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

Annotations declare a safe read, but the description adds substantial context they cannot: price $0.60, free refusal on coverage_gap with no_charge, the 3-free-calls-per-IP-per-day limit, api_key vs x402 fallback, optional Ed25519 receipt, and the not-legal-advice disclaimer. This is exactly the beyond-annotation disclosure expected.

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 one-call purpose, then constraints and options. Dense but every sentence carries operational information; the receipt clause duplicates the schema slightly but restates the payment caveat usefully.

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?

A 14-parameter paid tool with annotations, high schema coverage and an output schema; the description covers cost, payment path, refusal semantics, receipts and verification. Nothing an agent needs before calling 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 86% and the description still adds meaning: the due_date vs invoice_date+terms_days alternative, receipt being included at no extra USDC on paid success, and api_key bypassing x402. Only lightly beyond the schema, so a 4 rather than 5.

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?

Specific verb+resource: "Check an invoice in one call" plus the three concrete checks (VAT correctness, overdue/late interest, tax ID format). This clearly differentiates it from siblings compute_vat, compute_statutory_interest and validate_tax_id, which each do one part.

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?

Says how to invoke either branch ("Send due_date, or invoice_date with terms_days"), enumerates supported seller countries, and explains the paid/free/refusal routing. It does not explicitly say when to prefer the single-purpose siblings over this composite tool, so it falls 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.