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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds real behavioral context beyond them: the annual calculation basis and its few-pounds-a-year divergence from a payslip, the regional coverage, and the 'information, not financial advice' caveat. It still doesn't state response shape beyond a high-level list, which keeps it at 4.

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?

The opening sentence is long but front-loads what is computed and what is returned, and each subsequent sentence earns its place by adding a caveat, a coverage limit, or a disclaimer. Slightly dense punctuation over four sentences keeps it from a 5.

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 ('by the year, month and week, with each tax band, the sources and a link to the worked answer'). Combined with the accuracy caveat, regional coverage and scope exclusions, an agent has everything needed 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 description coverage is 100%, so all ten parameters are already documented and the baseline is 3. The description adds only holistic context (NI category A, tax-calculated-on-annual-basis), and its 'year, month and week' phrasing omits the 'hour' enum value present in 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 and resource with full scope: net UK pay after income tax, National Insurance (category A), student loan and workplace pension, broken down by year/month/week. It is immediately distinguishable from siblings like student_loan_repayments or check_refund_rights.

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?

It states the scope conditions and explicit exclusions (not K codes, benefits in kind, other income, or the self-employed) and clarifies the annual-vs-payslip calculation basis. It does not route the agent to a named alternative such as student_loan_repayments for loan-only queries, 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.