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

Annotations already cover safety (readOnly, idempotent, non-destructive). The description adds real behavioral context the annotations don't: it automatically probes several well-known selectors before falling back to a user-supplied one, which explains why a result may appear even when the caller knows no selector.

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 and resource and then qualifies the selector behavior. Nothing is redundant or padded.

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 read-only DNS lookup the safety story is complete via annotations, but with no output schema the description says nothing about what a result looks like — e.g. whether it reports which selector matched or what a negative finding means. Adequate but leaves that gap.

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%, so both params are already documented, including the 'URL or email also works' flexibility. The description's remark about auto-tried common selectors mirrors the schema rather than extending it, so the baseline 3 applies.

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?

Specific verb + resource ('look up a domain's DKIM public key') that clearly separates a diagnostic lookup from the sibling generate_dkim_record. It never names a sibling or explicitly states the contrast, so it stops short of a 5.

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 implies usage ('trying the selectors ... plus one you name') and tells the agent it may optionally supply a selector, but gives no when-to-use/when-not or routing against check_domain_email_setup/check_dmarc. Usage is inferred rather than stated.

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