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

Goes well beyond the readOnlyHint/openWorldHint annotations: discloses the 5-number batch cap, that some member states withhold names/addresses, that VIES can be down for an entire country at a time, and the exact billing model ($0.002 USDC per successful call via x402 on Base/Solana/Arc, failures uncharged). For a paid external call, this is precisely the context an agent needs 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?

Front-loads the action and scope in the first clause, then layers caveats and pricing in a tight, waste-free sequence. Every sentence carries decision-relevant information; nothing is padding.

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?

An output schema already covers return shape, so the description need only add what structured fields can't: batch limit, partial-data member states, per-country outages, and cost/failure billing. All of those are present, making the definition complete for a single-parameter paid call.

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% and the single parameter already documents the comma-separated, 2-letter-country-code format. The description only reinforces this with an example ('DE143208032'); it adds format-adjacent value but no new semantics beyond the schema, 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: checks up to 5 EU VAT numbers against the European Commission's VIES service, and enumerates the returns (valid yes/no, name, address, member-state error). No sibling in the catalog shares this domain, so an agent can identify it immediately without ambiguity.

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?

Usage context is implied by the batch-limit phrasing ('up to 5') and the VIES framing, but the description never states when to choose this over alternatives (e.g. a single-number lookup or the LEI registry) or any prerequisites such as needing a funded x402 wallet before calling. Adequate but leaves routing to inference.

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