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skanfirmy.pl

by io.github.bartosz-kuc
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sprawdz_vies

Read-onlyIdempotent

Validate a counterparty's EU VAT number in the VIES system (European Commission). For Polish (PL) numbers the address is reduced to the town, because the number may belong to a natural person (GDPR).

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": {
      "countryCode": "<countryCode>",
      "vatNumber": "<vatNumber>"
    },
    "name": "sprawdz_vies"
  }
}

Input Schema

NameRequiredDescriptionDefault
vatNumberYesVAT number without the country prefix.
countryCodeYesTwo-letter EU country code (e.g. DE, IE, FR).

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A3.7/5.0
Behavior4/5

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

Annotations already cover the safety profile (readOnly, idempotent, openWorld), so the description only needs to add context, and it does: it discloses a concrete output-behavior quirk for PL numbers, where the returned address is truncated to the town because the number may belong to a natural person under GDPR. That is exactly the kind of non-obvious behavior an agent cannot get from structured fields.

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?

Two sentences, both front-loaded and load-bearing: the first establishes what is validated and against which external system, the second flags the single non-obvious behavioral caveat. No filler or restatement of the tool name.

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?

There is no output schema, so the description carries more of the burden for describing the response; it hints that an address is returned and that validity is assessed, but never states the result fields or what an invalid/unavailable VIES response looks like. For a two-parameter external lookup this is serviceable but leaves return-shape gaps.

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%, with both countryCode and vatNumber documented and an enum on the country code, so the baseline is 3. The description adds no syntax, format, or prefix guidance beyond what the schema states, and its PL note concerns the response rather than the inputs.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource: validate a counterparty's EU VAT number against the VIES system, with the authority (European Commission) named. The EU-VAT scope implicitly separates it from the Polish-register siblings (sprawdz_nip, sprawdz_regon, sprawdz_lista_nip), though no sibling is named explicitly.

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?

Usage is only implied by the purpose statement: an agent can infer this is the tool for EU VAT validation, but there is no explicit when-to-use, when-not-to-use, or routing to an alternative such as sprawdz_nip for Polish NIP numbers. Adequate but leaves the agent to reason about the boundary itself.

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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