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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely useful behavior — an implicit fallback sweep across common selectors when selector is omitted, implying extra DNS lookups and latency — but says nothing about what a failed or missing lookup yields.

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 short sentences, no filler, with the core action front-loaded and the selector-fallback rule immediately after. Nothing could be removed without losing information.

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?

A simple read-only lookup with full param coverage and annotations needs little more, and the fallback behavior is disclosed. However, with no output schema, the agent is left guessing whether the result is a boolean, the key text, the resolved selector, or an error, which matters for a validation 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%, so both parameters are already documented, including that a URL or email address is reduced to its domain. The description's selector note largely restates the schema's "Leave out to try the common ones" and adds no new syntax or format detail, making baseline 3 correct.

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 and resource: "Looks up and validates a DKIM public key." An agent can tell this apart from dmarc_check or spf_check by resource, but the description never names those siblings or draws an explicit boundary, so it falls short of full differentiation.

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?

The only guidance is operational, not comparative: supply a selector if known, otherwise common selectors are tried. There is no statement of when DKIM validation is the right check versus dmarc_check/spf_check/bimi_check, or of prerequisites such as needing a signed message context.

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