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

Annotations already declare read-only, idempotent, non-destructive behavior, and the description adds substantial context: what checks are performed, possible status values, score range, confidence/source fields, warnings, disclaimer, and address non-logging. This is well beyond what the annotations provide.

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?

The description is front-loaded with the core checks and use case, and it is generally dense rather than chatty. The lengthy enumeration of response fields is partly redundant because an output schema exists, which keeps it from being maximally concise.

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 read-only lookup with an output schema and safety annotations, the description provides enough context to invoke it correctly: use cases, checks, status semantics, confidence/source metadata, warnings, and privacy behavior.

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 schema explains that a full address is accepted with only the domain part used. The description adds an example domain but no additional format or validation semantics, so baseline 3 is 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 states specific domain-level checks (mail acceptance, MX hosts, disposable/free-mail status, SPF/DMARC) and the intended decision context. Its domain scope clearly differentiates it from sibling tools like verify_email and verify_emails, which operate on individual addresses.

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 gives clear usage contexts: before trusting a sender domain or when onboarding a company. However, it does not explicitly name alternatives or state when not to use it, such as routing single-address checks to verify_email.

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