tool or a when-not condition, so it falls short of the top band but leaves no ambiguity about the situation it is for.

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