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

Goes well beyond the readOnly/idempotent annotations: discloses the $0.60 price, that unanswerable cases are refused free with coverage_gap and no_charge, the shared 3-free-calls-per-IP-per-day quota, the x402 fallback behavior, and the receipt guarantee. It also bounds expectations by disclaiming that the receipt is not a certified extract or legal advice.

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-loads price and purpose, and every clause carries information (pricing, refusal policy, quota, receipt caveats). It is a dense single paragraph with heavy parentheticals and the certified-extract aside is slightly digressive, which costs a point on scannability, but nothing is pure filler.

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?

For a 14-parameter paid composite tool with an output schema, the description covers the gaps that matter: cost, quota, failure mode, payment mechanics, receipt semantics, and verification path via verify_answer_receipt. An agent has everything needed to decide and pay correctly.

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% so the baseline is already high, but the description still adds real meaning: the mutually exclusive due_date vs invoice_date+terms_days pattern, the semantics of receipt on paid successes, and api_key as a metering bypass. The seller-country allowlist (BS, CA, CR, HN, MX, PA, SV) is a constraint the schema does not encode.

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 verb and resource and enumerates the three checks it performs (VAT correctness, overdue/late interest, tax ID format), which is enough for an agent to tell it apart from the narrower siblings compute_vat and validate_tax_id. The 'in one call' framing makes the composite nature explicit.

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?

Gives concrete invocation routing ('Send due_date, or invoice_date with terms_days') and the payment path (pass api_key, otherwise x402 requirement). It never explicitly says when to prefer this over calling compute_vat + validate_tax_id + compute_statutory_interest separately, so the alternative-selection guidance is implied rather than stated.

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