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 enumerates the return statuses including the member_state_unavailable downtime case with a retry instruction, warns that name/address may be withheld by some states (DE/ES), and notes consultation_number only appears when requester is set. That is exactly the failure-mode and edge-case disclosure an agent needs.

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 purpose, then usage, then input format, then returns, then alternatives — a logical order with no filler sentences. It is dense and slightly long, but every clause adds actionable information.

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?

No output schema exists, so the description must describe returns, and it fully covers status values, optional name/address, and the audit-proof field. Combined with input format guidance and sibling routing, an agent has everything needed to call and interpret this tool.

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 0%, so the description must compensate. It does well for 'vat' by specifying the country-prefixed format with concrete examples and tolerating spaces/dots, but 'requester' is only implied through 'consultation_number ... when requester is set' — it never says what a requester is or that the default is a VAT number.

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+resource (validate an EU VAT number) and names the exact upstream authority (European Commission VIES, 27 states + XI). It also distinguishes itself from siblings by naming HMRC for GB and verify_batch for multiple numbers.

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 triggering conditions ('asked whether a VAT ID is valid/active', 'before zero-rated reverse-charge B2B invoices') and explicitly routes two adjacent cases to alternatives (GB -> HMRC, >1 number -> verify_batch). Nothing is left to inference.

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