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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint, so safety is covered. The description adds genuinely useful behavioral context the annotations do not: DKIM is discovered by probing about 29 common selectors (Google, Microsoft 365, Mailchimp, SendGrid), and custom selectors may not be found. That sets correct expectations about completeness of results.

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, no waste. The core scope is front-loaded and the selector-probing caveat follows immediately where it is most useful.

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 not be explained, and the read-only annotations cover safety. The description adequately covers scope and result limitations; the only shortfall is the undocumented domain parameter format.

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 0% and the single 'domain' parameter has no description, so the description carries some burden. It says the tool works 'for a domain,' which implies a bare domain input, but it does not clarify accepted format (bare domain vs URL vs subdomain). Baseline 3 given limited added meaning.

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 specific resource and scope: 'Email authentication for a domain: SPF and DMARC records, and DKIM keys.' An agent can distinguish this from siblings like dns_records, ssl_certificate, or whois_lookup. It stops short of explicitly naming the sibling it is not, so it lands at 4 rather than 5.

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?

Usage is implied by the resource ('for a domain'), but there is no explicit when-to-use guidance and no mention of alternatives such as dns_records or domain_lookup. Nothing tells the agent when this tool is the wrong choice versus a general DNS lookup.

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