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

Goes well beyond readOnlyHint/openWorldHint by disclosing that exactly one CNAME hop is followed, that an empty p= means revoked, and the billing model (x402, $0.002 USDC per successful call, failures not charged) — a cost trait an agent must know before invoking. This is exactly the kind of context annotations cannot carry.

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?

Front-loaded with the verb and DNS target, and the exclusions and pricing come at the end where they belong. The first sentence is a dense run-on enumerating return fields that partly duplicates the output schema, but every clause is informative rather than filler.

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, yet the description still summarizes the meaningful returns (key type, revocation, decoded key size, strength hint, syntax check), and it covers the one real behavioral surprise (single CNAME follow) plus pricing. Nothing an agent needs to call this correctly is missing.

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 baseline is 3, but the description adds value by showing how the two params compose into a DNS name (<selector>._domainkey.<domain>), clarifying that 'domain' is the sending domain and 'selector' the DKIM selector prefix. That composition logic is not derivable from the schema alone.

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 (reads) and a precisely named resource (the DKIM public key at <selector>._domainkey.<domain>), then enumerates exactly what is extracted. The line 'Does not check a signed message' cleanly separates it from the broader sibling security_dmarc_spf_dkim, so an agent can choose without opening either 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?

'Does not check a signed message' is an explicit when-not boundary, and the mention of CNAME following tells the agent this works for large providers where a bare TXT lookup would fail. It stops short of naming the sibling tool to use instead for full message/signature validation, so no explicit alternative routing.

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