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

Annotations already cover the safety profile (readOnly, idempotent, closed-world), and the description adds genuinely non-obvious context: it is a free call but requires an InvoiceVerify account via OAuth sign-in, and it discloses the response content (causes and fix steps). No rate limits or fallback behavior for unknown rule IDs are mentioned.

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?

Two tight sentences, front-loaded with the capability and the outcome, then the auth/cost constraint. Minor redundancy in restating the three languages that the enum already enumerates.

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?

For a two-parameter, read-only lookup with no output schema, everything an agent needs is present: what the tool returns, which rule vocabularies are supported, the auth prerequisite, and the cost. No meaningful gap remains.

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 both parameters are already documented, including the language enum and the default. The description adds little beyond restating the language options and supplying example rule IDs; useful, but 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 (explain) and resource (EN 16931 / Peppol / XRechnung business rule) plus the shape of the answer: plain-language explanation with typical causes and fix steps. It is inherently distinct from the validate/parse/convert siblings, but the description never names them or draws the boundary explicitly.

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 usage context is only implied: the schema notes the rule_id comes 'as reported by validate_invoice', which hints this is a follow-up to a failed validation, but the description itself gives no when-to-use, when-not, or alternative routing.

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