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VerifyDesk

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Validate an EU VAT number (VIES)

verify_vat

Validates an EU VAT number live against VIES (European Commission) and returns whether it is currently valid, with the registered name and address when the member state provides them. Input: the full VAT number with its country prefix, e.g. 'FR40303265045'. Optional: requester_vat_number, your own EU VAT number; VIES then issues a consultationNumber, its official proof that you made this check at checkedAt. If VIES or the member state is down, returns the last answer VIES gave us for that number within the past 7 days, marked stale: true with its original checkedAt and never a consultation number; with no such answer, a retryable error. Fresh answers carry stale: false. Light read: costs 0.2 credit.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
vat_numberYesEU VAT number to check, with its 2-letter country prefix (EL for Greece, XI for Northern Ireland), e.g. 'FR40303265045'. Not an IBAN or a company registration number.
requester_vat_numberNoOptional: YOUR OWN EU VAT number, with country prefix, e.g. 'DE811569869'. When given, VIES returns a consultationNumber proving the check. VIES refuses a requester it does not know as valid.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changed
    • addedInput schema / properties / requester_vat_number
      Added value: +{
+  "description": "Optional: YOUR OWN EU VAT number, with country prefix, e.g. 'DE811569869'. When given, VIES returns a consultationNumber proving the check. VIES refuses a requester it does not know as valid.",
+  "minLength": 4,
+  "type": "string"
+}
    • changedInput schema / properties / vat_number / description
      Previous value: -"Full VAT number with country prefix, e.g. 'FR40303265045'"New value: +"EU VAT number to check, with its 2-letter country prefix (EL for Greece, XI for Northern Ireland), e.g. 'FR40303265045'. Not an IBAN or a company registration number."
  2. First observed

TDQS

A4.6/5.0
Behavior5/5

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 thoroughly: it discloses the 7-day stale-cache fallback, that stale answers carry stale:true, their original checkedAt, and never a consultation number, that no cached answer yields a retryable error, and that a fresh check costs 0.2 credit. These are exactly the behavioral traits an agent cannot infer from schema alone.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the core action and result before detailing fallback and cost semantics. It is dense and semicolon-chained, but each clause (fallback, staleness markers, cost) conveys distinct operational information rather than filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

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 describe returns — and it does: validity, registered name and address when provided, stale flag, checkedAt, consultationNumber, and error behavior. 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.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the baseline is 3. The description goes beyond it by explaining the functional consequence of requester_vat_number — that VIES issues a consultationNumber as official proof, and that an unknown requester is refused — which the schema only states in passing.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource (validates an EU VAT number live against VIES) and names the authoritative source. It is trivially distinguishable from sibling validate_iban, and the schema even reinforces the boundary by stating the input is not an IBAN.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives clear context: use it to check current VAT validity, and supply requester_vat_number when you need VIES's official proof of the check. It does not explicitly position itself against verify_company or validate_iban, so routing between siblings is left partly to inference.

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

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