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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint, so the safety profile is covered. The description adds material behavioral context beyond that: the cost model (0.01 USDC via x402 or 5 credits of a Factline API key), that the checks run in parallel, that inputs are combinable ('Send any combination'), and the four possible verdict values.

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?

A single dense, front-loaded block that leads with the tool's identity, then capabilities, inputs, eligibility, and finally cost. Every sentence carries information an agent needs, with no padding or repetition.

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 no output schema, the description carries the return-value burden and does provide the four verdict outcomes, plus cost, auth path, and input flexibility. It does not describe the shape of the per-check sub-results or explain the semantics of 'review' vs 'block_review', which is a minor remaining gap for a tool this complex.

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 only 29% (7 params, only vat and name have schema descriptions), so the description must compensate. It adds meaning for name, email, phone and vat ('send any combination'), but leaves type, country and birth_year completely undocumented, so it only partially fills the gap.

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 specific verb and resource ('One-call KYC/KYB onboarding check') and enumerates exactly what it screens (sanctions, email MX/disposable/role/typos, phone line type, EU VAT in VIES). Because it aggregates the individual capabilities exposed by siblings like email_verify, phone_verify, eu_vat_check and eu_sanctions_screen, an agent can readily tell it apart as the composite tool.

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 clear usage contexts ('signup, marketplace seller, payout and B2B onboarding agents'), which tells the agent when this tool is appropriate. It stops short of explicitly naming alternatives or when-not conditions (e.g., 'use email_verify alone if you only need email validation'), so it stays at a strong 4 rather than a full 5.

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