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

Annotations already declare readOnly, idempotent and non-destructive, and the description adds substantial context beyond them: pricing (0.003 USDC via x402 or 2 credits), a free tier of the first 5 paid-tool calls per day with no key needed, the returned data shape, and the regulatory deadlines driving the lookup. Auth and cost information is exactly what an agent needs before calling a paid tool.

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-loaded with the trigger, then a single dense sentence covering lookup inputs, outputs, regulatory context and pricing. Efficient overall, though the pricing/regulatory tail could be split for faster scanning.

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?

No output schema exists, and the description does explain the return contents (participant ids, accepted document types), plus cost and free-tier mechanics. The remaining gap is the zero-required-parameter schema: the description never states that a name, VAT, id or country must be supplied, which could let an agent attempt an empty lookup.

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%, so the description is at baseline. It lists the same identifier schemes (KBO, SIREN, KvK, Peppol id) already present in the schema descriptions, adding no selector logic such as which identifier takes precedence when several are supplied, or the fact that at least one of vat/id/name must be provided despite zero required parameters.

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 (looks up a company in the Peppol Directory) and names the concrete outcome (participant ids and accepted document types). An agent can immediately distinguish it from siblings like eu_vat_check or company_lookup, which validate identity rather than e-invoicing reachability.

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?

Opens with an explicit trigger: 'Use when you need to send an e-invoice or check if a company can receive one.' That gives clear context but names no alternative or exclusion, e.g., whether to use eu_vat_check first to validate the VAT number before attempting the Peppol lookup.

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