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

Annotations already declare readOnlyHint and openWorldHint, so safety is covered. The description adds genuinely new behavioral context: a 3-credit cost per successful call and that schema-invalid input is rejected before billing — non-obvious commercial semantics an agent should know before calling.

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: purpose first, then return shape and cost. Nothing is padded, though the inline field list is dense and could be slightly better separated for scanning.

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 usefully enumerates the returned fields (domain, spf, dmarc, dmarcPolicy, findings, pass) and discloses billing/rejection behavior. Only the meaning of the 'findings' codes and 'pass' semantics remain unexplained, a minor gap for a read-only check.

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 and schema coverage is 100%, with the schema already explaining that IP literals and private suffixes are refused. The description adds no further detail about the domain argument, so it sits at the baseline for fully documented schemas.

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 the specific resource (a domain) and the exact checks performed: SPF and DMARC presence and DMARC policy strength. An agent can tell this is a DNS-record-focused email auth check rather than a generic lookup. It does not, however, name the nearby siblings (dns-lookup, header-audit) it might be confused with.

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 only implied by the subject matter; there is no explicit statement of when to choose this over dns-lookup or security-txt, nor any prerequisites. The billing note hints at cost consideration but not tool selection.

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