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

Beyond the readOnly/idempotent annotations, the description discloses price ($0.40), the free-refusal rule (coverage_gap, no_charge), a shared rate limit (3 free calls per IP per day with the answer tools), auth paths (api_key vs x402 402 response), the receipt-issuance behavior on paid successes only, and explicit non-legal-advice / not-a-certified-extract caveats. That is unusually rich behavioral context.

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?

Front-loaded with price and the core question, then inputs, then payment/refusal mechanics — a sensible order. It is dense but most sentences earn their place, with only mild duplication of the receipt explanation already present in the schema.

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?

An output schema exists, so return values need not be described; the description still covers pricing, auth, refusal, receipt verification, and disclaimers thoroughly. The one missing piece is a comparison to the overlapping sibling tool, which leaves the selection decision under-supported for this complexity.

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 the baseline is 3, but the description adds the either/or rule for supplying the date (due_date, or invoice_date plus terms_days) and clarifies that receipt on an unpaid 402 or free refusal yields nothing. Those are semantic relationships the schema fields state only individually.

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 opening sentence states a specific question set — is the invoice overdue, by how many days, and what statutory late-payment interest is owed — with the calculation method (Actual/365 simple) and supported regimes (FJ, NZ, SB, TO, WS). It is clear what the tool does, but it never distinguishes itself from the sibling compute_statutory_interest, which an agent would reasonably consider an overlapping alternative.

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 gives concrete invocation guidance ('Send due_date, or invoice_date with terms_days') and a refusal policy for unanswerable inputs, which implies scope boundaries. However, it names no alternative tool and does not say when to prefer this over compute_statutory_interest or the certified/answer tools, so routing remains inferential.

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