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

Annotations already mark it readOnly/idempotent/non-destructive, and the description goes well beyond them with real behavioral content: VAT is never taxed under CIS, materials come off before the rate applies, the deduction is rounded down to the penny, and the return contains each step in pence and pounds. That is exactly what an agent needs to trust and interpret a calculation tool.

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?

Three tightly packed sentences, front-loaded with the calculation and direction, then the rules, then the return shape and exclusions. No filler, and the semicolon clauses each carry a distinct rule rather than padding.

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?

There is no output schema, yet the description covers the return shape ('each step of the working, money in pence and pounds'), the money-unit convention, and the domain rules. Combined with annotations covering safety, nothing an agent needs to call it correctly is missing.

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%, so each parameter (gross, vat, materials, rate enum) is already documented in the schema. The description restates the inputs and their relationships but adds no new per-parameter syntax or format detail beyond what the schema carries; 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+resource and pins down the direction explicitly: 'Works out the CIS deduction on a payment to a subcontractor, from the gross bill to the net payment.' The gross-to-net direction is exactly the axis that distinguishes it from the cis_gross_up sibling, so an agent can route without opening either 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?

Gives explicit scope exclusions — 'Does not cover retentions, or decide which rate applies (verify the subcontractor with HMRC for that)' — which routes rate-determination work toward cis_needs_verification. There is no positive 'use this when...' trigger, but the direction statement plus exclusions give the agent enough to select correctly.

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