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

Annotations already cover safety (readOnly, openWorld, non-destructive), and the description adds real behavioral context beyond them: a live official check against VIES, distinct retryable-error semantics for registry outages, and the caveat that Spanish/German registries may withhold the name. It also explains what registry_match actually represents (our own register data, not VIES confirmation), preventing a plausible misinterpretation.

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?

Front-loaded with purpose and scope, then edge cases in descending importance. It is dense but each sentence carries information an agent needs; the final registry_match sentence is slightly convoluted but resolves a real ambiguity about return values.

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 burden of describing returns — and it does, covering the valid-case payload and the registry_match object's fields. Combined with the scope limits and error semantics, an agent has everything needed to call and interpret this single-parameter tool.

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% and the schema already explains the country prefix and space/punctuation handling, so baseline would be 3. The description adds one genuinely useful format rule beyond the schema — Greek numbers must use the EL prefix — which prevents a common malformed call.

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 and resource ("Validate an EU VAT number against VIES") plus the return payload (registered trader name and address). It also pins the exact geographical scope (27 EU states + XI, EL prefix for Greece, GB out of scope), so an agent knows precisely what this tool will and will not accept.

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 conditions for use and explicit exclusions (GB post-Brexit not covered, Greek numbers need EL), plus the crucial guidance that a retryable error means a down registry rather than an invalid number. It stops short of naming a sibling alternative for out-of-scope lookups, but no sibling covers VAT validation.

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