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

Annotations already declare readOnly/idempotent/openWorld, so the bar is lower, and the description adds genuine behavioral context beyond them: the external Bank Rate dependency read daily (explaining the open-world flag), simple day-count interest from the day after due, the default of today when 'paid' is omitted, and size-banded fixed compensation. It stops short of describing the returned figure breakdown.

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 clause, and the dense legal/arithmetic detail that follows is largely load-bearing for a calculator of this kind. The long parenthetical rate clause is heavy but reordering it would lose precision.

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 but a fully covered input schema and rich annotations, the description supplies the calculation method, the edge case (unpaid, defaults to today), and the external data source. It could state the shape of the computed figures, but nothing needed to invoke 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 coverage is 100%, so the baseline is 3, but the description adds real meaning beyond the schema: it clarifies that interest accrues from the day after the due date to the paid date, that omitting 'paid' resolves to today, and that the fixed compensation is banded by the invoice's size (tying back to 'amount'). This is more than the schema restates.

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 names a concrete verb and resource ('statutory interest and fixed compensation on a late business invoice') and grounds it in a specific statute, so it is unambiguously distinguishable from every sibling (none of which computes late-payment interest). An agent knows exactly what this tool produces before opening the 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?

Scope is implicitly constrained to commercial/business invoices by the Act citation, which quietly excludes consumer debts, but the description never states when to reach for this tool versus siblings such as will_they_pay or payment_practices. Usage must be inferred from the domain rather than stated.

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