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

With no annotations provided, the description carries full responsibility for behavioral disclosure. It explicitly states that no real mail is sent, lists all the per-check evidence including the SMTP RCPT TO probe, STARTTLS, reverse DNS, and Spamhaus ZEN, and notes that checks are best-effort. This is thorough and honest about the tool's probing nature.

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?

Although the description is long, every clause carries new information: the core no-send behavior, the return shape, the full list of evidence checks, the best-effort caveat, and the owner-key gate. It is front-loaded with the most important semantic distinction before enumerating supporting details.

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?

There is no output schema, so the description compensates by enumerating the return fields and the individual checks an agent can expect. It also covers the authentication requirement and the non-sending behavior, leaving little ambiguity about how to invoke or interpret this tool correctly.

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%, and both address and owner_key are already well documented in the input schema. The description adds no additional parameter-level semantics beyond restating the no-real-mail guarantee and owner-key gating, 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 opens with a specific verb and resource: 'Check whether an email address looks reachable without sending real mail.' It then enumerates the returned verdicts and checks, making the tool's role distinct from siblings like send_email_reply and isdomainreachable even without naming them.

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?

The description implies usage by emphasizing that no real mail is sent and that the tool is owner-key-gated, but it does not explicitly state when to prefer this tool over alternatives or when not to use it. There is no direct mention of isdomainreachable for domain-only checks.

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