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

Annotations already cover readOnly/idempotent/non-destructive, so the description's job is to add what they can't — and it does: it discloses the return shape (0-100 score, letter grade, prioritized fixes) and, critically, the x402 payment requirement of $0.02 USDC per call on eip155:8453. Undisclosed are failure modes (invalid domain, missing records) and whether partial results are still billed.

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 checks, then return values, then price — a sensible order with no filler. The only waste is the first clause, which restates the title verbatim before the more informative sentence begins.

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 correctly supplies the return contract (score, grade, prioritized fixes) and the payment precondition. For a two-parameter, read-only audit tool, an agent has everything needed to call it and interpret the result.

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 50%: dkimSelectors is documented in the schema, while the required 'domain' parameter carries only a maxLength. The description's mention of 'DKIM keys at common selectors' usefully implies a built-in selector list that dkimSelectors appends to, but no format detail (bare domain vs FQDN vs IDN) is added for the required parameter.

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?

The description names a concrete verb and resource ('audits a domain's mail setup') and enumerates exactly what is inspected: MX, SPF tree and lookup count, DMARC policy, DKIM selectors, MTA-STS, TLS-RPT, BIMI. It implicitly separates itself from the granular siblings (dns_mx, dns_spf, dns_dmarc) by framing this as one aggregate call, but it never names those siblings explicitly.

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?

The phrase 'One call audits a domain's mail setup' hints that this is the aggregate alternative to the per-record DNS siblings, but it is inference rather than instruction. There is no explicit when-to-use/when-not statement and no named alternative, so guidance is only implied.

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