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

With no annotations, the description carries the full burden. It usefully discloses what comes back (registered name/address plus a VIES consultation number as audit proof), but says nothing about behavior on invalid numbers, error cases, rate limits, or the payment-requirement handshake implied by payment_signature. Solid on output content, thin on operational traits.

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?

Two dense sentences with the purpose front-loaded, then the usage condition and the proof variant. Every clause earns its place; nothing is padded or redundant.

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 no output schema, the description partially compensates by describing the return (name/address + consultation number) and the audit-proof path. It leaves the payment_signature flow and failure behavior unaddressed, but for a 3-parameter validation tool it is nearly complete.

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%, so the baseline is 3. The description adds business meaning to the proof parameter ('official VIES consultation number as audit proof') beyond the schema's generic 'higher price' framing, lifting it above baseline, though vat and payment_signature are not elaborated further.

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/confirm) and resource (VAT number) with explicit geographic scope (EU 27 + XI and UK). An agent immediately knows this is a read/validation tool for VAT numbers, not a general lookup or write operation.

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 concrete when-to-use conditions ('before reverse-charge invoicing or B2B onboarding') and ties a variant (proof=true) to the audit-proof use case. There are no siblings to route against and no explicit when-not guidance, so it falls just short of a 5.

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