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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description usefully discloses that live MX/SMTP lookups and reputation heuristics occur, which is behavioral context beyond the annotations. It stops short of saying the SMTP check is a live network probe that may be blocked, slow, or inconclusive, and says nothing about what a failed validation returns.

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?

One sentence, front-loaded with the verb and resource, followed by the check list. No filler or restatement of the tool name.

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 should convey the shape of the result — boolean verdict, per-check breakdown, reason codes — and it does not. For a validation tool whose value depends on how the verdict is reported, that leaves a meaningful gap, though the check enumeration does tell an agent what dimensions are evaluated.

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?

Single parameter with 100% schema coverage and a maxLength constraint already documented, so the schema carries the burden. The description only echoes "email address" and adds no format or normalization guidance (e.g., trimming, case handling). 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 and resource ("Validate an email address") and then enumerates the exact checks performed: syntax, MX, SMTP, disposable, role-based. That enumeration distinguishes it cleanly from siblings like dns_lookup or whois, which cover only part of the same ground.

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 listed checks imply when the tool is appropriate (you need a deliverability/quality verdict on an address), so usage is inferable. But there is no explicit when-to-use/when-not statement, no mention of when a plain dns_lookup would suffice, and no prerequisites or caveats for the SMTP probe.

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