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

Beyond the readOnly/idempotent annotations, the description discloses price ($0.60), the free-refusal behavior (coverage_gap, no_charge), the shared 3-free-calls-per-IP-per-day quota, the x402 fallback, and the receipt semantics including what a receipt is NOT (not a certified extract, not legal advice) plus how to verify it. This is exactly the behavioral context annotations cannot carry.

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 cost and purpose are front-loaded well, but the prose is dense and run-on, packing pricing, quotas, receipts, disclaimer language, and a pointer to a non-MCP SKU into long sentences. Much of the receipt/legal-advice caveat duplicates the receipt parameter description in the schema, so some sentences don't fully earn their place.

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?

With an output schema present, return values needn't be explained, and the description covers the remaining agent-facing concerns: cost, quotas, payment modes, refusal semantics, and receipt verification. It is thorough for a 14-parameter, high-complexity composite tool, with only minor redundancy against the parameter docs.

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 schema already documents most parameters; the description still adds cross-field constraints the schema only hints at (due_date vs. invoice_date+terms_days) and, critically, enumerates the supported seller countries, which is a real usage constraint not expressible via the free-form country field. It adds genuine meaning over the schema baseline.

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 names a concrete action ("Check an invoice") and enumerates the three checks it bundles: VAT correctness, overdue status plus statutory late interest, and tax-ID format validation. It implicitly distinguishes itself from the granular siblings (compute_vat, compute_statutory_interest, validate_tax_id) by framing itself as "in one call," though it never names those siblings explicitly.

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 invocation guidance: "Send due_date, or invoice_date with terms_days," and explains the payment path (api_key if you have one, otherwise the x402 payment requirement). It also states the coverage boundary — unsupported seller countries are refused free with coverage_gap. What's missing is an explicit statement of when to prefer this composite over the individual sibling tools.

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