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

Annotations already cover readOnly, idempotent, non-destructive and closed-world. The description adds genuinely new behavioral context: malformed input containing '@' is refused, and the same engine/keys/limits apply as the REST endpoint. That said, it doesn't elaborate on output or failure modes, and output schema exists anyway.

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?

Front-loads verb, resource and scope, then enumerates return signals, then the use condition. Every sentence carries weight, though the parenthetical list of detection types is somewhat dense.

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 needn't be explained, and the description covers input constraints, detection scope, and rate-limit parity. An agent has what it needs to call this correctly, with only minor gaps in error/edge-case detail.

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 there is a single required parameter, so the schema already documents format and length. The description reinforces the no-'@' rule but adds no syntax or value details beyond the schema – the baseline 3 for full schema coverage 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 (domain) and enumerates the four things it returns: disposable, free webmail provider, look-alike, and typo suggestion. It also explicitly scopes to a domain with no local part, which cleanly separates it from the check_email sibling.

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 a clear use condition: 'Use this when you do not need the part before the @.' This routes the agent away from the email-checking sibling, though it does not name check_email explicitly for a fully explicit either/or. No exclusions beyond the @ restriction.

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