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

Annotations already declare the safe, read-only, idempotent, closed-world profile, so the safety burden is lifted. The description adds real value beyond that: it discloses the return type (true or false) and the important scope boundary that deliverability/mailbox existence are not verified.

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?

Three short sentences, front-loaded with the core purpose, then usage, then return/boundary behavior. No filler, no repetition of the tool 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?

For a one-parameter boolean validator with no output schema, the description covers purpose, usage context, return values, and the key limitation. An agent has everything needed to call it correctly and interpret the result.

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% and the single parameter already documents the format, an example, and the ASCII-only constraint. The description's 'conservative ASCII' phrasing reinforces but does not extend the schema, so 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 (Check) and resource (email address syntax) with a scoping qualifier ('conservative ASCII') that defines the validator's strictness. The resource name cleanly distinguishes it from the many sibling validators like is-url-safe, is-uuid-safe, and is-card-safe.

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?

Explicitly names the intended contexts: rejecting malformed addresses in forms and test fixtures. It also carves out what it is not for ('does not check mailbox existence or deliverability'), which pre-empts a common mis-invocation, though it names no alternative sibling tool.

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