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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so safety is covered structurally. The description still adds real behavior: 'Reads public DNS only' (no auth, no mutation), SPF includes expanded and counted against the 10-lookup limit, and 'about 40 common selectors' checked in addition to user-supplied ones. It does not discuss rate limits or failure modes.

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?

Front-loaded with the plain-language outcome, then the check list, then the output shape and the read-only constraint. Dense and mostly non-redundant, though the middle sentence is a long list that could be tightened slightly.

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 return-shape burden and does so: every finding includes a severity, a one-sentence fix and the RFC/provider document behind it, and detail controls whether passing checks appear. Combined with 'reads public DNS only', an agent has everything needed to call 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 description coverage is 100%, so domain and dkim_selectors semantics (including URL/email inputs) and the detail enum are already fully documented in the schema, anchoring the baseline at 3. The description only marginally reinforces this by noting the ~40 always-checked selectors and that findings are filtered in summary mode.

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 (check/explain) and a tightly scoped resource (email authentication for a domain), then enumerates the exact record families inspected (MX, SPF, DMARC, DKIM, BIMI, MTA-STS, TLS-RPT, bulk-sender rules). That enumeration makes it unmistakable next to check_ssl, check_security_headers and check_domain_registration.

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?

There is no when-to-use guidance and no reference to alternatives, even though the sibling set includes full_domain_report, which plausibly overlaps. The reader gets rich description of what is checked but nothing about when this tool is the right choice or when a sibling is better.

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