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

Annotations already cover the safety profile (readOnlyHint, openWorldHint), and the description adds real behavioral value beyond them: the 1-credit-per-successful-check cost model and the fact that results are returned per-source plus an overall summary. Return/pagination details for large source sets are the only gap.

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?

Four sentences, all carrying distinct payload (target scope, return shape, cost, use cases) with no filler, and the core purpose is front-loaded. Slightly dense but every sentence earns its place.

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 read-only, 3-parameter lookup with no output schema, the definition covers the target types, the source-selection option, the cost, and the shape of the result. An agent has enough to call it correctly; only the detailed per-source response format is unspecified.

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 description coverage is 100%, so all three parameters, including the sources override and the target_type enum with 'auto' default, are already documented in the schema. The description adds no syntax or format detail beyond that, which is the expected baseline when the schema does the heavy lifting.

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 address, domain, or URL against major global blacklists and spam databases (DNSBL/RBL sources)'), plus what it returns. It is clearly distinguishable from most siblings, though it never explicitly contrasts itself with the closely related check_spam_score.

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 concrete usage contexts ('deliverability debugging, reputation checks, and abuse triage') so an agent knows when to reach for it. It offers no exclusions or named alternatives, so it lands at clear-context-without-routing rather than a full 5.

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