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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint false, so the description builds on that. It adds crucial interpretation guidance: read the `note` on each result, DMARC p=none passes checks but does nothing, DKIM may be published under an unknown selector, and 'FAILED' does not mean 'missing'. This goes far beyond what annotations provide and prevents common misinterpretations.

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 well-structured: purpose first, then when to use, then critical caveats and interpretation rules. Every sentence adds value; there is no fluff. The critical warning about reading notes and interpreting FAILED vs missing is front-loaded within the paragraph, making it hard to miss.

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 tool has no parameters and no output schema, the description fully equips the agent to use it correctly. It explains what the tool does, when to use it, and how to interpret results (the note field, DMARC p=none, DKIM selector issues, and the FAILED/missing distinction). The annotations cover safety, so the description is complete for a zero-argument tool.

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 tool has zero parameters, so the schema is trivially complete. The description states 'It takes no arguments', which is redundant but reinforces the schema. Since there are no parameters to explain, the baseline for 0-param tools is 4, and the description doesn't need to add anything more.

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 the tool checks DNS records (SPF, DKIM, DMARC, MX) for the connected mailbox's own domain, which is a specific verb and resource. It distinguishes itself from all siblings (none are deliverability-related), so there's no ambiguity about what it does.

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?

It explicitly lists when to use it: when the user asks why mail goes to spam, why a recipient didn't get something, or whether the domain is set up properly. It also clarifies a key limitation: it cannot look up anyone else's domain. This gives clear guidance on when and when not to use it.

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