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

Annotations already declare readOnly/idempotent/openWorld, and the description adds real operational context on top: recursive 10-lookup counting behavior, return shape (grade A-F, score, spoofable flag, parsed records, plain-language issues), error codes (400 invalid domain, 404 NXDOMAIN), and mandatory per-call pricing with the x402 retry mechanism. For a paid third-party API, the payment/retry disclosure is exactly the behavioral detail an agent needs.

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-loads purpose and scope before moving to use cases, return values, errors, and payment, which is a sensible ordering. It is dense and long for a description, but nearly every clause carries distinct information; the record-type enumeration could be tightened slightly without losing meaning.

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?

Despite having no output schema, the description specifies the return payload, the failure modes, and the payment/quota model, so an agent can both decide to call it and handle its result. Nothing material for correct invocation is missing for a two-parameter read-only audit.

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% and both parameters are fully documented there, including the comma-separated selector list, the max of 12, and the default of 12 common selectors. The description only echoes 'DKIM selector probe' and that the domain field accepts an email address, adding little beyond structured fields — 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?

Names a specific verb and resource ('Email authentication DNS audit for a domain or email address') and enumerates exactly what gets checked: SPF with 10-lookup count, DMARC policy/pct/rua, DKIM selector probe, MTA-STS, TLS-RPT, BIMI, MX. No sibling in the list overlaps with this DNS-audit scope, so an agent can route to it unambiguously.

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 use contexts — deliverability checks, Google/Yahoo bulk-sender readiness, spoofing risk — which tells the agent what classes of user request this answers. It stops short of explicit when-not or named alternatives, though the sibling list contains no comparable tool, so the practical gap is small.

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