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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, open world), and the description adds materially more: live DNS via Cloudflare DoH, no email is sent, the $0.01 USDC x402 charge with the exact _meta payment key, and that failed/invalid calls are not charged. It also discloses a real limitation — DKIM is only probed on common selectors, so 'not found' is not proof of absence.

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 core purpose, then checks, then usage trigger, then caveats, then cost/failure semantics. Dense but every clause carries information an agent needs — nothing is padding.

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?

No output schema exists, so the description carries the return burden and does: a rough 0-100 score plus plain-English fixes, and the specific record classes examined. It also resolves the payment and error semantics that would otherwise be invisible.

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?

Only one parameter, and the schema already documents it at 100% coverage including the hostname-or-URL example. The description adds no format or parsing detail beyond the schema, 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 ('Review the email authentication setup of a domain') and enumerates exactly which checks are run: SPF + lookup count, DMARC policy/pct/reporting, DKIM selectors, MTA-STS, TLS-RPT, BIMI. This clearly separates it from siblings like check_dns and check_security_headers, which cover unrelated record types.

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 an explicit trigger context: 'Use it before sending mail from a domain or when mail lands in spam.' That is strong when-to-use guidance, though it names no sibling alternative (e.g. check_dns vs. this tool) and states no when-not-to-use condition.

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