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

Annotations already signal non-read-only and open-world behavior, and the description adds a precise cost ($0.002 USDC), which is the most important behavioral trait: this is a paid call. It doesn't elaborate on rate limits or other external side effects, but it covers the key transaction detail.

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?

Two short fragments with no filler; the payment cost is front-loaded, and every listed item adds signal. This is appropriately sized for a one-required-parameter tool.

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 simple paid lookup, the description plus schema covers the input and quote-first payment flow; the listed checks also hint at output fields even though no output schema exists. The main gap is the absence of any guidance about result interpretation or alternative choices.

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?

The schema documents payment_signature well, and the description connects email_or_domain to deliverability checks, but it doesn't specify accepted formats (e.g., 'user@example.com' vs 'example.com') or response details. At 50% schema coverage, the description only partially compensates for the undocumented parameter.

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 lists concrete deliverability checks (MX, SPF, DMARC, disposable flag, trust score), so an agent can see what the tool computes. It lacks an explicit verb and does not differentiate from domain-related siblings like genesis402_whois, so it doesn't reach 5.

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?

The phrase 'Email/domain deliverability' implies the tool is for deliverability checks, but there is no when-to-use or when-not-to-use guidance and no mention of alternatives. Given the sibling tool list includes other domain/web tools, explicit routing would help.

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