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

With no annotations, the description carries the full burden and does well: it discloses the verdict taxonomy, that MX hosts are returned, and the important caveat that catch-all domains may accept non-existent addresses. It omits operational traits like latency of the live MX lookup, rate limits, and whether the lookup is cached.

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?

Three sentences, front-loaded with the core purpose, then the check list, then the returned verdicts, ending with the key limitation. No filler and every clause carries information.

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 single-parameter, no-annotation tool with no output schema, the description covers purpose, the check pipeline, the return shape (verdict values plus MX hosts), and the catch-all caveat — everything an agent needs to call it and interpret results correctly.

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 coverage is 0% and there is a single required 'email' parameter. The description makes clear an email address is being validated but adds no format or normalization guidance (e.g., whether it should be trimmed/lowercased). The parameter is largely self-evident, so this sits at the baseline.

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+resource ('Email address validation') and enumerates the exact checks performed: syntax check, disposable/temporary-domain detection, and live MX-record lookup. An agent can immediately tell this apart from the unrelated sibling tools (fx-rates, weather, web-search), since none of them touch email validation.

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 concrete use contexts ('signup screening, lead cleanup and form verification') and a meaningful boundary ('Does not probe individual mailboxes'). There are no siblings that overlap, so no alternative-tool routing is needed, but it also states no exclusions for when not to use it.

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