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

Well beyond the readOnly/idempotent annotations, it discloses price ($0.60), the free-refusal-on-coverage-gap behavior (coverage_gap, no_charge), shared 3-free-calls-per-IP-per-day quota, api_key vs x402 payment flow, receipt semantics, and explicit scope limits (not a certified extract, not legal advice). For a paid, open-world read this is exactly the behavioral context an agent needs.

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?

Purpose is front-loaded in the first sentence and nearly every subsequent clause carries real information (pricing, refusal behavior, receipt caveats). It is dense and mixes payment/legal caveats into one block rather than structuring them, which slightly hurts scannability but no sentence is 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?

With 14 params, an output schema, and full annotations, the description fills the remaining gaps: cost, quota, refusal semantics, payment modes, and the receipt/legal-advice boundaries. An agent has everything needed to decide and call 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 high (86%), so the baseline is 3, but the description adds cross-parameter logic the schema does not: the due_date-vs-(invoice_date+terms_days) choice, the receipt flag's effect on paid success, and the api_key/x402 branching. That relational guidance justifies a bump above 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?

States a specific verb and resource ('Check an invoice in one call') and enumerates the three sub-checks: VAT correctness, overdue/interest, tax-ID format. This clearly distinguishes it as a composite invoice check rather than any single sibling. It stops short of naming the siblings it bundles (compute_vat, validate_tax_id, compute_statutory_interest), so an agent must infer the differentiation.

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 one concrete calling rule ('Send due_date, or invoice_date with terms_days') and explains the free-refusal and payment paths. It never says when to prefer this bundled check over the individual compute_vat / validate_tax_id / compute_statutory_interest tools, so usage relative to alternatives is only implied.

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