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

Annotations already cover safety (readOnly, idempotent, openWorld, non-destructive), yet the description still adds real behavioral detail: domain names go to domain lists while A/AAAA addresses go to IP lists, and unreachable lists are reported as not tested rather than listed. The return-format sentence is partially redundant with the output schema, keeping this just below a 5.

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?

Front-loaded with the core purpose and list coverage, then usage triggers, then the important caveat and routing to the sibling. Slightly dense but every sentence carries information an agent needs.

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 only needs to cover invocation context and edge behavior, and it does: input formatting, domain vs IP handling, false-negative caveat, and tool routing are all present.

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 that the input must be the public IPv4/IPv6 of the sending mail server or a domain, and explains the divergence in how each input type is evaluated.

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 (check) and resource (IP/domain against email blocklists), names the concrete lists queried, and explicitly distinguishes itself from the sibling check_email_domain for SPF/DKIM/DMARC needs.

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?

Gives explicit triggers ('when mail bounces with listed/blocked errors or lands in spam, or after a server was compromised') and names the alternative tool for the adjacent-but-different task. Nothing is left to inference.

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