Klarbon

Check EU VAT numbers in VIES

check_vat_numbers

Read-onlyIdempotent

Checks up to 25 EU VAT numbers (and Northern Ireland's XI numbers) in the European Commission's VIES service: valid or not valid now, with the registered name and address where the member state gives them. Each number's format and published check digits are checked first, and one that fails is not sent. unavailable means the member state did not answer, never that the number is invalid. Free: without an API key the anonymous daily limit applies, with one 500 numbers a day. Nothing about the numbers is kept.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "numbers": "<numbers>"
    },
    "name": "check_vat_numbers"
  }
}

Input Schema

Name	Required	Description
`country`	No	The country of numbers given without their prefix.
`numbers`	Yes	VAT numbers as written, with or without the country prefix (EL for Greece, XI for Northern Ireland).
`requester`	No	Your own VAT number, so VIES returns a consultation number for each answer.

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 10, 2026

TDQS

A4.5/5.0

Behavior5/5

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

Annotations already declare readOnly/idempotent/non-destructive/openWorld, and the description adds real behavior on top: local format and check-digit validation happens first and failures are never sent upstream, 'unavailable' is disambiguated from 'invalid', quota tiers are quantified, and it states nothing about the numbers is retained.

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

Conciseness5/5

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

Front-loaded with what is checked and what comes back, then validation order, then the unavailable caveat, then cost/quota, then data retention. Five dense sentences with no 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?

Although there is no output schema, the description explains the return shape itself (valid/not-valid, registered name and address where the member state supplies them, and the unavailable case). Combined with full schema coverage and annotations, an agent has everything needed to call and interpret the result.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

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

Schema description coverage is 100%, so `country`, `numbers` and `requester` are already documented. The description reinforces the 25-item cap and the prefix conventions but adds no syntax or format detail beyond the schema, so the baseline 3 applies.

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 ('Checks ... EU VAT numbers ... in the European Commission's VIES service') and bounds the scope ('up to 25', including Northern Ireland XI numbers). An agent can immediately distinguish this from the invoice-conversion siblings, which handle documents rather than VAT validation.

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 operating conditions: batch of up to 25, anonymous daily limit without an API key versus 500/day with one, and the crucial rule that 'unavailable' means no answer from the member state, not an invalid number. It does not explicitly name a sibling alternative or a when-not-to-use case, 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.

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