SiteCheck

Validate an EU VAT number with VIES

vat

Read-only

EU VAT number check: format and checksum are tested locally first (a malformed number is rejected with HTTP 400 and no charge), then the official VIES service answers whether it is valid, with the registered name and address where the member state discloses them. All 27 EU states plus Northern Ireland (XI). VIES can take 6-25 seconds when a member state's register is slow; if it does not answer within 25 s the call returns HTTP 503 and is not charged. Source: European Commission VIES. Price: $0.002 in USDC per successful call, paid with x402 (Base, Solana or Arc); failed calls are not charged.

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": {
      "number": "<number>"
    },
    "name": "vat"
  }
}

Input Schema

Name	Required	Description	Default
`number`	Yes	VAT number with its 2-letter country prefix, e.g. DE811907980 (EL for Greece, XI for Northern Ireland). Spaces and dots are ignored
`country`	No	Optional: country code, if the number has no prefix

Output Schema

Name	Required	Description	Default
`name`	No
`input`	No
`valid`	No
`checks`	No
`source`	No
`address`	No
`formatted`	No
`vatNumber`	No
`viesStatus`	No
`countryCode`	No
`requestDate`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 14, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "properties": {
+    "address": {},
+    "checks": {},
+    "countryCode": {},
+    "formatted": {},
+    "input": {},
+    "name": {},
+    "requestDate": {},
+    "source": {},
+    "valid": {},
+    "vatNumber": {},
+    "viesStatus": {}
+  },
+  "type": "object"
+}

First observedOct 8, 2026

TDQS

A4.4/5.0

Behavior5/5

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

Beyond the readOnly/openWorld annotations, the description discloses the two-stage validation flow, HTTP 400 with no charge for malformed numbers, 6–25 s VIES latency, HTTP 503 on 25 s timeout with no charge, the returned name/address where disclosed, and the $0.002 USDC x402 pricing on successful calls only. That is exactly the billing and failure-mode context an agent needs before calling a paid, slow external service.

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 validation flow, then latency/failure behavior, then source and price — a sensible ordering where each sentence carries distinct information. It is dense and slightly run-on in the pricing clause, but nothing is 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?

With an output schema present, the description need not explain return values, and it still covers coverage, latency, error codes, charging rules, and provenance. Nothing an agent needs to call this paid, potentially slow tool correctly is missing.

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 coverage is 100%, so the schema already documents both parameters including the country-prefix rule and EL/XI exceptions. The description reinforces coverage (27 states + XI) but adds no new syntax or format guidance 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?

The description names a specific verb (validate/check) and resource (EU VAT number), states the scope (all 27 EU states plus Northern Ireland XI), and makes the validation chain explicit (local format/checksum, then VIES). It is clearly distinguishable from siblings like eori, iban, or the generic validate tool.

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?

Usage context is strong: EU VAT numbers only, with the EL/XI prefix exceptions called out, and the local pre-check explains what happens to malformed input. It stops short of naming alternatives or explicit when-not-to-use conditions (e.g., versus eori or iban), so it is clear context without exclusions.

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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