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

Annotations already establish read-only and idempotent behavior; the description adds that validation is deterministic and returns a signed XDR-1 receipt for accounting defense. This is useful behavioral context beyond the annotations, with no contradiction.

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 core validation sentence is clear and front-loaded, but 'CRITICAL TAX COMPLIANCE CHECK' and 'Eliminates cross-border invoicing errors, invalid billing, and tax audit penalties' are marketing overclaims that do not add operational value. The description is short but not every sentence earns its place.

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 read-only check with an output schema and annotations, the description is largely sufficient. It could be more precise about whether non-BE EU VAT numbers are accepted, since 'EU/BE' is ambiguous, but the schema's 'BE mod-97 checksum' phrasing partially resolves this.

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%: the single parameter 'vat_number' is already described as 'EU VAT id; BE mod-97 checksum (BE prefix optional).' The tool description adds no significant parameter-level meaning beyond the schema, so the high-coverage baseline of 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?

The description opens with 'Deterministically validates EU/BE VAT identifiers with MOD-97 checksums,' giving a specific verb and resource. It clearly distinguishes this tool from sibling validation tools such as iban_check, gstin_check, and luhn_check.

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 clearly frames the tool as a tax-compliance check for cross-border invoicing, so an agent can infer when to use it. However, it does not name alternatives or specify when not to use it, such as when a live VAT registration lookup is required.

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