validate_eu_vatValidate an EU VAT ID (27 member states + XI): format and check digit offline. Set live=true to also query the EU VIES registry for registration status, company name and address (slower, may be throttled; failures return vies.checked=false, never a false 'invalid').
| Name | Required | Description | Default |
|---|---|---|---|
| live | No | Also check the official VIES registry | |
| value | Yes | VAT ID with country prefix |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the annotations (which already cover readOnly/idempotent/openWorld), the description discloses operationally important behavior: offline vs registry-backed checking, throttling risk on the live path, the fields the live path can return, and — most valuable — that failures surface as vies.checked=false rather than a false 'invalid'. That failure semantic is exactly the kind of context annotations cannot carry.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
Two sentences with zero filler; the default behavior is front-loaded and the live-mode caveats are packed into a single parenthetical. Every clause carries information the agent needs.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
There is no output schema, and the description partially compensates by naming the fields the live path returns and the failure signal. It does not describe the shape of the offline result (e.g. what a valid/invalid format check returns), leaving a small gap for a tool whose callers must interpret the response.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Schema coverage is 100%, so the baseline is 3, but the description earns extra credit by explaining the tradeoff of live (slower, throttled, extra returned fields) and clarifying that value carries a country prefix and is validated against 27 states + XI. It adds real meaning over the one-line schema descriptions.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
States a specific verb and resource (validate an EU VAT ID) plus the exact scope (27 member states + XI) and the two modes of operation. The scope cleanly separates it from siblings like validate_gb_vat and validate_ein without the agent needing to open any schema.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
Explicitly tells the agent when to flip live=true and why not to by default: it is slower and may be throttled, while the offline check is the default. It does not explicitly route the agent to sibling validators (e.g. GB numbers belong to validate_gb_vat), so it falls just short of full alternative guidance.
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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