Check MTD VAT Status

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

The annotations already declare readOnlyHint=true, destructiveHint=false, idempotentHint=true. The description adds significant behavioral context: default sandbox connection, production override via HMRC_API_BASE, required OAuth credentials, and error behavior when credentials are missing. It also includes a caution not to infer status. No contradiction with 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 (roughly 80 words) and well-structured: a clear usage directive in the first sentence, followed by return details, environment setup, and credential requirements. Every sentence adds necessary information without 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?

For a simple tool with one parameter, annotations covering safety, and an output schema (context indicates it exists), the description covers all critical aspects: purpose, return content, environment configuration, credential prerequisites, and error conditions. There are no gaps given the tool's complexity.

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 provides a detailed description for the single parameter 'vrn' (format, examples, handling of GB prefix), giving 100% coverage. The tool description does not add additional parameter information beyond what the schema provides. Per the guidelines, high schema coverage warrants a baseline of 3.

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: checking MTD VAT status for a given VAT registration number. It uses a specific verb ('check') and resource ('MTD status'), and the title reinforces this. Among sibling HMRC tools (e.g., hmrc_get_vat_rate, hmrc_search_guidance), this tool is uniquely identified, so no confusion.

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 states when to use the tool ('USE THIS TOOL WHEN you have a 9-digit VAT Registration Number and need that business's Making Tax Digital VAT mandate status.'). It also provides context about environment setup (sandbox vs production) and credentials. However, it does not explicitly state when not to use it or mention alternatives, but given the sibling tools cover different functionality, no exclusion is needed.

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