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

Adds substantial context beyond the annotations: no email is actually sent, checks include syntax/MX/disposable/role, results are cached 6 hours, and the call costs $0.002 USDC via x402 with named supported chains. The payment and caching traits are exactly the kind of behavior annotations cannot convey.

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-loads the core purpose, mechanism, and return statuses before pricing. Every sentence carries information, though the pricing/x402 sentence is dense and slightly long; still no wasted content for a paid tool.

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 compensates by enumerating all five status values and noting the raw SMTP reply. For a single-parameter tool, nothing an agent needs in order to call and interpret it 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?

Schema coverage is 100% and there is only one parameter, so the baseline is already met. The description additionally marks it required and supplies a concrete example input ('{"email":"support@microsoft.com"}'), which clarifies the expected format 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 and resource ('checks whether an email address really exists') and explains the mechanism (SMTP RCPT TO probe). No sibling overlaps, so an agent can select it unambiguously from the listed tools.

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?

Gives three concrete use contexts: before sending outreach, cleaning lead lists, blocking fake sign-ups. There are no competing alternatives among siblings, so no exclusions are needed, but it stops short of stating when not to use it (e.g., bulk volumes, privacy-sensitive cases).

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