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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=true), so the bar is lower. The description still adds real context beyond them: the exact check set, the important caveat that DKIM is only probed via 'common selectors' (a false-negative risk), and the interpretation that results flag spoofable 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?

Two compact sentences with zero filler, front-loaded with the action and the scope of checks. Every clause earns its place, including the parenthetical caveat about selectors.

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 no output schema, the description carries the burden of explaining findings and does so at a high level ('flags spoofable domains'). It leaves the actual result shape and severity semantics unspecified, but for a one-parameter read-only check it is close to sufficient.

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?

One parameter with 100% schema description coverage, so the schema fully documents 'domain' with an example. The description adds no format, normalization, or subdomain-handling guidance beyond it, making the baseline 3 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?

States a specific verb and resource ('Check a domain's email authentication') and enumerates exactly what is inspected: SPF, DKIM, and DMARC. It is unambiguous on its own, though it never contrasts itself with siblings such as check_security_headers or check_ssl, so the agent must infer the boundary from names alone.

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?

Usage is implied by the description ('Flags spoofable domains') but there is no explicit when-to-use, when-not-to-use, or named alternative. For a single-purpose, low-ambiguity read tool this is adequate rather than helpful.

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