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For an MCP server that checks VAT numbers, a useful design is to expose one tool for local input and format checks and another for a VIES lookup. That split lets callers distinguish malformed input from a result returned by an external service. It is an architectural choice—not an MCP requirement—and a single tool can also work if it reports those outcomes clearly.
How do I build an MCP server for VAT validation?
An MCP tool has a declared name, description, and input schema, followed by a handler that implements the operation. The MCP Java SDK documentation explains that schema validation can happen before the handler runs: “When validation fails, the call returns a CallToolResult with isError set and a textual error, rather than reaching your handler.” That gives the server a natural place to reject missing or structurally invalid arguments without making a network request.
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A VAT-validation server can then separate three kinds of outcome for its clients: input rejected locally, input accepted and a VIES result returned, or a lookup problem that may need follow-up. Treating these as distinct outcomes is implementation guidance; the cited sources do not prescribe a VIES outage taxonomy, exact response fields, timeout codes, or retry policy.
Define the local-check tool
Give this tool a narrow description: it checks whether the supplied value meets the input and format rules your server implements. Its schema can require a VAT-number string and reject absent or incorrectly typed arguments before the handler runs. Be explicit that a local format check does not establish registration or eligibility for intra-EU transactions.
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Define the VIES lookup tool
Give the second tool a description that states the external source’s scope: VIES checks VAT numbers issued by EU Member States and Northern Ireland. The handler performs the remote lookup only after receiving suitable input, then reports the outcome as a VIES result or a lookup problem. Avoid presenting a lookup result as a broader tax determination.
Why split format checks and VIES lookup?
The split makes the boundary between local reasoning and an external status check visible. A client can handle an input error differently from a response based on national VAT data, and a malformed value need not trigger a remote request. Whether that clarity is worth another tool in the client’s tool list depends on how often callers need the operations independently and how clearly a combined interface can explain its outcomes.
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| Design consideration | Two tools | One tool |
|---|---|---|
| Malformed arguments and network requests | Local checks can reject malformed input before the VIES tool is called. | The handler must perform local checks before attempting the external lookup. |
| Distinguishing outcomes | The interface separates input-check results from VIES lookup outcomes. | The tool must label local validation, VIES results, and lookup problems unambiguously. |
| Describing external scope | The VIES tool’s description can focus on the source and its coverage. | The description must explain both local checks and VIES coverage. |
| Client discovery | Separate operations make the boundary clearer but add tool surface area for clients. | One operation is simpler to discover, but its description and output need to remain understandable across multiple stages. |
Neither design is mandated by the MCP sources cited here. Choose two tools when callers benefit from explicitly selecting local checks or remote lookup; choose one when the workflow is inherently a single operation and its outputs can still distinguish each stage.
What does an invalid VIES result mean?
An invalid result does not, by itself, fully explain a business’s registration situation. The European Commission’s VAT guidance says it can mean that the number does not exist, has not been activated for intra-EU transactions, or that registration is not complete. A server should therefore report the result as returned, without turning it into a definitive claim about why the number failed.
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If the result conflicts with what a business says about its VAT registration, the Commission advises contacting the relevant national authority. Keep validation records where appropriate, and treat VIES as a checking mechanism—not a substitute for understanding the tax rules that apply to a transaction.
Does VIES validate UK VAT numbers?
Not all UK VAT numbers. The European Commission says its VIES web service stopped validating UK Great Britain (GB) numbers on 1 January 2021. It identifies Northern Ireland numbers with the XI prefix and includes Northern Ireland in the service’s coverage. Do not label VIES coverage simply as “UK VAT validation”: GB and XI have different treatment. For GB checks, the Commission directs traders to the UK tax administration. See the European Commission’s VIES service.
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What should the tool descriptions and results say?
Make the source and limits clear in the tool names, descriptions, and returned content. For example, describe the local tool as a format check rather than a registration check, and describe the remote tool as a VIES lookup for EU Member State and Northern Ireland VAT numbers. Return the external result as an external result; do not imply that it proves a complete tax position.
- Reject structurally invalid arguments before the handler or remote lookup where the SDK’s schema validation supports that behavior.
- Keep locally detected input problems distinct from VIES outcomes.
- Represent lookup failures separately from a VIES response so callers can decide whether follow-up is needed. The available sources do not establish a particular failure taxonomy or retry policy.
- When reporting an invalid result, avoid inferring the underlying registration cause; direct unresolved cases to the relevant national authority.
The Official MCP Registry has a listing described as offering EU VAT validation via VIES alongside offline rates and format checks. That establishes that this is an existing software category, not that any particular listing is endorsed, verified, or required for an MCP server. See the Official MCP Registry search result.
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