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

Beyond the annotations (readOnly, idempotent, openWorld), the description reveals operational details: no network/register/account/cost, per-row checks for empty/malformed/duplicate/not_covered, and the critical caveat that well-formed does not mean registered. These are behavioral traits an agent must know, and they are not present in the 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?

The description is information-dense yet efficient. It front-loads the core distinction (local vs register), enumerates the per-row outcomes, then gives usage guidance in a final sentence. No wasted words; every clause carries meaning.

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 single-parameter, no-output-schema tool, the description fully covers what an agent needs: input constraints, per-row result semantics, and the relationship to the sibling. The absence of an output schema is mitigated by the description's explicit statement of what it reports for each row.

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%, and the schema itself already documents vatId format, reference echoing, and row limits in detail. The description adds no new parameter semantics beyond what the schema provides; it focuses on tool behavior instead. Baseline 3 is 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?

The description states a precise action ('normalise and check up to 20 VAT numbers locally') and immediately distinguishes itself from the sibling check_vat_list by calling itself the 'FREE half' and explicitly excluding register checks. It also clarifies edge cases such as 'not_covered' vs 'invalid', leaving no ambiguity about scope.

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?

It gives explicit sequencing guidance: 'Use it to clean a list first, then send the well-formed rows to check_vat_list.' It also implicitly states when not to use it (when register verification is needed) by clarifying that 'register is always not_run'. This is direct, actionable routing.

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