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

No annotations exist, so the description carries the whole burden. It does disclose useful behavior, notably that unsupported countries return 'unsupported' instead of inferring a pattern and that it never reports registration or active status, but the final clause ('and a pass is ') is cut off mid-sentence, so the meaning of a passing result is never delivered. Batch semantics (up to 100 per call) are stated, which is a genuine addition over the raw schema.

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 scope, country list and limits are front-loaded with an em-dash separator, and most sentences are dense and non-redundant. The one structural defect is the dangling final clause 'and a pass is ', which is a truncated fragment that occupies space without conveying anything.

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 tool with no annotations, no output schema, and a nested items object, the description covers purpose, scope, batch size and key limitations, but never describes what a per-item result looks like (valid/invalid/reason codes) and is truncated before explaining what constitutes a pass. An agent would still need to discover the response shape empirically.

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?

The description explains the batch shape implicitly via 'up to 100 EU VAT numbers per call', which matches the items array's maxItems, but adds nothing about the per-item object, the optional two-letter country field, or the accepted input forms (prefix, spaces, dots, hyphens) beyond what the schema already declares. With effectively low schema-description coverage at the top level, this leaves a gap, though not a fatal one for a single-parameter tool.

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 names a specific verb and resource ('Checks the structure and check digit of ... EU VAT numbers') and pins the scope to exactly five countries (AT, BE, DE, DK, FI) with an explicit batch ceiling of 100 per call. An agent can tell instantly what this tool validates and what it will not.

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 states the triggering situation ('for an agent about to put a supplier or customer VAT number on an invoice or into a record'), an exclusion ('every other country answers unsupported rather than guessing a pattern'), and redirects the registration/active-status question to VIES or the national register. That is explicit when-to-use, when-not-to-use, and alternative routing.

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