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

Annotations already establish this is a safe, non-destructive, closed-world read-only calculation, so the safety bar is low. The description usefully adds that the deduction applies to labour only with materials excluded, that rates vary by CIS status, and that the result also reports the net amount received and whether annual deductions will over- or under-shoot the tax due - context the annotations cannot convey.

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 opens with the verb and resource before trailing into rate variants and secondary outputs. It is information-dense with no filler, though the long clause chain makes it slightly run-on.

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 compensates reasonably by naming the three things returned: the deduction, the amount received, and the annual over/under comparison. Six parameters at 100% coverage leave little else unexplained; only the exact return shape remains unspecified.

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% and includes the rate mapping (registered 20% / unregistered 30% / gross 0%) and per-parameter explanations, so the schema does the heavy lifting. The description's 'labour only, materials excluded' framing lightly clarifies how materialsAmount interacts with grossPayment, but adds little beyond the schema.

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 states a specific verb (Calculate) and a precise resource (the CIS deduction on a payment), plus the rate basis and the two ancillary outputs. It is unmistakably distinct from the other calculate_* siblings such as calculate_vat or calculate_take_home_pay, though 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?

There is no when-to-use or when-not-to-use guidance, no prerequisites (e.g. that this is for contractor/subcontractor CIS payments only), and no pointer to adjacent tools like estimate_self_employed_tax for the final tax position. Usage must be inferred entirely from the described scenario.

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