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

With no annotations, the description carries the full burden. It usefully discloses that queries are read-only DNS lookups and explains the MX-then-A resolution ordering, but it does not state which blocklists are consulted, whether results are aggregated, or anything about rate limits or failure modes.

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?

Three tight sentences with the core purpose front-loaded and the resolution rules following in priority order. Slightly dense with em-dashes, but no sentence is padding.

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 single-parameter, no-annotation, no-output-schema tool, the description covers resolution behavior and read-only safety adequately. It stops short of describing what a positive/negative result looks like or the edge case of a domain with neither MX nor A record.

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 coverage is 100%, so the baseline is 3. The description goes slightly beyond the schema by spelling out the fallback chain (MX IPs, then A record when no MX, literal IP checked directly), which helps an agent predict what the single `target` parameter will actually resolve to.

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 gives a specific verb and resource ('Check whether a mail server is listed on common email DNS blocklists'), making the email-reputation focus clear. However, it never names or differentiates itself from the sibling ip_blocklist_check, so an agent must infer the distinction on its own.

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 DNSBL/email-reputation framing hints at when this tool applies, and the MX-vs-A logic clarifies scope. There is no explicit when-to-use, when-not-to-use, or routing to the obvious alternative (ip_blocklist_check).

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