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

No annotations are provided, so the description carries full burden. It discloses the distinction between 'invalid' and 'service unavailable' (national DB offline), and mentions pricing and payment method ($0.01 per call, x402 payment, USDC on Base mainnet). This goes beyond basic functionality, though it omits rate limits or authentication requirements.

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?

The description is concise at around 100 words, front-loaded with the core purpose, and structured with key details (returns, distinction, use cases, input format, pricing). Every sentence adds value without redundancy.

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 tool with two parameters and no output schema, the description fully explains what it does, what it returns (valid/invalid, company name, address), and handles edge cases (unavailable). It is complete and clear.

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 descriptions for both parameters. The description adds meaning by stating 'Input: 2-letter country code (EL for Greece) + VAT number' and 'VAT number without country prefix', which clarifies the expected format beyond the schema alone.

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?

The description clearly states 'Validate an EU VAT number against the official EU VIES registry across all 27 member states' with specific verb and resource. It uniquely distinguishes this tool from siblings, as no other sibling tool performs VAT validation.

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?

The description explicitly lists use cases: 'For invoicing, B2B onboarding, KYB and tax-compliance agents.' It also clarifies the distinction between 'invalid' and 'unavailable', providing context for when the result might be ambiguous. However, it does not explicitly state when not to use this tool.

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