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

Beyond the annotations (readOnly, openWorld, non-idempotent), the description discloses genuinely useful behavior: the check is live against VIES, coverage varies by member state ('where the member state shares them'), sole traders' names are withheld for privacy, and the cost is $0.005 per call via x402 or credits. It doesn't state latency, rate limits, or failure modes for an unreachable VIES.

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?

Front-loaded with the core action and result, then coverage caveats, then pricing. The pricing sentence is housekeeping rather than capability, but it is short and the whole description is dense without filler.

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 one-parameter, read-only external lookup whose annotations already carry the safety profile and which has an output schema, the description covers everything needed: what it validates, input format expectations, source system, coverage limits, and cost. Nothing material 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% and the single parameter already includes the country-prefix example (DE811569869), so the description's 'with its country prefix' adds little. The mention that XI (Northern Ireland) numbers are accepted is a mild clarification of accepted input space, keeping this at baseline.

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 ('Check an EU VAT number live against ... VIES') and names the exact return values (validity, registered company name, address). It is clearly distinguishable from siblings like eu-fx and eu-lei, which cover different EU data domains.

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?

Gives clear context for when the tool applies — live VAT verification, including Northern Ireland (XI) numbers — which tells an agent what inputs qualify. It stops short of naming alternatives or stating exclusions (e.g. when a cached/offline check would be preferable, or what to do for non-EU numbers).

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.