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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint, so the safety/idempotency profile is covered. The description adds that this is an offline format/checksum validation rather than a VIES registration lookup, which is useful context, but the bulk of its content is negative scoping that overlaps usage guidelines.

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 tightly packed sentences with the positive purpose front-loaded and the exclusions following. Appropriately sized for a single-parameter validator, though the second sentence reads as a list of negatives rather than tightly structured prose.

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?

An output schema exists, so return values need no explanation, and the single parameter is fully documented in the schema. The description covers scope and exclusions completely; only the meaning of a failed check (versus a malformed input) is left to the output schema.

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?

With a single parameter and 100% schema description coverage, the schema already documents the vat_number field, its format and the optional BE prefix. The description's 'BE only' and VIES exclusion concern the tool's scope rather than adding parameter-level meaning, so the 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 (Check) and resource (Belgian VAT format and MOD-97 checksum) and bounds the scope with 'BE only'. It distinguishes itself conceptually from other validators via 'not VIES, other EU VAT formats', though it does not name a specific sibling tool.

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?

Gives clear negative guidance: 'not VIES, other EU VAT formats, registration status or tax compliance', which tells the agent when this tool is the wrong choice. It lacks an explicit positive statement of when to prefer it over a live registry lookup, but the exclusions are unusually complete.

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