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

With no annotations, the description carries the disclosure burden; it does add a useful behavioral boundary ('without WHOIS or ownership data') and implies a non-destructive text operation. It does not, however, disclose that non-host parameters are discarded or describe the result shape. The discard behavior is only recoverable from parameter descriptions, not the tool description.

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 one tightly scoped sentence with a clear verb and no filler. It could be improved by front-loading the return format, but it is efficient for what it states.

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?

There is no output schema and no annotation, so the description should explain the result format and invalid-input behavior; it only says 'split into labels.' The definition also does not account for the nine-parameter schema, leaving an agent guessing which parameters are meaningful and what a 'shape check' returns.

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 even with no parameter detail in the tool description. The word 'hostname' aligns with the host parameter, but the description adds no meaning beyond what the input schema already provides.

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 exactly what the tool does: 'Split a hostname into labels' and explicitly excludes a related concern ('without WHOIS or ownership data'). This is specific enough to distinguish domain-shape from siblings like normalize-url, jira-key-shape, or wiki-title-ok without opening their schemas.

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 implies usage when you need hostname labels and not ownership data, but it never names an alternative ('use X instead') or states a condition like 'for full URL validation use normalize-url'. Sibling tools exist, so this is a gap, but the negative scope provides some routing guidance.

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