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

Annotations already declare readOnlyHint=true, destructiveHint=false, and openWorldHint=true, so safety is covered. The description adds genuinely useful behavioral context beyond them: the validation stops at MX records and does not perform SMTP mailbox verification, so the agent knows a pass does not confirm deliverability. It does not discuss rate limits or latency.

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 sentence that front-loads the verb and resource, then qualifies scope and the key limitation. No filler, no restatement of the name or title.

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?

With an output schema present, return values need no explanation, and annotations carry the safety profile. The description supplies the decisive missing context (no SMTP verification) for a simple one-parameter tool, leaving little an agent still needs before calling it.

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?

Only one parameter exists and schema description coverage is 100%, so the schema fully documents the 'email' argument. The description's mention of syntax/domain/MX matches the schema description and adds nothing new about parameter format or constraints, making the baseline 3 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 gives a specific verb (Validate) and resource (email) plus the exact scope of checks performed: syntax, domain, and MX records. The 'BASIC' framing and the 'without SMTP mailbox verification' clause distinguish it from a deeper validation variant, though no such sibling is named in the provided list.

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?

It states a clear use context ('basic automated checks') and a boundary condition ('without SMTP mailbox verification'), which tells the agent this is for lightweight validation rather than deliverability confirmation. It stops short of naming an alternative tool for full verification, so it lands just below the explicit when/when-not tier.

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