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

Annotations already declare the tool as read-only, idempotent, open-world, and non-destructive. The description adds behavioral context: it queries multiple external RBLs (Spamhaus, Barracuda, etc.), performs reverse DNS checks, and relies on caching (with force_refresh bypass). These are useful beyond the annotations, though it does not specify cache TTL or behavior when MX records are absent.

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?

The description is two concise sentences. The first delivers the core action with specific details (RBLs, reverse DNS), and the second provides usage context and sibling differentiation. No wasted words, fully front-loaded.

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 an output schema present, the description doesn't need to detail return values. It covers what the tool checks (blocklists, rDNS), caching behavior, and when to use it. Minor gaps: no mention of what happens if the domain has no MX records, but the output schema likely handles that. Overall complete for a reputation check tool.

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 adds meaning by explaining that 'domain' is used to resolve MX records and that the tool checks blocklists/reverse DNS, which gives context beyond the schema's simple descriptions. This raises the score slightly.

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 clearly states the tool checks mail server MX IPs against spam blocklists and verifies reverse DNS. It explicitly distinguishes from the sibling tool 'check_rbl' by stating it targets MX hosts rather than a specific IP, leaving no ambiguity about its purpose.

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 provides explicit usage guidance: 'Use when you want to know if your mail server IP is blacklisted, or if your MX is on any blocklist.' It directly contrasts with check_rbl, which checks a specific IP, giving clear context for when to choose this tool over its sibling.

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