Check whether a Swiss VAT (MWST) number is valid and currently active via the UID Webservice. Use it to confirm business VAT status in the Zefix MCP Server.
Validate a Swiss VAT (MWST) number using the UID Webservice. Checks if the VAT number is valid and currently active.
| Name | Required | Description | Default |
|---|---|---|---|
| vatNumber | Yes | VAT number to validate in format CHE-XXX.XXX.XXX |
Changes observed during successful MCP inspections.
v1.0.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full burden. It does disclose that validation goes through an external UID Webservice and that the result reflects current active status rather than mere format validity, which is useful. However, it says nothing about network/rate-limit behavior, failure modes, or permissions.
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 tight sentences with the core action and the validation scope front-loaded; nothing is redundant or padded.
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 single-parameter, read-only lookup with no output schema, the description covers the essential behavior. The one gap is that it doesn't convey the shape of the result (boolean validity vs. structured status/details), which an agent must infer.
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% and the single parameter is fully documented with a regex pattern and example format. The description adds no syntax or edge-case detail beyond the schema, so the baseline of 3 applies.
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 (validate) and resource (Swiss VAT/MWST number) and names the backing service (UID Webservice), so the agent knows exactly what it does. It does not explicitly differentiate itself from the sibling validate_uid, which is the main thing keeping it from a 5.
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?
There is no when-to-use guidance, no mention of when to prefer validate_uid versus this tool, and no statement of what to do with a negative result. Usage is only implied by the wording 'checks if valid and currently active'.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.