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

Annotations already provide readOnlyHint, openWorldHint, idempotentHint, and destructiveHint, so the safety profile is covered. The description adds the 'curated' qualifier, which implies the list is maintained and possibly non-exhaustive—useful context beyond annotations. However, it does not disclose edge cases, return format, or what happens with invalid domains.

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?

A single sentence that front-loads the action ('Test whether') and the object ('domain') followed by the precise list qualifier. There is no filler or redundant phrasing; every word earns its place.

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, boolean-style lookup with annotations covering safety, the description conveys the core behavior adequately. It could mention that it returns a boolean or doesn't handle full email addresses, but 'Test whether a domain belongs' strongly implies a boolean result and the scope is clear. Minor gap, but not critical.

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 parameter description already explains 'Domain to test, e.g. gmail.com.' The tool description adds no extra meaning beyond what the schema provides, so the 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 a specific verb ('Test whether'), a clear resource ('domain'), and a precise criterion ('belongs to the curated free consumer-email-provider list'). It distinguishes this tool from siblings like email_disposable (disposable domains) and email_validate (general email validation) by highlighting the curated provider-list scope.

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 description makes the purpose clear but does not explicitly state when to use this tool over alternatives such as email_disposable or email_validate. Usage context is implied ('free consumer-email-provider') but no exclusions or alternative routing are provided.

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