Validate a single EU/UK VAT identifier against VIES (EU) or HMRC (UK). Returns name + address when available.
| Name | Required | Description | Default |
|---|---|---|---|
| vat_id | Yes | Full VAT identifier including country prefix (e.g. 'DE123456789' or 'GB123456789') |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations provided, the description carries the full burden. It discloses the core behavior (external validation via VIES/HMRC and returning name/address when available), but it does not mention possible failures, network dependency, latency, or what happens for invalid or unregistered VAT numbers.
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?
The description is two sentences with no filler. It front-loads the main purpose and adds the key return detail without unnecessary elaboration.
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?
For a simple single-parameter tool, the description covers the essential information: what is validated, against which systems, and what is returned. It lacks explicit sibling differentiation and error-handling details, but nothing needed to invoke it correctly is missing.
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 description coverage is 100%, so the schema already documents vat_id fully, including the country-prefix requirement. The description adds only that the VAT identifier is 'single' and 'EU/UK', which is marginal beyond the schema. Baseline 3 is appropriate.
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?
The description clearly identifies the action ('Validate'), the target resource ('a single EU/UK VAT identifier'), and the validation sources (VIES/HMRC). It does not explicitly distinguish itself from the similar-sounding sibling vat_registration_check, so it misses full sibling differentiation.
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?
The usage is implied by the description: use it to validate a single EU/UK VAT identifier against VIES or HMRC. However, it does not explicitly state when to prefer this tool over vat_registration_check or vat_compliance_full, nor does it mention any exclusions or alternatives.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Add one secure layer between your agents and this server.
Each tool has a distinct primary purpose: registration, validation, rate lookup, reverse-charge certification, and full compliance analysis. There is slight overlap because reverse_charge_certificate also validates VAT IDs via VIES, but the descriptions clearly differentiate the outputs and use cases.
Most tools follow a descriptive snake_case pattern with a vat_ prefix, such as vat_number_check and vat_registration_check. reverse_charge_certificate and vat_compliance_full deviate slightly from the prefix/pattern but remain consistently readable.
Five tools is a well-scoped size for a VAT compliance server. Each tool covers a distinct compliance need without unnecessary redundancy or bloat.
The set covers the core VAT compliance workflow: registration eligibility, VAT number validation, rate lookup, reverse-charge eligibility, and a broad gap analysis. Some edge cases like reverse charge for goods or detailed place-of-supply rules are not explicitly covered, but the core domain is well served.
