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

Beyond the readOnly/idempotent/openWorld annotations, it discloses price ($0.40), the 3 free calls per IP per day shared with the answer tools, the api_key vs x402 fallback, and the Ed25519 receipt option plus how to verify it. Refusal behavior (coverage_gap, no_charge) is also spelled out, which annotations cannot convey.

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 purpose and price are front-loaded and most sentences carry load-bearing information. It loses a point for duplicating the receipt/certified-extract language already present in the receipt parameter schema, and for a dense run-on in the receipt sentence.

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. Coverage is complete for a paid, multi-country calculation tool: inputs, country-regime caveats, pricing, refusal semantics, and receipt verification path are all addressed.

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 a genuine constraint not expressed in the schema: due_date is mutually exclusive with the invoice_date + terms_days pair. It also enumerates the country regime split, adding meaning to the country enum.

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 precise deliverable set: whether an invoice is overdue, by how many days, and the statutory late-payment interest owed, with the calculation convention (Actual/365 simple). It is instantly distinguishable from generic siblings like compute_statutory_interest because it is framed around an invoice payable.

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 input routing ('Send due_date, or invoice_date with terms_days'), states the free-refusal policy for unanswerable cases, and lists which countries fall under the full late-payment regime versus statutory-interest-only. It never names a sibling alternative (e.g. compute_statutory_interest) or explicitly says when NOT to use this tool, so it stops 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.