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

With no annotations provided, the description carries the full burden of behavioral disclosure. It does explain the core behavior—checking a domain's mail-relevant IPs or a single IP against 24 DNSBLs—and states that it returns per-IP listing status. However, it does not mention network side effects, rate limits, or limitations such as DNSBL false positives.

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?

Two concise sentences with no filler. The core operation is front-loaded, the return behavior is stated, and the 'free tool' note adds relevant context without bloating the description.

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 tool with a fully described schema and no output schema, the description is nearly complete. It explains what input is acceptedшение, how domains are handled, and what the response conveys. It could be more explicit about the exact output format, but the per-IP listing status summary is sufficient for correct invocation.

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 schema already describes the 'target' parameter as a domain or IPv4 address (100% coverage). The description adds meaningful nuance by clarifying that a domain is checked via its mail-relevant IPs, which is not present in the schema.

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 ('Check') with a clear resource ('a domain... or a single IP address') and a precise scope ('against 24 DNS blacklists'). Naming specific lists like Spamhaus ZEN and Barracuda makes it easy to distinguish from siblings such as whois_lookup or check_spoofability.

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 clearly implies when to use the tool: when you need DNS blacklist status for a domain or IP. It does not explicitly name alternatives or exclusion criteria, but the context is strong enough for an agent to select it appropriately among the listed siblings.

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