Check MTD VAT Status

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

Annotations already indicate readOnlyHint=true and idempotentHint=true. The description adds critical behavioral details: it connects to an HMRC sandbox by default, requires specific environment variables for OAuth 2.0 authentication, raises errors if credentials are missing, and warns not to infer status from absence. These disclosures provide substantial value beyond the annotations.

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: it starts with a clear usage condition, states what is returned, and then provides necessary environment setup information. Every sentence adds value, with no redundant text.

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?

With an output schema present, the description appropriately summarizes the return fields (mandate status, effective date, trading name). It covers the core behavior but could briefly mention error scenarios beyond missing credentials (e.g., invalid VRN handling). Overall, it is sufficiently complete for a read-only tool.

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% coverage with a detailed description for the 'vrn' parameter (9 digits, GB prefix handling). The tool description does not add additional parameter-specific information beyond what the schema already provides. Baseline 3 is appropriate as 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 purpose: 'Check MTD VAT Status', and elaborates that it returns MTD mandate status, effective date, and trading name for a business given a VAT registration number. This distinguishes it from sibling tools like hmrc_get_vat_rate and hmrc_search_guidance.

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 9-digit VAT Registration Number and need that business's Making Tax Digital VAT mandate status', providing explicit usage context. However, it does not explicitly exclude alternatives (e.g., using hmrc_get_vat_rate for rate queries) or mention when not to use it, leaving a minor gap.

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