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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint, so the safety profile is clear. The description adds useful behavioral context beyond the annotations: it returns records in priority order, indicates whether any MX records exist, and explains the real-world consequence of no records (mail bounces). No contradiction with 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 tightly packed into three sentences with no filler: the core behavior, a key consequence, and routing guidance. It front-loads the primary purpose and stays well within a readable size.

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 an output schema available, the description does not need to explain return values in detail. It already mentions ordering and existence, adds the bounce implication, and points to the alternative for broader checks. For a single-parameter read-only tool, nothing essential 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?

The input schema fully documents the only parameter, 'domain', including an example and the note that an email address is accepted. The description itself adds no parameters, but that is acceptable because the schema coverage is 100%, so the baseline 3 applies.

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 uses a specific verb-object pair: 'Returns the MX hosts in priority order and whether any exist.' It clearly identifies the resource (mail exchanger records for a domain) and distinguishes itself from check_email_domain by stating it covers deliverability rather than spoofing posture.

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 tells the agent when to use this tool ('Use when you only need deliverability') and when to consider the alternative ('use check_email_domain for the full picture'). This is direct, actionable guidance that resolves routing ambiguity.

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