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

Annotations already cover the safety profile (readOnly, idempotent, openWorld), and the description adds materially: pricing (0.003 USDC via x402 or 2 Factline credits), a free tier of 5 paid-tool calls per day, and the accepted-document return contents. It omits failure behavior when a company is not found in the directory, which keeps it below 5.

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 purpose and trigger are front-loaded in the first clause, followed by return values, regulatory context, and pricing. Dense and nearly waste-free, though the regulatory mandate sentence is more motivational than operationally necessary.

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 five optional parameters, no output schema, and no required fields, the description supplies what the structured data cannot: what the response contains, what payment/auth paths exist, and how to avoid payment via the free daily quota. Nothing essential for a correct first call 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 coverage is 100% and each field already documents itself, including the 'Or a company number...' alternative phrasing and the api_key flow. The description largely restates those identifier options rather than adding new semantics such as precedence when id and vat are both supplied, so the baseline of 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 action (look up in the Peppol Directory) and its output (Peppol participant ids and accepted document types), with the accepted identifier schemes spelled out. It is clearly distinguishable from siblings like eu_vat_check or company_lookup, which validate registrations rather than resolve Peppol participation.

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 opening 'Use when you need to send an e-invoice or check if a company can receive one' gives a concrete trigger, reinforced by the Belgium 2026 / France 2026-09 mandates. It does not name an alternative tool or state when not to use it (e.g., vs eu_vat_check for pure VAT validation), 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.