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converters_vat_validate

Read-onlyIdempotent

EU VAT number validation: layout check per member state plus a live VIES lookup

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": {
      "vatNumber": "<vatNumber>"
    },
    "name": "converters_vat_validate"
  }
}

Input Schema

Name	Required	Description
`country`	No	Two-letter country code when vatNumber has no prefix (Greece is EL).
`vatNumber`	Yes	The EU VAT number, with or without the two-letter prefix and with spaces or dots, for example DE123456789 or 'DE 123.456.789'.
`offlineOnly`	No	true checks only the number layout and skips the live VIES lookup.

Schema Changelog

Changes observed during successful MCP inspections.

AddedNov 1, 2026

TDQS

A3.9/5.0

Behavior4/5

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

Annotations already cover read-only, idempotent, non-destructive; the description adds the material behavioral fact that a live external VIES call occurs unless skipped, which matters for latency and availability. Note a mild tension with openWorldHint=false, since a live VIES lookup does reach an external service, but this is not a direct contradiction of the stated purpose.

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 clause with zero filler; the mechanism is stated before any caveat, which is exactly what an agent needs for triage.

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?

With no output schema, the description says nothing about what a result looks like (valid/invalid, VIES status, error when VIES is unreachable) or how failures are surfaced. For a validator this is a noticeable gap, though the tool is simple enough that the core contract is inferable.

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, vatNumber formatting and offlineOnly are already documented. The description's phrase 'layout check per member state' only loosely reinforces the country parameter and adds no syntax or format detail beyond the schema.

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) and resource (EU VAT number) and names the two-stage mechanism (member-state layout check + live VIES lookup). This cleanly separates it from siblings like converters_vat_rates and converters_vat_gst, which compute tax rather than verify identifiers.

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 description implies the context (checking an EU VAT number's format and existence) but never states when to prefer it or what to do for non-EU numbers. The offlineOnly hint in the schema, not the description, is what tells an agent how to degrade gracefully.

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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