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

Annotations already establish this is a safe read-only, non-destructive, closed-world operation. Beyond that, the description adds two genuinely useful behavioral facts: 'No credit used' (cost) and 'Available on the Qualisend Pro plan' (plan gating), neither of which is derivable from the schema or annotations.

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 compact sentences, front-loaded with the core purpose and followed by the two operational notes. Every clause carries information; no filler or repetition.

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 simple one-parameter, read-only lookup with no output schema, the description covers purpose, the categories returned, cost, and plan availability. The only mild gap is that it does not describe the shape of the result beyond the category names, but that is largely implied.

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, so the schema fully documents the input. The description adds no format, syntax, or validation detail (e.g., normalization of the email) beyond what the schema already provides, which is the baseline 3 case.

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 a specific verb and resource ('Tell whether an email address...') and enumerates the three distinct categories it detects (throwaway/temporary, generic team inbox, free consumer), with concrete examples. It is clear and self-contained, though it never explicitly differentiates itself from siblings like check_domain or check_typo.

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 offers a use case ('useful for spotting low-quality sign-ups'), which implies when the tool is relevant, but gives no explicit when-to-use/when-not guidance and does not name any alternative. An agent must infer that this is distinct from check_domain or verify_email.

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