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

Beyond the readOnly/openWorld annotations, the description discloses genuinely useful operational traits: no SMTP probing, a 1-hour cache, and a concrete price ($0.005) with the x402 payment mechanism (USDC on Base or Solana). These are exactly the details an agent needs before calling a paid third-party lookup.

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?

Roughly three sentences, front-loaded with the check list, followed by the exclusion, caching, and pricing details in order of decision relevance. Every clause carries information; nothing 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?

Cost, caching, payment method, and the set of checks are all covered, which is strong for a paid external lookup. However, with no output schema, the description does not indicate the shape of the result (e.g., booleans per check vs. a score), leaving the return contract underspecified.

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?

With 50% schema coverage, the `email` parameter is described only by examples and length constraints, and the description adds no format guidance for it. The payment line implicitly contextualizes the `tx` parameter, but the description never explains the tx field or its relationship to the X-Payment-Tx header, so it only partially compensates for the coverage 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 resource (email address) and enumerates exactly the checks performed: syntax, MX, disposable list, role accounts, and free providers. No sibling tool overlaps this domain, and the scope is unambiguous from the first line.

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 clearly states the applicable context (validating an email for deliverability signals) and explicitly bounds it with 'No SMTP probing,' which tells the caller what it will not do. There are no alternative tools to route to, so the lack of explicit exclusions is a minor gap rather than a real deficiency.

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