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

Annotations already declare readOnlyHint=true and openWorldHint=false, and the description adds genuinely new context: it is a paid endpoint ($0.005/call via x402 or an API key) and it computes from "verified UK Rules data," which speaks to data provenance. It does not disclose rate limits or the response shape, keeping it below 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?

Purpose is front-loaded in the first sentence and the text is short. Minor redundancy: "Paid." is emitted as a standalone fragment and then repeated as "Paid: $0.005 USD per call," which wastes a line.

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?

For a calculation tool with no output schema, the description should say what comes back (per-employee NI amounts, totals, employment-allowance handling). It covers payment and data provenance well but leaves the return values and result semantics 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%, so date format, salary bounds, the 1-200 employee limit and the employment-allowance flag are all already documented in the schema. The description only restates salaries and date at a high level, adding no format or semantic detail beyond the structured fields, so 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 (calculates) and resource (employer Class 1 National Insurance) plus scope (one or more annual salaries, on a date). It is clearly distinguishable from the sibling tools, which are rule/rate/company lookup utilities rather than a calculator.

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?

"Use this instead of doing the maths yourself" gives a clear directive for when to reach for the tool, and the cost note frames the trade-off. It stops short of naming exclusions or alternatives (e.g. when a generic get_rate lookup would suffice), so it is strong context without full routing guidance.

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