Validate a VAT number's format locally against country-specific rules without network calls or fees. Confirm structure before using VIES.
纯本地 VAT 号格式校验（零网络、零成本）：判断号码形态是否符合该国规则。Free local format-only check (no network).
| Name | Required | Description | Default |
|---|---|---|---|
| vat | No | 完整 VAT 号，可含国家前缀 / Full VAT incl. country prefix | |
| vatNumber | No | 不含前缀的税号 / VAT number without prefix | |
| countryCode | No | ISO 国家代码 / 2-letter country code |
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, and it does disclose the crucial behavioral trait: the check is purely local and syntactic, so it cannot confirm the number actually exists or is registrable. It does not describe the response shape or behavior on malformed input, which keeps it out of 5 territory.
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?
Very short and front-loaded with the key constraint (zero network, zero cost). The bilingual restatement duplicates content, which is defensible for i18n but means every claim appears twice.
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 three-parameter check the description covers the essential scope, but with no annotations and no output schema it says nothing about return values or how errors are surfaced — notable given the sibling get_error_codes implies a structured error surface the agent should know about.
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 all three parameters (vat, vatNumber, countryCode) are already documented in the schema, including the prefix/no-prefix distinction. The description adds nothing about how these fields interact (e.g. whether countryCode is required when vat includes a prefix), so the baseline 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 and resource (VAT number format validation) and pins down the scope with "format-only" and "纯本地". The contrast with the sibling validate_vat is inferable from "format-only / 零网络", but the sibling is never named, so it stops short of a full 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?
Usage is implied rather than stated: "zero network, zero cost, format-only" suggests a cheap syntactic pre-check, but the description never says when to prefer this over validate_vat or when it is insufficient. No exclusions or prerequisites are given.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.