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

Beyond the readOnlyHint and destructiveHint annotations, the description discloses the add-on requirement, that no credit is used, and critically explains the semantics of `listed_on`, `undetermined_on`, and `clean`. The warning that a non-empty `undetermined_on` means UNKNOWN—not clear or listed—is essential behavioral context.

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 with no filler: the first states purpose, the second adds requirements and cost, and the third delivers the critical interpretation caveat. Every sentence earns its place, and key info is 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?

For a one-parameter tool with no output schema, the description fully covers what an agent needs: how to interpret ambiguous results, what `clean` really means, and operational constraints like the add-on requirement. Nothing essential is missing.

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% for the single `target` parameter, and the description mainly echoes the domain-or-IPv4 value type. Since the schema already documents the parameter fully, this is the appropriate baseline; the description adds no new parameter-level meaning.

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') and resource ('a sending domain or IP against email blacklists'), naming concrete lists like Spamhaus, SpamCop, and Barracuda. It clearly positions this as a point-in-time reputation check and differentiates it from sibling tools like check_disposable and check_domain.

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?

It gives clear context: use this to check whether a sender is on major spam-affecting blacklists, as a point-in-time reputation check. It does not explicitly state when not to use it or name alternative tools, but the intended use case is clear enough relative to the sibling list.

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