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

Annotations already provide readOnlyHint, idempotentHint, and destructiveHint=false. The description adds behavioral detail: it probes common selectors, validates algorithms, and reports key strength. There is no contradiction. The description adds value beyond the annotations by explaining the internal probing and validation behavior.

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?

The description is three sentences, front-loaded with the core purpose, followed by specific capabilities and usage guidance. Every sentence earns its place with no redundancy or 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?

Given the 4 parameters (100% schema coverage), clear annotations, existence of an output schema, and the family of sibling check_* tools, the description fully covers what the tool does, why to use it, and how it fits into the audit context. No missing information is needed for an agent to select and invoke it 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%, so the schema already describes all four parameters well. The description mentions 'probes common selectors' which adds context for the optional selector parameter, but does not significantly enhance parameter understanding beyond the schema. Baseline 3 is 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 clearly states it looks up DKIM records, probes common selectors, validates signing algorithms (RSA-1024/2048, Ed25519), and reports key strength. It distinguishes itself from the many sibling check_* tools by focusing specifically on DKIM verification.

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 description says 'Use to verify that outbound email signatures are cryptographically sound' and places it as part of scan_domain audit. While it doesn't explicitly list alternatives or when not to use, the domain-specific purpose and sibling names imply when to invoke it. A small gap in explicit comparative guidance keeps it from a 5.

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