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

With no annotations present the description carries the full load, and it discloses the upstream dependency (VIES), that format validation happens before the lookup, the geographic coverage limits, and that name/address are only returned where the member state shares them. It also flags a real cost of $0.003 USDC on Base via x402 — critical context for an agent deciding to call it. Missing only error/failure behavior and rate limits.

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?

Four tight sentences, front-loaded with what the tool does and what it returns, followed by coverage caveats and cost. Every sentence carries distinct information with no repetition of the name or filler.

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?

For a two-parameter, no-output-schema tool, the description covers return content (status, name, address), coverage boundaries, and the x402 payment requirement. Only the shape of failure or partial responses is left implicit, which is a minor gap for a validator of this simplicity.

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%, with both 'vat' and 'country' documented in the schema itself, so the description needs to add little. It adds only that country prefix matters and implies the country param is a fallback, which does not go beyond the schema. Baseline 3 is appropriate.

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 (EU VAT number) plus the authoritative data source (European Commission VIES). It also enumerates what comes back — registration status, company name, and address — which no sibling tool offers, so the agent can distinguish it from company_enrich or email_validate at a glance.

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 clear operating conditions: works for the 27 EU states and Northern Ireland (XI), while GB numbers only get a format check, so an agent knows when the answer will be partial. It does not name an alternative tool or state when not to use it, though for a narrowly scoped VAT validator there is little ambiguity to resolve.

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