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

Annotations already cover readOnly, idempotent, non-destructive and open-world, and the description still adds genuine context: it reads public DNS only, requires no credentials, and returns a graded report with a recommendation. What is missing is return shape detail and any rate-limit or failure behavior, but the annotation bar is already met.

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, zero filler. The capability list is front-loaded and the practical guidance follows.

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?

For a one-parameter, no-credential, annotated read tool this is essentially complete: it names the checks, the side-effect profile, and the output's character. Only minor gaps remain, such as what a failed/unreachable domain returns.

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 100% and the single `domain` parameter is documented in the schema, including URL/email normalization. The description adds nothing about the parameter itself, so 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) and resource (domain email setup) and enumerates exactly what is inspected: MX, SPF, DMARC, detected mail provider, and a graded report. There are no sibling tools, but the scope is still unambiguous and an agent knows precisely what the call produces.

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 a concrete usage context — diagnose a domain before suggesting a sign-up — and clarifies the prerequisite bar (no account, no API key, free to call repeatedly). It stops short of any explicit when-not-to-use condition, but with no siblings there is no alternative to route against.

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