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

Annotations already establish readOnly, closed-world and idempotent behavior; the description adds real signal beyond that by stating 'Free tier (no network)' and by listing what is explicitly NOT performed (MX, mail auth, domain age, verdict). It does not describe output shape or any rate/quota limits on the free tier, so it stops short of full disclosure.

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?

Two sentences, front-loaded with the caller's situation and the check list, then the escape hatch to the paid endpoint. Every clause carries information; nothing is repeated from the schema or annotations.

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 single-parameter, read-only local check this covers what is evaluated, what is not, and the cost model. The remaining gap is the return shape - no output schema exists, and the description does not say whether results come back as per-check booleans, flags, or a summary string.

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?

There is a single required parameter with 100% schema description coverage, which sets the baseline at 3. The description adds no extra semantics for the 'email' value (e.g. whether display-name forms like 'Name <a@b.com>' are accepted or how it is normalized), so it neither compensates nor detracts.

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 an email address') and enumerates exactly what is validated: syntax, disposable/throwaway provider, role account, free webmail. The email-vs-domain split from sibling check_domain is self-evident from the resource named, so an agent can pick this tool without opening the schema.

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 the triggering context ('an agent is about to store, email or trust') and explicitly routes elsewhere for the heavier checks: MX, mail auth, domain age and a verdict go to the paid endpoint. The condition that selects the alternative is stated outright rather than inferred.

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