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

Beyond the annotations (readOnlyHint, openWorldHint, idempotentHint), the description discloses the exact return contents: 'Returns the verdict, its explanation, and the published key record when one resolves.' It also explicitly states that no fix record is returned and why, adding behavioral context that complements the annotations without contradiction.

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 efficient and front-loaded. The first sentence establishes usage context, followed by the core action, return details, and a critical caveat about fix records. Every sentence earns its place; there is no fluff or redundancy.

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?

The tool has an output schema, so return structure is already defined; the description adds context on what those outputs mean (verdict, explanation, key record) and clarifies the absence of a fix record. It covers when to use it, how to call it, and what to expect, making it complete for an agent to invoke 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?

With 100% schema coverage, the baseline is 3, but the description significantly enhances parameter meaning. For domain, it clarifies acceptable formats ('Bare registrable names and subdomains both work; scheme, path or port do not belong here') and normalization ('Unicode accepted to punycode'). For selector, it explains the structure ('name before ._domainkey'), examples, and that it comes from the platform's settings page, providing actionable guidance.

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 states a precise action: 'Check ONE specific DKIM selector on a domain', with a clear resource (domain and selector). It distinguishes from other tools by noting it targets the exact selector the platform uses, which a full scan's common-selector sweep misses, and explicitly mentions it does not return a fix record, setting it apart from tools like scan_domain or build_* tools.

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 opens with explicit usage triggers: 'Use this when the user asks whether DKIM is set up... whether a specific selector exists, or why DKIM fails.' It also gives a key distinction from a full scan and explains that no fix record is returned because the fix is always platform-driven, guiding when NOT to expect a fix from this tool.

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