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

Annotations already declare read-only, idempotent, open-world, non-destructive behavior. The description adds substantial behavioral context beyond that: paid price, free full refusals, coverage_gap no-charge policy, 3 free calls per IP per day shared with answer tools, api_key/x402 flow, and receipt semantics. No contradiction with annotations.

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 description is dense and covers several concerns in one block, but it front-loads the paid nature and the core question before addressing inputs, refusal policy, and receipts. Most sentences carry necessary payment or coverage information, though the receipt/certified-extract disclaimers make it longer than ideal.

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?

Given a paid, refusal-prone tool with an output schema, the description covers what an agent needs: core purpose, required inputs, payment/refusal behavior, free-call sharing, api_key handling, and receipt verification. Return values are left to the output schema, which is appropriate.

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. The description still adds value by clarifying the due_date OR invoice_date+terms_days input alternative and by explaining that api_key bypasses x402 and that receipt adds an Ed25519 answer receipt on paid success. These additions go beyond what the schema alone states.

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 analytical question: whether an invoice is overdue, by how many days, and how much statutory late-payment interest is owed. It also names the calculation basis (Actual/365 simple, rate and workings cited) and scopes the statutory interest countries. This is enough to distinguish it from generic interest or date tools.

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?

The description gives practical input guidance ('Send due_date, or invoice_date with terms_days') and scopes countries, but it does not explicitly say when to use this tool instead of siblings such as compute_statutory_interest or the certified extract endpoint. Usage is implied rather than contrasted.

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