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

Annotations already declare read-only, idempotent, non-destructive, open-world behavior, and the description adds substantial context beyond that: no mail is actually sent, results are uncharged if a data source is down, payment is $0.003 USDC per call via x402 on Base, an unpaid call returns the payment requirement, and invalid input is not billed. That is exactly the billing and failure-mode transparency an agent needs before invoking a metered tool.

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 dense sentences that are front-loaded with the core action and output, followed by behavioral and pricing caveats. Every clause carries information, though the billing details make it slightly packed for a single-parameter tool.

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?

There is no output schema, but the description compensates by naming the per-check verdicts, findings, and 0-100 score. Combined with annotations covering the safety profile and explicit payment/failure semantics, an agent has everything needed to call it correctly.

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 the single 'domain' parameter is fully described in the schema, including the reduction of an email address or URL to its domain. The description only echoes this via 'a sending domain', adding no format or validation detail beyond the schema, so the 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?

States a specific verb (Check) plus the exact resource dimensions (MX, SPF, DMARC, DKIM over public DNS) and the output shape (pass/weak/missing per check, findings, 0-100 score). This is clearly distinct from siblings like domain_rdap_lookup or domain_trust_score, which cover registry data and reputation rather than DNS mail authentication.

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 clear framing context — checks are performed over public DNS, no mail is sent, and it works on a domain derived from an email or URL. It does not, however, explicitly say when to prefer this over sibling tools like domain_trust_score, so the routing guidance is implicit rather than stated.

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