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

Annotations already declare readOnly, idempotent, non-destructive and closed-world. Beyond that, the description adds a genuinely non-obvious trait: the output is speculative guesses, not verified addresses, and must be validated. It still omits return shape (count, ordering) but the caveat is real added value.

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 short sentences, zero waste. The core behavior and the example formats come first, and the verification caveat is correctly placed last as a qualifier.

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 no output schema, the description should sketch what comes back; 'list the likely email address formats' gestures at it but gives no detail on format or volume. Combined with the undocumented 'middle' parameter, the definition is adequate but leaves clear gaps for a 4-parameter tool.

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 0%, so the description must carry parameter meaning, and it only partially does. The permutation examples imply how 'first' and 'last' are combined, but the optional 'middle' parameter is never mentioned and nothing explains that only first+domain are required or what the maxLength limits mean.

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 ('list ... likely email address formats') and resource, with concrete example patterns (first@, first.last@, flast@). It is immediately distinguishable from the check_*/generate_* siblings, which all do verification or record generation rather than permutation.

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 caution 'Guesses only: verify before sending' implies a usage context (use before actual outreach, treat output as unverified), but it never states when to prefer this over alternatives like check_mx or check_domain_email_setup, nor any prerequisites. Usage is implied rather than specified.

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