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

With no annotations, the description carries the full burden and does well: it declares the operation is free, requires no sign-in, sends no email (non-destructive), and notes that big providers skip DNS so results vary by domain. It omits rate limits and any auth/quota caveats, but the behavioral profile is unusually complete for an annotation-free tool.

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 the core action and free/no-sign-in qualifier, then the capability list, then the return shape. The long DNS record enumeration is dense but informative; a slightly tighter grouping would be ideal.

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 describing the return report: a 0-99 score and a verdict with named enum-like values (likely_valid, risky, likely_invalid, unknown). The single parameter is fully covered; only edge-case behavior (errors, rate limits) is unaddressed.

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?

One required parameter at 100% schema coverage, which is the stated baseline of 3. The description adds no format or syntax guidance beyond the schema's 'Email address.', though none is really needed for a self-evident single string input.

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 ('check of an email address') and then enumerates the exact checks performed (format, provider rules, disposable/role detection, MX, SPF, DMARC, DKIM, MTA-STS, TLS-RPT, BIMI). That enumerated depth implicitly separates it from shallower siblings like validate_email or verify_check, so an agent can route correctly.

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 'free, no sign-in needed' framing gives context for choosing it over paid/auth-gated siblings, and 'no email is sent' hints at the non-intrusive path. However, it never explicitly names when to prefer validate_email or verify_start, so selection is left to inference.

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