Agent Decision & Evidence Tools (MCP + x402)

EU VAT identity check (free)

validate_vat

Read-onlyIdempotent

FREE live validation of an EU VAT ID against VIES, including the registered name and address when the member state publishes them. Use this as the no-cost entry point before paid VAT, counterparty or invoice decisions.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`vat_id`	Yes

Output Schema

TableJSON Schema

Name	Required	Description
`hint`	No
`name`	No
`tool`	No
`error`	No
`input`	No
`price`	No	No payment is required for this tool.
`source`	No
`address`	No
`charged`	No	When present on an error, false means the failed result is uncharged.
`country`	No
`message`	No
`missing`	No
`settled`	No	When present on an error, false means no settlement occurred.
`unknown`	No
`accepted`	No
`retryable`	No
`checked_at`	No
`error_code`	No
`normalized`	No
`registered`	No
`status_code`	No
`did_you_mean`	No
`format_valid`	No
`vies_checked`	No
`missing_checks`	No
`partial_evidence`	No
`suggested_vat_id`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed6 schema fields changedSep 6, 2026

addedInput schema / additionalProperties
```
Added value: +false
```
addedInput schema / properties / vat_id / maxLength
```
Added value: +32
```
addedInput schema / properties / vat_id / minLength
```
Added value: +4
```
removedInput schema / properties / vat_id / title
```
Removed value: -"Vat Id"
```

removedInput schema / title

Removed value: -"validate_vat_toolArguments"

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "additionalProperties": true,
+  "anyOf": [
+    {
+      "required": [
+        "input",
+        "normalized",
+        "country",
+        "format_valid",
+        "price"
+      ]
+    },
+    {
+      "required": [
+        "error"
+      ]
+    }
+  ],
+  "description": "Structured result contract for validate_vat. A success satisfies the documented success branch. A domain or tool error includes error and is not a successful settlement; unpaid x402 requirements are carried by the transport wrapper.",
+  "properties": {
+    "accepted": {
+      "items": {
+        "type": "string"
+      },
+      "type": "array"
+    },
+    "address": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "charged": {
+      "description": "When present on an error, false means the failed result is uncharged.",
+      "type": "boolean"
+    },
+    "checked_at": {
+      "type": "string"
+    },
+    "country": {
+      "type": "string"
+    },
+    "did_you_mean": {
+      "additionalProperties": true,
+      "type": [
+        "object",
+        "null"
+      ]
+    },
+    "error": {
+      "oneOf": [
+        {
+          "type": "string"
+        },
+        {
+          "additionalProperties": true,
+          "properties": {
+            "code": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "message": {
+              "type": "string"
+            },
+            "param": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "type": {
+              "type": "string"
+            }
+          },
+          "required": [
+            "message"
+          ],
+          "type": "object"
+        }
+      ]
+    },
+    "error_code": {
+      "type": "string"
+    },
+    "format_valid": {
+      "type": "boolean"
+    },
+    "hint": {
+      "type": "string"
+    },
+    "input": {
+      "type": "string"
+    },
+    "message": {
+      "type": "string"
+    },
+    "missing": {
+      "items": {
+        "type": "string"
+      },
+      "type": "array"
+    },
+    "missing_checks": {
+      "items": {
+        "type": "string"
+      },
+      "type": "array"
+    },
+    "name": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "normalized": {
+      "type": "string"
+    },
+    "partial_evidence": {
+      "additionalProperties": true,
+      "type": "object"
+    },
+    "price": {
+      "const": "free",
+      "description": "No payment is required for this tool.",
+      "type": "string"
+    },
+    "registered": {
+      "type": "boolean"
+    },
+    "retryable": {
+      "type": "boolean"
+    },
+    "settled": {
+      "description": "When present on an error, false means no settlement occurred.",
+      "type": "boolean"
+    },
+    "source": {
+      "type": "string"
+    },
+    "status_code": {
+      "type": "integer"
+    },
+    "suggested_vat_id": {
+      "type": "string"
+    },
+    "tool": {
+      "type": "string"
+    },
+    "unknown": {
+      "items": {
+        "type": "string"
+      },
+      "type": "array"
+    },
+    "vies_checked": {
+      "type": "boolean"
+    }
+  },
+  "type": "object"
+}

First observedJul 31, 2026

TDQS

A4.3/5.0

Behavior4/5

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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds useful behavioral detail beyond annotations: it performs a live VIES lookup and may return the registered name and address conditionally, which helps the agent set expectations.

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?

The description is two compact sentences with no filler. The key attributes ('FREE', 'live validation', 'VIES', 'registered name and address') are front-loaded, and the use-case guidance is clearly stated in the second sentence.

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?

For a single-parameter validator with an output schema and safety annotations, the description covers the tool's purpose, behavior, and recommended usage context. No critical information for selecting or invoking the tool 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 description coverage is 0%, so the description must compensate for the vat_id parameter. It clarifies that the value is an EU VAT ID and that the check is against VIES, which adds meaning beyond the raw schema. However, it does not specify expected format details like country code prefix or examples, leaving some ambiguity for agents unfamiliar with VAT ID formats.

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 clearly states the action ('live validation'), the target resource ('an EU VAT ID'), and the lookup source ('VIES'), which distinguishes it from sibling tools like validate_iban or check_counterparty_eu. It leaves no ambiguity about what the tool does.

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?

The description explicitly positions this as the 'no-cost entry point before paid VAT, counterparty or invoice decisions,' giving clear when-to-use guidance. It does not explicitly name alternatives or exclusions, but the context strongly implies this is the first free validation step.

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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