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

With annotations present, this description goes well beyond them: it discloses that no message is sent and the mail server is not contacted, that results are DNS/syntax signals rather than proof of mailbox existence, and that each successful call costs $0.002 USDC on Base via x402. The cost and payment mechanism are exactly the kind of context annotations cannot express.

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?

Four tight sentences: what it checks, what it returns, what it does not do, and how to pay. The most decision-relevant facts (checks performed, no side effects, price) are front-loaded with no 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?

No output schema exists, yet the description enumerates the return categories (likely-deliverable, risky, undeliverable, invalid) and their reasons, and it covers cost and the key caveat. An agent has everything needed to invoke it and interpret the result.

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?

There is a single parameter and schema description coverage is 100%, so the schema already documents 'address' fully. The description adds no format, normalization, or syntax guidance 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?

The description opens with a specific verb and resource ('Check email syntax, DNS and MX records, disposable providers, role addresses...') and states the exact scope of the check. It is unmistakably distinct from sibling validators like validate_iban or validate_luhn and from domain/domain-report.

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 gives clear positive context ('Use before sending mail or saving a signup'), which tells the agent when this tool is the right choice. It does not name a when-not condition or an alternative sibling to use instead, so it stops short of the 5-level routing guidance.

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