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

Annotations already mark this as a read-only, idempotent, closed-world, non-destructive operation, so the safety bar is low. The description adds genuine context beyond them: it performs no HMRC lookup and verifies nobody, and it explicitly scopes out group companies and business transfers. It does not, however, describe what the returned answer looks like.

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?

Two tight sentences: the verdict and its legal basis come first, the qualifying rule second, and the negative boundaries last. Every clause carries information; there is no filler or repetition of the title.

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?

The decision rule, its legal source, the required inputs and the out-of-scope scenarios are all covered, which is ample for a 3-parameter closed-world predicate. The one gap is the absence of an output schema combined with no statement of what form the answer takes (boolean, plus reason, etc.).

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 formats (YYYY-MM-DD) and the array/comma-separated convention already documented, so the baseline is 3. The description restates the three inputs and reinforces that they are caller-supplied (supporting openWorldHint=false), but adds no syntax or edge-case semantics the schema lacks.

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 ('says whether a contractor must verify a subcontractor with HMRC before paying them') and anchors it to a legal rule (SI 2005/2045 reg 6). It is clearly distinguishable from the sibling tools, which deal with deductions, turnover tests, grossing up, penalties, nil returns and tax months rather than a verification obligation.

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 the positive condition for 'not needed' (subcontractor on one of this contractor's CIS returns, or verified by this contractor, in the payment tax year or the two before) and explicit exclusions ('does not cover group companies or business transfers'). It does not name a sibling alternative for the excluded cases, so it stops short of full when/when-not routing.

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