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

Goes well past the readOnlyHint/openWorldHint annotations: it discloses data provenance and reliability limits (community compilation, not the official EU TEDB), coverage gaps, and a per-call price of $0.002 USDC via x402 on Base/Solana/Arc with failed calls free. The payment/authorization requirement is exactly the kind of context an agent must know before invoking.

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?

Three sentences, front-loaded with what is returned, then limitations, then cost. No padding; the pricing sentence is long but carries non-redundant, decision-relevant 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?

With an output schema handling return values and annotations covering safety and openness, the remaining agent-critical gaps — data trustworthiness and payment mechanics — are both addressed. Nothing needed to invoke it correctly 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%, so both parameters are already documented, and the description adds only that exactly one member state is returned per call and that rates are as of a given date. No format, default, or validity detail beyond the schema — baseline 3 is appropriate.

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 — EU standard and reduced VAT rates for one member state on a given date — and names the exact underlying dataset (ibericode/vat-rates on GitHub). The scope is bounded by the explicit exclusion of IOSS registration and de minimis rules, which separates it from customs-duty and de-minimis siblings without needing the schema.

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 gives a clear negative boundary (does not cover IOSS or de minimis) but never names the sibling to use instead (e.g. logistics_us_de_minimis_section321) or states a positive use case such as feeding a duty calculation. Usage is implied by the resource itself rather than stated.

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