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

Annotations already declare readOnly/idempotent/openWorld/non-destructive, so the safety profile is covered. The description adds genuinely useful behavior beyond that: RFC 7505 null-MX detection, implicit A/AAAA fallback, provider detection, and the $0.005 USDC x402 payment requirement (eip155:8453) — cost and payment prerequisites are exactly the kind of context an agent needs before invoking.

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 sentences, both front-loaded with substantive content; the feature enumeration in the second sentence and the price note each carry information an agent cannot get from annotations or schema. No filler.

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 does explain what comes back — priority-sorted MX records, null-MX and implicit-MX cases, provider label — plus the payment model. It is close to complete; a note on error behavior for nonexistent domains would close the remaining gap.

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?

Only one parameter (domain) at 100% schema description coverage, so the schema already does the work. The description adds nothing about the domain argument (e.g., IDN handling, subdomains), leaving this at the baseline.

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+resource: retrieve the MX records of a domain, sorted by priority, with provider detection. This is clearly distinguishable in content from siblings like dns_lookup or email_security, but the description never explicitly contrasts itself with them.

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?

No explicit when-to-use or when-not-to-use statement, and no mention of dns_lookup or email_security as alternatives. The described output (MX priority, null-MX, provider) implicitly signals the intended use case, which is only minimal guidance.

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