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

Annotations already cover readOnly, idempotent, openWorld, and non-destructive behavior. The description meaningfully adds the commercial behavior: a limited number of free samples per day, then $0.003 USDC per call via x402, which an agent must know before invoking. It stops short of describing return shape or failure modes.

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 tight sentences, with the output contents front-loaded before the pricing caveat. The output list is slightly comma-heavy but every element is informative; nothing is wasted.

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?

An output schema exists, so return values need no prose explanation, and the cost model plus result scope are documented. For a simple one-parameter lookup tool this is essentially complete, with only the missing sibling-routing guidance as a minor 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?

Single required 'domain' parameter whose schema description coverage is 100%, so the schema fully carries parameter meaning. The description adds no syntax, format, or validation detail for the domain value, so the baseline of 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?

Specific resource (a domain's email authentication) plus an enumerated result set: MX, SPF, DMARC records, mail provider, issues, and a grade. This clearly separates it from siblings like domains__dns or domains__facts, which do not cover mail authentication grading.

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 description implies usage (checking a domain's email auth posture) and adds a valuable cost/rate-limit condition, but never states when to pick this over domains__dns or other domain tools, nor any exclusions or prerequisites. Usage is inferable rather than directed.

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