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

Annotations already declare read-only, idempotent, non-destructive, open-world behavior. The description adds material context beyond that: live external lookup in VIES, inclusion of registered name/address where shared, and a precise payment model (0.003 USDC via x402 or 2 credits, with five free paid-tool calls per day).

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-loads the trigger and scope, then adds coverage, input format, and pricing in a compact block. It is dense and slightly run-on at the end, but every clause carries useful information.

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 read-only verification tool with no output schema, the description supplies what an agent needs: data source (VIES), geographic coverage, return content (validity plus registered name/address where shared), input format, and cost/auth requirements.

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 both vat and api_key are already fully documented in the schema. The description repeats the VAT example and country-prefix requirement but adds no new syntax or constraints, so 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: a live validity check of an EU VAT number against VIES, explicitly covering all 27 member states plus Northern Ireland. This clearly distinguishes it from siblings like eu_eori_check or eu_company_check without needing to name them.

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 a clear trigger condition: 'Use when the user gives an EU VAT number and you need to know if it is valid.' It does not name when-not to use it or point to alternatives (e.g., eu_eori_check for EORI numbers), so it stops short of a 5.

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