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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint, covering the safety profile. The description adds that the check is deterministic and returns a signed XDR-1 receipt—useful behavioral context beyond the annotations, but no details on authentication, rate limits, or error behavior.

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 description is front-loaded with the core purpose but includes marketing-style benefit claims ('Eliminates cross-border invoicing errors, invalid billing, and tax audit penalties') that do not add operational value. The signed receipt detail is useful, but the overall text could be tightened.

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 low complexity (one required parameter), the presence of annotations and an output schema, the description covers the essentials: what it does, how it does it, and that it returns a signed receipt. It lacks usage guidance, but the tool is otherwise well-enough specified to be invoked correctly.

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 100%, so the single parameter is fully documented in the schema. The description adds no parameter-level information beyond what the schema provides, making the baseline score of 3 appropriate.

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 (validates), resource (EU/BE VAT identifiers), and mechanism (MOD-97 checksums), making the tool's function immediately clear. However, it does not explicitly differentiate itself from the many sibling validators (e.g., gstin_check, iban_check), so it falls short of a 5.

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 description emphasizes benefits ('Eliminates cross-border invoicing errors...') but provides no explicit when-to-use guidance, prerequisites, or alternatives. It does not help an agent decide between this tool and the other validation tools in the sibling list.

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