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

Adds substantial context beyond the annotations: the paid x402 flow ($0.01 USDC on Base), the required two-step call pattern with params._meta["x402/payment"], the 10-DNS-lookup SPF budget, and the exact shape of the returned grading data. The payment prerequisite in particular is behavior an agent cannot infer from readOnlyHint/idempotentHint alone.

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?

Information-dense and front-loaded: capability first, then grading output, then usage conditions, then payment mechanics. The final payment sentence is long and slightly run-on, but every clause carries operational value rather than 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?

With no output schema, the description still specifies the return shape (A-F grade, 0-100 score, coded issue identifiers) and the unique payment handshake needed to actually get a result. Nothing essential for a correct first invocation is missing.

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 fully documented, including the reduction of URLs/emails to a domain and the common DKIM selector list. The description's mention of 'your selector or 15 common ones' largely restates the schema, so baseline 3 is appropriate.

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 ('checks a domain's email authentication') and enumerates exactly what is parsed: SPF, DMARC, DKIM keys and MX. It also names the concrete return artifacts (A-F grade, 0-100 score, coded issues), which clearly separates it from siblings like dns_lookup and email_mx_verify.

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 three explicit use cases (judging spoofability, debugging deliverability, vetting a sender/vendor domain), which is well above generic 'use this tool' filler. It does not, however, contrast itself with the nearest alternatives such as email_mx_verify or dns_lookup, so a routing decision still requires inference.

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