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

Even though readOnlyHint and openWorldHint are already present, the description adds valuable behavior: it names the Cloudflare DNS-over-HTTPS source, the 5-second timeout, the exact status response variants, and how to interpret not_applicable vs error. This goes well beyond the annotations.

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?

The description is front-loaded with the core purpose and use case, then compactly covers source, timeout, response shapes, and interpretation. Every sentence adds needed information with no fluff or repetition.

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 only one parameter and no output schema, the description fully compensates: it documents success, absence, timeout, and error response shapes, and clarifies operational semantics. An agent has what it needs to call the tool and interpret results 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 schema already explains that domain accepts public names and rejects IPs, ports, paths, and protocol prefixes. The tool description adds no additional parameter-level meaning, 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?

The description states an exact verb and resource: it lists a domain's MX records sorted by priority. The phrase 'see where a domain's inbound mail goes' and the linkage to SPF/DMARC distinguish it from generic sibling tools like dns_lookup and dossier_dns.

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?

It gives clear when-to-use guidance: use it to inspect inbound mail routing or before checking SPF and DMARC. However, it does not explicitly name alternatives or state when not to use it, so the guidance stops short of a full when/when-not comparison.

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