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

Annotations already cover safety (readOnly, idempotent, non-destructive) and open-world behavior, and the description adds genuinely useful detail: the 10-DNS-lookup limit on SPF, that problems are returned ranked by severity with a fix each, and that used email services are detected. It doesn't mention response time or failure modes for unreachable domains, keeping it below 5.

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?

Two sentences, zero waste. The full check surface is front-loaded and the return shape follows immediately, so an agent can grasp both purpose and output at a glance.

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?

With no output schema, the description carries the return-value burden and does so: ranked problems, per-issue fixes, and detected email services. For a two-parameter read-only tool with full annotation coverage, nothing an agent needs to call it 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 both parameters are already documented, including the URL/email-address tolerance for 'domain'. The phrase 'DKIM keys on common selectors' hints at the dkim_selectors parameter but adds no format guidance beyond the schema's own example.

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 ('Check') and concrete resources (SPF, DMARC, DKIM, MX) for a domain. The email-DNS scope is unmistakable and clearly different from check_email_headers and check_sending_ip, which operate on messages and IPs respectively.

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?

Usage context is implied by the enumerated checks, but the description never says when to reach for this tool instead of check_email_headers or check_sending_ip, nor any prerequisites. The agent must infer routing from the subject matter.

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