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

The annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), and the description adds genuinely useful behavioral context: the specific validation passes performed, including format, branch/government/health-authority type handling, and the HMRC mod-97 / 9755 checksum. It does not describe what the tool returns, which is a minor gap given there is no output schema.

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?

A single front-loaded sentence that names the action, the target, and the exact checks performed. Every clause earns its place with no filler.

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 one-parameter, read-only validation tool with full schema coverage, the description gives enough to invoke it correctly and understand what it checks. The only shortfall is the absence of any hint about the response (e.g., boolean or reason codes), which matters because no output schema exists to fill that gap.

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?

There is a single parameter with 100% schema coverage, so the schema already documents 'value' including an example ('GB980780684'). The description adds the format prefix context (GB) but no syntax or normalization details beyond what the schema and its example convey, which is the expected baseline 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 uses a specific verb ('Validate') plus a precisely scoped resource ('a UK (GB) VAT registration number'), and the parenthetical GB qualifier cleanly separates it from the validate_eu_vat sibling. An agent can identify exactly what this tool operates on without opening the schema.

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?

Usage context is only implied: the agent infers it should be used when validating a UK VAT number. Given the sibling list contains validate_eu_vat and validate_ein, an explicit statement of when to prefer this over the EU variant would have earned a higher score, but no such routing is provided.

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