NewKwLab Tools

validate_vat

Validate an EU (or UK) VAT number against the official VIES registry and return the registered business name and address when the registry provides them.

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": "validate_vat"
  }
}

Input Schema

Name	Required	Description	Default
`number`	Yes	VAT number, with or without the country prefix
`country`	No	Two-letter country code, e.g. DE, FR, GB

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 11, 2026

TDQS

A3.7/5.0

Behavior3/5

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

With no annotations, the description carries the full burden; it does disclose the external dependency (VIES) and that name/address are returned only when the registry provides them. It omits failure modes an agent needs — what happens for an invalid or unregistered number, and whether registry outages or rate limits surface as errors.

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?

A single front-loaded sentence that identifies the action, the data source, and the return content with no filler. Nothing could be removed without losing information.

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

Completeness3/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a simple two-parameter lookup with no output schema, the description covers purpose and returned fields. It is still incomplete on the validator's outcome semantics — whether an invalid number yields an error or a negative result — which is the key behavior for this class of tool.

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%, including the note that the number may be given with or without a country prefix, so the schema already does the work. The description adds nothing about the optional country parameter or how it interacts with an embedded prefix.

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 (validate), a specific resource (EU/UK VAT number), and the authoritative source (official VIES registry). The resource is distinct from sibling validators like validate_card and validate_email, so an agent can select it without opening the schema.

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

Usage Guidelines3/5

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

The scope (EU/UK VAT numbers only) implies when the tool applies, but there is no explicit when-to-use or when-not-to-use guidance and no reference to alternatives for other identifier types. Usage must be inferred from the resource name.

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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