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

Annotations cover only readOnly/idempotent/openWorld; the description adds substantial behavior: results are cached for a day, a format pre-check runs, the source is the official VIES service, and it discloses the cost model (0.003 USDC via x402 or 2 Factline credits). Payment requirements and caching are exactly the kind of context annotations cannot convey.

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 purpose and coverage, and dense with useful facts rather than filler. The opening sentence strings several clauses together, but each carries distinct information, so nothing reads as waste.

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?

No output schema exists, yet the description explains what comes back (valid/invalid plus registered name/address where the country shares them). With cost, caching, format rules, and return values all covered for a single-param tool, nothing an agent needs 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 coverage is 100% with a single required parameter already documented. The description restates the input as 'VAT number with country prefix, e.g. IE6388047V,' mirroring the schema example, so it adds no meaning beyond structured fields – baseline 3.

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 (EU VAT number) with precise scope: all 27 member states plus Northern Ireland, via the official VIES service. It clearly distinguishes itself from siblings like kyc_check and email_verify through resource coverage.

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 explicit target use cases (invoicing, accounting, B2B onboarding, KYB, e-commerce), which frames when to reach for it. It stops short of naming an alternative sibling or stating when-not-to-use, but the context is clear.

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