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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered. The description adds real value beyond them by naming the parsed fields (presence, p= value, raw record) and stating the single-domain-per-call constraint. It does not mention behavior when no DMARC record exists or any rate/lookup limits, which is the remaining gap.

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?

Three tight sentences, front-loaded with the verb+resource, then usage examples, then the routing alternative. Every clause carries routing or scoping information; nothing is redundant padding.

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 an output schema present, the description needn't explain return values, and annotations cover the read-only nature. The one-domain constraint and the alternative-tool routing make it essentially callable as-is; only absent-record / error behavior is left unstated, which is a minor omission for a simple lookup tool.

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 coverage is 100% and the single domain parameter is fully documented in the schema with an example. The description's only addition is the repeated emphasis on 'ONE domain per call,' which reinforces cardinality but adds little beyond what the schema already provides; 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 pair (fetch and parse) and resource (DMARC policy) with an explicit scope of ONE domain, plus what is returned (presence, p= policy, raw record). An agent can immediately distinguish this from email_security_check or dkim_discover without opening a schema.

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 concrete when-to-use examples in quotes ("does this domain have DMARC", "is it p=reject/quarantine/none") and names the alternative email_security_check with the exact condition that selects it (full graded SPF+DMARC+MX posture). It also adds a cardinality constraint: one domain per call.

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