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

Annotations already declare readOnlyHint and openWorldHint, so safety is covered. The description adds genuinely new behavioral context: the $0.01 USDC cost via x402 and, critically, that the charge only applies on success — information an agent cannot derive from annotations. It does not describe the shape of the validation result.

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 sentence, front-loaded with the verb and the validation scope, followed by the pricing caveat. Every clause carries useful information, though the dense enumeration of checks makes it slightly long for one sentence.

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 two-parameter, read-only, open-world tool with no output schema, the description covers purpose, scope of checks, and cost model. It stops short of explaining what a caller receives back (pass/fail, risk signals) and how email vs domain precedence works, which would complete the picture.

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 100%, so both parameters are already documented in the schema, including that domain is an 'alternative to email'. The description restates the email-or-domain choice but adds no new syntax, format, or mutual-exclusivity detail beyond the schema, so the baseline of 3 applies.

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 gives a specific verb and resource ('Validate an email address or domain') and then enumerates exactly what is checked (syntax, MX, SPF/DMARC, disposable/role-based detection, free-mail flag, domain age). No sibling tool covers email/domain validation, so an agent can identify this tool unambiguously.

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?

'before outreach' establishes a clear use context that tells the agent this is a pre-contact screening step. However, it names no alternatives or exclusions, and does not say what to do when both email and domain are supplied.

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