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

Annotations already establish that this is a safe, read-only, idempotent, open-world lookup. The description adds useful context beyond annotations: it identifies VIES as the member-state registry, describes return information (validity, registered name/address, check time), and explains the official-proof behavior when requester_vat is supplied.

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 short and front-loaded with the core question. The second sentence efficiently covers the external source, result content, and the requester_vat proof behavior without unnecessary repetition.

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 3-parameter lookup with no output schema, the description gives enough to call it correctly and understand the main return values and proof behavior. Minor omissions remain, such as invalid-number response behavior or rate-limit/error context, but these are not critical for basic invocation.

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 schema already documents all three parameters. The description reinforces the meaning of requester_vat and the consultation-number proof, but that meaning is already present in the schema, so it adds little beyond the structured parameter documentation.

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 a specific operation: checking whether an EU VAT number is currently valid via VIES, and it names the returned fields. It is distinguishable from siblings by domain/resource (EU VAT vs. EORI, company lookup), though it does not explicitly name sibling tools as alternatives.

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 context implies this tool is for EU VAT validation, and it explains why to pass requester_vat for a consultation number. However, it gives no explicit when-to-use/when-not guidance and does not route the agent away from near alternatives such as eori_check or company_lookup.

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