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

No annotations are provided, so the description carries the full burden. It discloses several important behaviors: no real mail is sent, checks are best-effort for some sources, Google Safe Browsing is skipped if the API key is missing, and the tool is owner-key-gated. It does not detail rate limits, timeouts, or failure behavior, but the disclosed behaviors are substantial and useful.

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?

The description is a single dense paragraph that front-loads the core purpose and then lists the return values and checks. It is efficient and every sentence adds value, though the long list of checks makes it slightly dense. It could be broken into shorter sentences for readability, but it is not bloated.

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?

For a read-only diagnostic tool with 2 parameters and no output schema, the description is quite complete: it explains the verdict scale, the evidence sources, the best-effort nature, the API-key dependency, and the owner-key gate. It doesn't describe the exact JSON structure of the response, but without an output schema, the description's list of return fields is a reasonable substitute. Minor gaps: no mention of timeouts or error conditions.

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%, so the schema already documents both parameters. The description adds context about the domain parameter ('No real email is sent') and the owner_key gating, but it doesn't add syntax, format, or edge-case details beyond the schema. Baseline 3 is appropriate.

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 states a specific verb ('Check whether'), a clear resource ('a domain'), and the key constraint ('without sending real mail'). It also distinguishes itself from the sibling isemailreachable by focusing on domain-level reachability rather than email addresses. The verdict and evidence list further clarify what the tool does.

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 when to use this tool: when you need to assess domain reachability without sending real mail. It does not explicitly name alternatives or exclusions, but the sibling context (isemailreachable) and the phrase 'without sending real mail' provide clear context. It lacks an explicit 'use X instead when...' statement, so it doesn't earn a 5.

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