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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive, so the safety profile is covered. The description adds real behavioral value by disclosing the return shape: per-check pass/warn/fail, what was found, and remediation guidance, plus the implicit fact that multiple DNS lookups happen against live external records.

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 sentences, both front-loaded and dense: the first states scope and output shape, the second states when to use it. Nothing is wasted, though the first sentence is a long compound clause carrying four check types plus three output elements.

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 compensates by describing the per-check status model (pass/warn/fail), findings, and fix guidance. Combined with fully covered parameters and rich annotations, an agent has everything needed to invoke and interpret 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 coverage is 100%, so both 'domain' and 'selector' are documented in the schema, including the note that common DKIM selectors are tried automatically. The description adds no parameter detail beyond what the schema already provides, so the baseline of 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 and resource (check a domain's email setup) and enumerates exactly what is verified: MX, SPF, DKIM and DMARC. This distinguishes it from the single-record siblings like check_spf/check_dkim/check_dmarc by framing it as the aggregated 'in one go' check.

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 triggering situations: 'why their email goes to spam', 'whether their domain is set up right', 'before sending cold email'. It does not explicitly name the alternative per-record tools (check_spf, check_dkim) or state when to prefer them over this aggregate, so it falls short of a 5.

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