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

Annotations already cover safety (readOnlyHint/openWorldHint), but the description adds real value: it discloses the no-email-sent side effect, the data basis ('9,000+ listed' domains), and the accuracy boundary that a pass does not prove mailbox existence. Return format is not described, keeping it from a 5.

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?

A single dense sentence with zero filler; the check list is front-loaded and the limiting caveat closes it. Every clause carries information an agent can use.

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 one-parameter read-only lookup with no output schema, the description covers what is checked and what is not, which is the crucial context. It leaves the response shape (per-check booleans vs. overall verdict) unspecified, a minor gap given no output schema exists.

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?

The single parameter is fully documented in the schema (100% coverage) as 'Email address to check.', so the schema does the heavy lifting. The description implies the input is an email address but adds no format or constraint detail beyond that. 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?

States a specific verb+resource ('email address check') and enumerates the five sub-checks performed (syntax, MX, disposable domain list, role address, typo detection). This specificity distinguishes it from generic siblings like dns_lookup and domain_rdap 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?

Gives a clear operational context ('without sending anything') and an explicit exclusion ('Does not confirm that a mailbox exists'), which tells the agent when the result is insufficient. It stops short of naming an alternative tool for mailbox verification, so it is short of a 5.

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