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

Annotations already indicate read-only, open-world, idempotent, and non-destructive behavior. The description adds context by explaining what the tool actually does with the MX records (identify servers and provider). It does not contradict annotations and provides useful behavioral insight beyond 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 three sentences, front-loaded with the main action. Every sentence adds value: purpose, what it identifies, usage guidance, and context. No unnecessary words. Highly efficient.

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's complexity (3 parameters, 1 required, output schema present), the description is sufficiently complete. It explains the core function, usage scenarios, and its role in the broader scan_domain audit. The presence of an output schema eliminates the need to describe return values. No gaps identified.

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 description coverage is 100%, so the baseline is 3. The description does not add parameter-level details beyond what the schema already provides. The description mentions 'domain' implicitly but no additional semantics for 'format' or 'force_refresh'. This is adequate given the schema's thoroughness.

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 verb 'Look up' and the resource 'MX records for a domain'. It specifies what it identifies (mail servers and email hosting provider) and provides concrete examples. It distinguishes itself from sibling tools like check_mx_reputation by focusing on the MX record lookup and provider identification. The purpose is unambiguous.

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 explicitly states when to use the tool: 'when asked which email provider hosts inbound mail for a domain, or to see MX record configuration'. It also mentions it's part of the scan_domain audit, providing context. However, it does not explicitly exclude other scenarios or mention alternatives, which is a minor gap.

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