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

With no annotations, the description carries the full burden and does well: it discloses the exact checks performed and the cost model (0.001 USDC per call via x402), with the 402 payment flow documented in the schema. It stops short of describing the response shape or failure semantics (e.g., how invalid vs. disposable results are reported).

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 compact sentences with zero filler, front-loading what is checked and then the pricing constraint.

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?

The core decision inputs — what is validated and what it costs — are covered, and payment mechanics live in the schema. Without an output schema, the description could have said more about what the result contains (validity flag, reason codes), which is the only notable gap for a validation tool.

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 coverage is 100%, so both parameters are already fully documented, including the paymentProof handshake. The description adds no syntax or format detail beyond what the schema provides, so the baseline 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?

States specific verbs and resources: syntax check, MX/A record lookup for mail reception, and disposable/role address detection. An agent can distinguish this from domain_email_security or address_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?

Usage is implied (validate an email address before using it), but there is no explicit when-to-use vs. when-not guidance and no pointer to the related sibling tools such as domain_email_security or address_check.

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