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

Goes well beyond the readOnly/openWorld annotations: it discloses the per-call price and payment rail (x402, $0.0045 USDC on Base/Solana/Polygon/Arbitrum), the no-charge behavior when a member state service is unavailable, and that company name/address are only returned when the member state discloses them. That is exactly the extra context an agent needs.

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 core purpose, then packs payment, fallback and sample information efficiently. It runs long and mixes billing details into the definition, but each sentence carries information an agent needs before calling a paid endpoint.

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?

With no output schema the description must convey returns, and it does indicate validity plus conditional company name/address disclosures. It stops short of describing the exact response shape or error semantics, but the payment and fallback behavior make it largely self-sufficient.

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 already 100%, so the baseline is 3, but the description adds concrete format examples ('NL004495445B01', 'DE811569869') and restates the country_code exception logic, giving an agent unambiguous input shapes beyond the schema's terse field descriptions.

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 identification number) against a named authoritative service (VIES), and clarifies scope as live validation with a national-format pre-check. It is trivially distinguishable from sibling validators such as iban_validate, email_validate and phone_validate.

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 EU VIES validation, accepted input shapes, a free sample invocation) but does not name alternatives or state when-not to use it (e.g., non-EU VAT or offline checks). Useful, but not full routing guidance.

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