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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is fully covered. The description adds one genuinely new behavioral fact beyond that: the tool actively resolves each MX target rather than only listing records. It does not say how resolution failures are surfaced (error vs. per-record status), whether lookups time out, or what happens for a domain with no MX records.

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?

A single front-loaded sentence with no filler; the core action (list MX records with priorities) precedes the secondary behavior (resolution check). Nothing could be removed without losing information.

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 does roughly describe the return content (servers, priorities, resolution status). For a simple single-parameter read tool that is nearly enough; only failure/empty-result behavior and the exact form of the resolution result are left unstated.

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?

Only one parameter, and the schema documents it at 100% coverage, including the useful note that a full URL or email address is reduced to its domain. The description contributes nothing beyond "of a domain," so the baseline 3 for full schema coverage 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?

States a specific verb and resource ("Lists the mail servers (MX records) of a domain") plus an extra action ("checks that they resolve"). That is clearly more specific than a generic DNS lookup, but it never contrasts itself with the sibling dns_lookup or email_deliverability_report, so an agent must infer the boundary.

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 is only implied: the tool is for mail-server/MX inspection. There is no explicit when-to-use statement, no when-not-to-use, and no pointer to dns_lookup for general record types or smtp_test for deliverability checks. An agent can guess the context but is not routed.

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