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

Annotations already cover read-only/idempotent/open-world, but the description adds meaningful operational context: price ($0.02 USDC on Base), the x402 payment flow, and that a call returns payment requirements unless payment_signature is supplied. It does not describe rate limits or behavior on invalid/unknown VAT numbers, so not a 5.

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 a category tag, purpose, and usage, followed by pricing and return behavior. Dense but every sentence carries information; the sample path and payment mechanics are useful rather than 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?

With no output schema, the description appropriately explains the two-phase return behavior (payment requirements vs. result) and the payment model. It is complete enough to invoke correctly, though it omits the response shape of a successful VAT verification.

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%, so the schema already documents both parameters, including the country-prefix format and the payment_signature semantics. The description's 'Returns the x402 payment requirements unless payment_signature is given' largely restates what the schema says, so it adds little beyond the baseline.

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 and resource: 'Verify an EU supplier's or customer's VAT number'. It is clearly distinguishable from the sibling check_eori (customs identifier) and supplier_check, giving an agent an unambiguous target.

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 concrete trigger conditions: 'before invoicing or paying', 'when applying the reverse charge', 'onboarding a B2B counterparty in the EU'. It lacks explicit when-not or a named alternative sibling, so it stops 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.