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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is fully covered. The description adds only the return shape ('Returns: {amount_net, amount_gross, vat_amount, vat_rate_pct}'), which is also redundant with the existing output schema. No extra behavioral context (rounding, currency handling, precision) is supplied.

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 compact sentence front-loads the verb, resource and operation, then the return shape, then a sibling pointer. No filler; every clause conveys information.

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-purpose calculator with 100% schema coverage, full annotations and an output schema, the description covers purpose, the net/gross duality, and the return fields. The only notable omission is explicit differentiation from the generic and reverse VAT siblings, which leaves a small ambiguity an agent must resolve by name alone.

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%, with enums and defaults documented for mode and rate, so the schema carries the parameter burden. The description's 'net (ex-VAT) / gross (inc-VAT)' phrasing loosely restates the mode parameter but adds no syntax or edge-case detail beyond the schema. Baseline 3 applies.

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?

States a specific verb and resource ('Calculate UK VAT') and the core operation ('convert between net (ex-VAT) and gross (inc-VAT) amounts'). The UK qualifier implicitly separates it from calculate_french_vat, calculate_belgian_vat and other country VAT siblings, but it never explicitly contrasts with calculate_vat_generic or calculate_vat_reverse, so differentiation is partial.

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?

Implied usage is clear (you use it to convert net/gross at UK rates), but there is no explicit when-to-use, when-not-to-use, or routing to alternatives like calculate_vat_generic or calculate_vat_reverse. The 'See list_bundles' pointer is a discovery aid, not selection guidance between siblings.

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