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

Annotations already declare readOnlyHint, idempotentHint, etc. The description adds value by specifying what is returned (validity, company name, registered address) and that it uses VIES. This enriches understanding beyond annotations, though it omits potential rate limits or service quirks.

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?

A single sentence efficiently conveys the tool's action, source, return data, coverage, and format example with zero wasted words. Excellent structure.

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 simple one-parameter validation tool with a rich output schema, the description covers all essential aspects: purpose, return data, geographic scope, and input format. It lacks discussion of edge cases or error conditions, but is adequate for confident selection and 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 coverage is 100% and the parameter description already includes examples. The description repeats the format and examples but does not provide additional semantic nuance beyond 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?

The description clearly states the tool validates EU VAT numbers via VIES, returns validity status, company name, and address, and explicitly lists coverage (all 27 EU member states + UK) with format examples. This distinguishes it from siblings (currencies/rates) and is specific and comprehensive.

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 description implicitly indicates usage for EU VAT validation, but does not explicitly state when to use versus alternatives or mention any prerequisites or exclusions. Since no sister VAT validation tools exist, the lack of alternatives is partially mitigated, but direct guidance is missing.

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