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

Goes well beyond the annotations: it states no message is sent and the mail server is never contacted, defines the limits ('DNS and syntax signals, not proof that a mailbox exists'), discloses pricing ($0.003/call, first 25 free, failed calls free) and auth (x402 USDC or card key), and proactively explains the non-obvious readOnlyHint=false ('reads or computes only; not marked read-only because each call is charged').

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?

Front-loaded with what it checks, then caveats, use guidance and pricing. Every sentence carries information, though the billing/auth and pricing details make it denser than a purely functional description.

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?

With no output schema, the description still enumerates the possible return verdicts (likely-deliverable, risky, undeliverable, invalid with reasons) and covers cost, auth and the meaning of the signals. Nothing needed to call it correctly or interpret results is missing.

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?

Only one parameter and schema coverage is 100%, so the schema already fully documents 'address'. The description adds a concrete example value (support@cloudflare.com), which is mildly helpful but not meaningful semantic depth beyond the schema.

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 a specific verb (Check) and resource (email syntax, DNS/MX, disposable providers, role/free providers, SPF/DMARC) and names the exact verdicts returned. The scope is distinctive from siblings like domain, sanctions_check or x402_check, so an agent can route to it 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?

Explicitly says to use it 'before sending mail or saving a signup,' giving a clear triggering context. It does not name alternative tools or state when not to use it, so it stops short of full when/when-not guidance.

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