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

Annotations already cover read-only, idempotent, non-destructive, open-world, so the description correctly spends its words elsewhere: cost ($0.05 USDC per successful call via x402 on Base), that only projected/processed evidence is returned and raw provider payloads never are, and that member-state outages leave coverage insufficient. It still omits failure/refund behavior when a check is unsuccessful despite charging "per successful call".

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 labeled structure ("Use when:", "Returns:", "Price:") is well front-loaded and scannable, with no filler in the English block. The full Japanese restatement roughly doubles the length and adds no information for an English-reading agent, though it is plausibly a deliberate bilingual contract.

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?

Returns, pricing, and safety profile are adequately covered given the readOnly annotations and the absence of an output schema. What remains thin for a 5-parameter, zero-required tool is parameter-level guidance (country_code, response_language, and mutual-exclusion rules among vat_id/participant_id), which no other structured field supplies.

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 0% — the schema offers only titles and types — so the description must carry parameter meaning. It implicitly maps name (name match), vat_id (VAT validity) and participant_id (Peppol discoverability), but leaves country_code (presumably the VAT prefix, 2 chars) and the response_language enum (ja/en/both) entirely unexplained, and never clarifies that all five parameters are optional in the schema despite the "and/or" phrasing.

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?

Uses a specific verb ("Check") plus a precise resource ("a known EU VAT ID and/or Peppol participant") and enumerates the three outputs: VAT validity, name match, Peppol discoverability. It is clearly separable from the jp_address_*/jp_invoice_* siblings by domain, but it never explicitly contrasts itself with the closest overlaps (jp_company_verify, jp_invoice_issuer_check).

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 "Use when:" clause states the condition for selection (need separate VAT validity, returned-name matching, and Peppol reachability evidence), and the caveat that "VIES is known-ID validation and member-state outages are insufficient coverage" functions as a when-not signal. It gives no explicit routing to a sibling alternative, so it stops short of a 5.

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