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

Goes well beyond the readOnlyHint/openWorldHint annotations by disclosing the fan-out behavior: for a domain it resolves and checks the domain itself, its A-record IP, and its MX IPs, and each verdict names the IP and its role. That is material context an agent cannot infer from annotations or schema. It stops short of noting rate limits, latency, or partial-failure behavior when some lookups fail.

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 sentences, front-loaded with the core action and provider list, then the domain fan-out behavior, then the return shape. Every sentence carries information; no filler.

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 one fully documented parameter, existing annotations, and an output schema covering return values, an agent has everything needed to call this correctly; the description supplies the only non-obvious piece, the multi-IP verdict semantics.

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% and the single parameter is well documented, so baseline is 3. The description adds value by emphasizing a single target per call ('ONE domain or IP'), implying lists or comma-separated input are not accepted, which the schema does not state.

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 (check one domain or IP against named email blacklists/RBLs) and enumerates the actual providers, which immediately separates it from siblings like dns_lookup, dmarc_check, and spf_analyze. The 'ONE target per call' constraint further pins down its scope.

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?

Clear context for when this is the right tool (reputation listing checks rather than DNS or authentication-record analysis) and an explicit usage constraint ('One target per call'). It does not, however, name a sibling alternative or state what it is not for (e.g., investigating DNS records generally).

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