validate_vat

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

Without annotations, the description carries the full burden. It states that the tool performs format and checksum validation only, not a live VIES lookup. It also clarifies the scope (EU/EFTA + UK). No destructive behavior is mentioned, and the note about passing full number or separate country code explains parameter behavior. A slightly higher score could be justified, but there is no mention of output format or error handling.

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 concise, using two sentences plus a note. It is well-structured, starting with an imperative 'USE THIS' to grab attention, followed by the action, scope, and limitations. Every sentence serves a purpose with no redundancy.

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?

Given the tool's simplicity (no output schema, no nested objects, 2 parameters), the description is complete. It covers what the tool does, its scope, its limitations, and parameter usage. No additional information is needed for correct 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?

The input schema has 100% description coverage, so the baseline is 3. The description adds value by explaining how to use the parameters: 'Pass the full number incl. country prefix... or the digits plus a country code.' This clarifies the relationship between the two parameters and provides concrete examples, enhancing usability.

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 that the tool verifies an EU/EFTA VAT registration number's format and checksum. It specifies the action (verify), the resource (VAT number), and the context (before invoicing or onboarding). It also distinguishes itself from a VIES lookup by noting what it does not do, ensuring no confusion with sibling validation tools.

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 explicitly says 'USE THIS to verify... before invoicing or onboarding a business' and tells when not to use it (i.e., not for live registration checks). It provides clear guidance on the intended use case and constraints, setting expectations for the agent.

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