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

No annotations are provided, so the description carries the full burden of behavioral disclosure. It reveals only one negative trait (no WHOIS/ownership data) and says nothing about side effects, return format, or behavior on non-hostname inputs. The schema's json parameter ('discarded after the check') sits uneasily beside a hostname-splitting purpose, and the description does nothing to resolve this tension.

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?

A single nine-word sentence, front-loaded with the action ('Split a hostname into labels') followed by a scope qualifier. Every word earns its place and there is no redundancy or filler.

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 4 unrelated parameters, no output schema, and no annotations, an agent cannot tell what the 'labels' output looks like, which parameters to populate, or whether full URLs are accepted. The mismatch between a hostname-splitting purpose and json/zone parameters leaves the tool underspecified for reliable invocation; a short example or input-form clarification would materially help.

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 the baseline is 3; per the rubric the description need not re-explain parameters. It adds a coarse hint that hostname-shaped inputs (host or url) are relevant, but it provides no mapping for the json and zone parameters, and its single-purpose framing partially conflicts with the multi-purpose schema. This nets out at baseline.

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 states a specific verb and resource ('Split a hostname into labels') plus a negative scope ('without WHOIS or ownership data'), making the core action clear. It is distinguishable from siblings like normalize-url and citation, though the contrast is slightly abstract since no listed sibling actually provides WHOIS data. The broad 4-parameter schema (url, host, json, zone) muddies which inputs this tool actually consumes, which keeps this from a 5.

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 offers no guidance on when to use this tool versus alternatives, and no indication of the expected input form (bare hostname vs full URL). The negative clause implies a contrast with ownership-enrichment tools but never names one or states the selection condition, leaving the when-to-use decision entirely to inference.

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