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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false), so the bar is lower, but the description still adds real value: it discloses a live external check, the specific record types inspected, that fix instructions are returned, and a 10-minute cache. It doesn't discuss rate limits or failure modes, keeping it short of a 5.

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 tightly packed sentences: capability, behavioral detail, then routing guidance. Front-loaded and waste-free, though the opening sentence repeats the title verbatim, which is mild redundancy.

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 describing returns and does so by enumerating MX, SPF, DKIM, DMARC, blacklist status, domain age, and actionable fix instructions. An agent knows what it will get and when to reach for it.

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 parameters are already documented (domain accepts URLs/emails, selector is an optional custom DKIM selector). The description's mention of DKIM records loosely relates to the selector but adds no syntax or format detail beyond the schema, 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?

States a specific verb (inspect) and resource (DNS/deliverability health of a domain) plus the distinguishing constraint 'without adding it'. This lets an agent separate it from add_domain and list_domains without opening any schema.

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?

Explicit USE THIS case (testing/grading arbitrary prospect or candidate domains without saving them) and two explicit DO NOT cases with named alternatives: add_domain for configuring, list_domains for viewing configured domains. Nothing is left to inference.

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