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

Annotations already declare readOnly/idempotent/non-destructive, so the bar is low, yet the description adds substantial non-schema behavior: $0.40 pricing, x402 payment requirement vs api_key bypass, free refusals with no charge on coverage gaps, a shared 3-free-calls-per-IP-per-day quota, and receipt semantics. It also clarifies the receipt is not a certified extract and not legal advice, which prevents misuse of the returned artifact.

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?

Most sentences carry load-bearing information about payment, refusals, quotas, and receipts, so it is not padding. But it opens with a price tag rather than the tool's purpose, and the certified-extract and legal-advice caveats could be compressed, making it dense and hard to scan for an agent.

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 no explanation, and annotations cover the safety profile. The description handles the remaining agent-facing concerns (payment path, coverage-gap refusals, quota sharing, receipt verification via verify_answer_receipt). It is close to complete, with only minor gaps such as the effect of the debtor and as_of inputs, which the schema already documents.

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 schema already documents all nine parameters and a baseline of 3 would be defensible. The description still adds meaning the schema does not carry cleanly: the due_date-versus-(invoice_date + terms_days) choice, the api_key bypass effect, and that receipt defaults to false and only appears on a paid success.

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 compound job: determine whether an invoice is overdue, by how many days, and the statutory late-payment interest owed under Actual/365 with rate and workings cited. This is far more than a restatement of the name and separates it from generic calculation siblings. It stops short of naming the sibling it most overlaps with (compute_statutory_interest), so sibling differentiation is only partial.

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 real invocation conditions ('Send due_date, or invoice_date with terms_days') and states that unanswerable cases are refused free with coverage_gap, which tells the agent what to expect. However, it never says when to prefer this tool over compute_statutory_interest or the other interest/deadline siblings, leaving routing to inference.

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