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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered; the description adds genuinely new context by warning that private selectors won't be surfaced and that "none found" is not proof of "no DKIM". It doesn't mention rate limits or how many selectors are typically probed, but the key interpretive caveat is present.

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?

Four short sentences, each doing distinct work: purpose/returns, trigger conditions, a false-negative caveat, and a scope constraint. Purpose is front-loaded and nothing is padded.

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 the description needn't document return shape, and it still gives a one-line summary. Combined with the false-negative caveat and the single-domain constraint, an agent has everything needed to call it and interpret results correctly.

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 schema already states "The single domain to check, e.g. 'acme.com'". The description's "One domain per call" restates that constraint rather than adding new semantics, so 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+resource ("Discover the DKIM signing keys published for ONE domain") and even summarizes the return payload (keys, mapped mail providers, selectors). The DKIM focus makes it trivially separable from siblings like dmarc_check, spf_analyze, and dns_lookup.

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 triggering phrasing ("Use when the user asks about DKIM, signing keys, or 'does this domain sign its mail?'"), which is strong when-to-use guidance. It does not name alternative tools or state when to prefer dmarc_check/spf_analyze instead, so it stops short of full routing guidance.

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