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

Annotations only say this is a safe read; the description adds what an agent cannot get from them: the pricing model (0.002 USDC via x402 or 1 Factline API key credit), a free tier of 5 paid-tool calls per day with no key required, and the exact three-way verdict taxonomy. Cost/auth behavior is precisely the kind of disclosure readOnlyHint 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?

Front-loaded with the usage trigger, then checks, then verdict, then payment terms. Every clause carries decision-relevant information (pricing and free tier matter materially for choosing this paid tool); no filler or repetition of the title.

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?

No output schema exists, yet the description names the return values (deliverable / risky / undeliverable) and covers the auth/payment prerequisites in full. Nothing an agent needs before invoking a single-parameter paid verification tool 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?

One parameter at 100% schema description coverage ('Email address, e.g. jan@example.nl'), so the schema already carries the semantics. The description adds nothing about accepted formats or input edge cases, hitting the baseline for fully documented schemas.

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+resource ('verify an email address') and enumerates exactly what is checked (syntax, MX, disposable domains, role addresses, typo suggestions, SPF/DMARC) plus the single verdict returned. An agent can distinguish it from phone_verify or domain_check without opening the schema.

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?

Opens with an explicit trigger: 'Use when you need to know if an email address is real and deliverable before signup, outreach or onboarding.' Strong positive guidance, but it names no exclusions or alternative sibling tools (e.g. phone_verify) for adjacent validation needs.

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