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

Annotations already cover the read-only, non-destructive, closed-world safety profile, so the bar is lower. The description adds real behavioral value by disclosing the computation (standard vs Flat Rate comparison, inclusion of the first-year discount) and what it returns (the cheaper scheme and the difference).

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 opens with the primary check before layering in the comparison detail. It is dense with clauses but contains no filler, though splitting into two sentences would improve scannability.

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 no output schema, the description carries the return-value burden and does so adequately by stating the tool returns the cheaper scheme and the difference. Combined with 80% parameter coverage and clear safety annotations, an agent has enough to call it correctly.

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 coverage is 80%, so the schema already documents nearly all inputs. The description references turnover, tax year, sector rate and first-year discount, but adds no format or unit detail beyond the schema, and vatablePurchases is only implied by 'VAT payable under the standard scheme'. Baseline 3 is appropriate.

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 names a specific verb (check) and resource (UK VAT registration threshold) and adds a distinct second function: comparing standard scheme vs Flat Rate Scheme VAT payable. This differentiates it from siblings like calculate_vat, but it never names an alternative explicitly.

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 is only implied by the scenario (a business evaluating registration and scheme choice). There is no explicit when-to-use, when-not, or named alternative such as calculate_vat, so the agent must infer the boundary.

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