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

Annotations already indicate read-only and non-destructive, but the description adds that it is deterministic, performs checksum validation including Belgian modulo-97, returns normalized identifier and stable issue code, and gives no legal guarantee. No contradictions with annotations.

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 concise sentences that front-load the core purpose and immediately clarify limitations. Every word contributes meaning; no redundancy or filler.

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?

Given the tool's purpose (format validation) and the presence of an output schema, the description covers scope, limitations, and behavioral guarantees. It explicitly states what the tool does not do, which is critical for correct usage.

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 baseline is 3. The description adds context about the validation process and output (normalized identifier, issue code), which enhances understanding beyond the schema descriptions. However, it does not directly elaborate on parameter usage beyond what is in the schema.

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?

The description clearly states it performs deterministic structure/checksum validation of EU VAT numbers, distinguishing it from live VIES checks. It specifies it is format plausibility only, not a live result, which differentiates it from sibling tool verify_vat_vies.

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?

Explicitly states when to use (format validation) and when not to use (not a live VIES result, no legal/fiscal guarantee). Mentions it never sends a Peppol invoice, providing clear boundaries. Sibling tools imply alternatives for live checks.

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