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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint and destructiveHint=false, so the safety profile is fully covered elsewhere. The description adds only the shape of the result ('with their preferences'), and says nothing about behavior for nonexistent domains, empty MX sets, timeouts, or whether results are sorted by priority.

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?

One sentence, front-loaded with the verb and resource, and every clause ('MX records', 'of a domain', 'with their preferences') contributes information. There is no filler or redundant restatement of the title.

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?

For a one-parameter read-only lookup with full annotation coverage and no output schema, the description is nearly sufficient: it says what is fetched and what is returned alongside each record. The remaining gap is edge-case behavior (no MX records, resolution failure), which keeps it short of a 5.

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?

There is a single required parameter with 100% schema description coverage, so the schema already carries the load; the baseline of 3 applies. The description's phrase 'of a domain' restates the parameter without adding format guidance (e.g., whether to pass an apex domain, a subdomain, or an IDN/Unicode form).

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 names a specific verb and resource ('List the mail exchanger (MX) records of a domain') and even states the returned attribute ('with their preferences'). It does not, however, distinguish itself from overlapping siblings such as dns_lookup or dns_report, so an agent cannot tell from the text alone which one to pick.

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?

There is no when-to-use guidance, no exclusions, and no mention of alternatives. With siblings like dns_lookup and dns_report in the catalog, a sentence such as 'use for MX-only lookups; use dns_lookup for arbitrary record types' would have earned a higher score.

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