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

Annotations provide openWorldHint and idempotentHint, but the description adds meaningful behavioral detail beyond them: the state change to 'verified,' the relay switching to the custom From with DKIM signing, fallback to the platform address until verified, and DNS propagation causing transient failures. This contextualizes side effects and retry behavior without contradicting the annotations.

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 front-loaded with the core action, then efficiently explains the verification outcome, fallback behavior, retry guidance, and the smtpRelay exception. Every sentence carries useful information and none is filler or redundant with the schema.

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?

For a single-parameter tool with an output schema and rich annotations, the description is complete: it covers the DNS records checked, the success/failure transition, retry rationale, and the case where this tool is unnecessary. An agent can correctly select and invoke the tool without missing key expectations.

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?

The schema already documents the sole parameter siteName with 100% coverage, describing it as 'The site whose custom sender domain to (re)check.' The tool description reinforces the same concept but adds no new parameter-level detail, formats, constraints, or default behavior, so the 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 a specific verb and resource: 'Check the DNS records for a site's custom sender domain (DKIM TXT + SPF include).' It also identifies the domain type (forms custom sender domain) and the verification workflow. It does not explicitly contrast with the sibling verify_custom_domain, so it stops short of full sibling 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 description gives clear when-to-use context: check DNS records, mark verified once DKIM is observed, and rerun if DNS propagation causes a first failure. It also provides a when-not-to-use condition: 'Not needed when the site uses a custom smtpRelay.' It does not explicitly name an alternative tool for that case, so it lacks a full alternatives statement.

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