Check Vat Number

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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint false. The description adds crucial behavioral context: it queries a live external register (consistent with openWorldHint), explains that a 'not registered' result is a real answer not an error, and instructs the agent to report the registered name and highlight mismatches with the contact's name. This goes beyond annotations and is essential for correct usage.

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 well-structured, front-loading the core purpose before diving into usage and behavioral nuances. Every sentence adds meaningful information: purpose, geographic scope, usage triggers, result interpretation, and reporting guidance. No wasted words; it is concise yet comprehensive.

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 only two parameters, no output schema, and annotations covering safety, the description fully covers what an agent needs: the exact function, the meaning of edge-case results, and how to handle the response (report name, flag mismatches). There is no missing critical information for correct invocation and use.

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 description coverage is 100%, so the baseline is 3. The description adds value by clarifying that vat_number can include or omit the GB prefix, and that contact_id is optional and used for reporting results against the contact. These details help the agent understand the intended usage beyond the raw schema.

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 checks a UK VAT registration number against HMRC's live register and returns the registered trader name and address. It specifies 'GB only' and the exact use cases, effectively distinguishing it from sibling tools like get_vat_summary by focusing on validation against the live register.

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 provides explicit when-to-use conditions: when the user asks about validity, when adding/editing suppliers/customers with VAT numbers, or when confirming a VAT number on a document. It also clarifies that 'not registered' is a legitimate answer. It does not explicitly mention alternative tools, but the 'GB only' constraint and the specific use cases make usage clear enough.

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