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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds real context beyond that: figures come from HMRC's published rates for the stated tax year, no account is required, and the response contains effective/marginal rates and payments on account. It doesn't discuss rounding, rate-update lag, or limitations of the estimate, so not a 5.

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?

One dense sentence, front-loaded with the core purpose and inputs before the output list. Every clause carries information, though the trailing enumeration of outputs makes it slightly long; still efficient overall.

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 8 parameters, no output schema, and read-only annotations, the description compensates well by enumerating the returned figures (take-home, effective/marginal rates, payments on account, tips) and clarifying the no-account scenario. Minor gaps remain around assumptions and edge cases for out-of-range or unsupported tax years.

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 88%, so the schema already documents nearly every parameter, including enums for taxYear and studentLoanPlan. The description only echoes 'turnover and allowable expenses' and adds no format, unit, or interaction detail (e.g. how businessMiles interacts with expenses) 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?

Names a specific verb (Estimate), precise resource (UK income tax and Class 4 NI for a sole trader), and the inputs it works from (turnover, allowable expenses). It also enumerates what comes back (take-home, effective/marginal rates, payments on account, tips), which clearly separates it from sibling tools like get_tax_position or check_tax_code.

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 clause 'Works from the figures given, without a TapTax account' tells the agent when this tool is appropriate: standalone what-if estimation rather than account-based lookups. It does not explicitly name an alternative sibling (e.g. get_tax_position) or state exclusions, so it falls short of a 5.

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