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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds useful context with the 1-credit cost and output categories, but it does not say whether results can be missing or what external lookup behavior entails beyond the openWorldHint annotation.

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?

It is short and has no wasted sentences, and the cost note is useful. However, the fragment style reads as a field list rather than a front-loaded explanation of what the tool does, weakening structure.

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 lookup with an output schema and read-only/open-world annotations, the description gives cost and output categories. It still omits address semantics and sibling routing, so it is only minimally complete.

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?

The sole required parameter is address, but schema description coverage is 0% and the description never mentions address format, validation, or how the input maps to the returned facts. With one undocumented parameter, the description does not compensate for the schema gap.

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 lists concrete data returned (mail server, disposable/free provider, role mailbox, sender domain facts), but it is a noun-phrase fragment and never states the action or resource itself; the name and title carry 'look up email address.' It also does not distinguish this tool from the sibling lookup_domain despite overlapping domain-related output.

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 gives no when-to-use guidance, prerequisites, or exclusions, and only mentions cost. An agent must infer from sibling names whether lookup_email or lookup_domain is appropriate.

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