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

Beyond the annotations (readOnlyHint, idempotentHint, destructiveHint), the description discloses that the data comes from a static lookup table current as of 22 Nov 2023, that rates may have changed, and that results include effective date and conditions. This adds valuable behavioral context about data freshness and scope.

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 and well-structured: a usage directive, a clear statement of return value, and an important caveat about static data with an alternative. Every sentence adds meaningful information without redundancy, and it is front-loaded with the primary use case.

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 single-parameter tool with an output schema, the description sufficiently covers the core purpose, return expectations, and a critical limitation (static data). It also points to an alternative for verification, making it complete for the tool's complexity and context.

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 already provides 100% coverage for the single parameter (commodity_code) with a detailed description and examples. The tool description adds only the 'UK' qualifier and switches 'code' to 'description,' which is minimal additional semantic value. The baseline of 3 is appropriate when the schema does the heavy lifting.

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's function: to get the VAT rate category for a UK commodity or service, with a specific verb ('get') and resource ('VAT rate'). It distinguishes itself from sibling tools by focusing on VAT rate lookup and explicitly mentions the return fields (rate, effective date, conditions).

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 begins with 'USE THIS TOOL WHEN you have a UK commodity or service description and want its VAT rate category,' providing a clear usage condition. It also names an alternative tool (hmrc_search_guidance) for time-sensitive advice, thus offering both when-to-use and when-not-to-use guidance.

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