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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive, so the safety profile is covered. The description adds genuine behavioral context beyond that: it names the backing registry (VIES), states what comes back (validity, registered name and address, check time, consultation number), and notes the check is free when a registry is down. Missing only failure-mode detail for invalid numbers.

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 leading question front-loads the purpose, followed by the registry and return values, then the optional-param tip and a cost note. Sentences are short and mostly earn their place, though the final sentence ('A registry that is down costs nothing') is slightly cryptic phrasing for a no-charge guarantee.

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?

There is no output schema, so the description carries the return-value burden and does so: validity, registered name and address, check time, and the consultation number. Combined with annotations covering safety and idempotency, an agent has everything needed to call this 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 coverage is 100%, so the schema already documents all three parameters, including prefix handling and the purpose of requester_vat. The description re-explains requester_vat as proof of the check in your name, which reinforces but largely restates the schema. 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?

States a precise verb and resource: validates an EU VAT number against VIES, the member state registry. This is clearly distinguishable from siblings like eori_check, lei_lookup, and company_lookup, which cover different identifiers. An agent knows exactly what this tool answers.

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?

Usage is implied by the resource (check a VAT number) and the description does explain when to pass the optional requester_vat (to obtain a consultation number). However, it never states when NOT to use this tool or names an alternative sibling for overlapping identifier checks. Adequate but with a clear gap.

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