OpenAccountants

Validate a business VAT / tax-ID number (live registry lookup)

validate_vat_number

Read-onlyIdempotent

Check whether a business's VAT / tax-registration number is real and registered, in real time against the official government registry — and return the registered business NAME and ADDRESS when the authority discloses them. This is an authoritative, unfakeable confirmation you cannot get from a knowledge lookup or from training data. Use it whenever a user wants to verify a supplier's or customer's tax ID: before applying a B2B reverse charge, zero-rating a cross-border sale, or onboarding a vendor. COVERAGE TODAY IS EU ONLY — the 27 EU member states plus 'XI' (Northern Ireland), live via the European Commission's VIES registry (use 'EL' for Greece). Other jurisdictions (e.g. UK/HMRC and beyond) are NOT supported yet but are on the roadmap — for a non-EU number, tell the user live validation isn't available for that country yet rather than guessing a verdict. This is a LIVE external lookup — if the registry is temporarily unavailable it says so plainly rather than inventing an answer.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`vat_number`	Yes	The VAT number, with or without the country prefix and spaces, e.g. 'IE6388047V', 'IE 6388047V', or '6388047V'.
`country_code`	No	2-letter EU member-state code of the VAT number, e.g. 'DE', 'IE', 'FR'. Use 'EL' for Greece and 'XI' for Northern Ireland. May be omitted if the country prefix is already part of vat_number.

Output Schema

TableJSON Schema

Name	Required	Description
`valid`	No	true/false verdict from VIES, or null if the service could not be reached (an availability issue, not a verdict).
`source`	No
`address`	No
`message`	No
`summary`	No
`checked_at`	No
`source_url`	No
`vat_number`	No
`vies_error`	No
`full_vat_id`	No
`next_action`	No
`country_code`	No
`business_name`	No
`human_backing`	No	Named accountant(s) who review that country's VAT guidance (guidance-level, not a per-figure sign-off).
`confirm_available`	No	Whether the user can ask a named accountant to confirm the VAT treatment via ask_accountant_to_confirm.

Schema Changelog

Changes observed during successful MCP inspections.

AddedJul 8, 2026

TDQS

A5/5.0

Behavior5/5

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

Beyond the annotations (readOnlyHint, openWorldHint, idempotentHint, destructiveHint=false), the description adds crucial behavioral context: it is a live external lookup, may be temporarily unavailable and will say so plainly rather than inventing an answer, and covers only EU/XI jurisdictions. This gives the agent realistic expectations about failure modes and scope.

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 appropriately sized given the tool's complexity, with every sentence conveying necessary information. It is front-loaded with a clear purpose, then covers usage, coverage, and failure behavior efficiently without redundancy.

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?

The description covers purpose, usage scenarios, geographical coverage and limitations, behavior on unavailability, and the nature of the output (business name/address). With an output schema present and rich annotations, this description provides all context an agent needs to invoke the tool correctly.

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

Parameters5/5

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

The input schema already describes both parameters with 100% coverage. The description adds further meaning by explaining accepted input formats (with/without country prefix, with/without spaces) and clarifying the 'EL' for Greece and 'XI' for Northern Ireland special cases, going well 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?

The description clearly states the tool checks whether a VAT/tax number is real and registered via a live government registry, and returns the registered business name and address when disclosed. This specific verb+resource phrasing distinguishes it from sibling tools like search_rules or list_jurisdictions, which serve different purposes.

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

Usage Guidelines5/5

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

The description explicitly states when to use the tool ('whenever a user wants to verify a supplier's or customer's tax ID' before reverse charge, zero-rating, or vendor onboarding) and when not to use it (non-EU numbers, telling the user to avoid guessing). It also contrasts with 'knowledge lookup or training data', giving clear usage guidance vs alternatives.

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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