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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so safety is covered structurally. The description adds genuine semantic context by naming exactly what the check evaluates (the 10-lookup limit, all-mechanism treatment of unlisted senders), which goes beyond the annotations without contradicting them.

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?

A single, front-loaded sentence that names the subject first and then lists the three evaluated facets. No filler and no redundancy with the schema or annotations.

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 present, the description must signal what comes back, and it does by enumerating the three result dimensions (existence/validity, lookup count, all-mechanism behavior). It is nearly complete; it could still hint at how a count-over-limit or missing record is reported, but an agent has enough to call and interpret it.

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?

There is a single parameter with 100% schema description coverage ('Domain name, for example example.com.'), so the schema already carries the semantics. The description adds no format, normalization, or edge-case detail for the domain argument, making the baseline 3 appropriate.

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 states a specific verb (Check) and resource (a domain's SPF record) and immediately enumerates the exact facets examined: existence/validity, DNS lookup count, and all-mechanism behavior. This is far more precise than a sibling like check_dkim or check_dmarc, so an agent can distinguish it by resource without opening the 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?

Usage is implied by the resource (you call it to audit a domain's SPF configuration), but there is no explicit when-to-use/when-not guidance and no routing to or from the sibling check tools. Nothing tells the agent whether this is a standalone check or part of a combined email-domain audit.

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