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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), so the description earns credit for adding domain semantics instead: it explains that ~60% of registrars are reseller shells and that parent_id reveals the actual operator. That is genuine behavioral context beyond the structured fields, though pagination behavior is left unmentioned.

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, front-loaded with the action and the return shape, followed by the parent_id insight that actually affects interpretation. The shell-statistic framing is somewhat verbose but it is the highest-value content in the definition, so little is wasted.

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?

An output schema exists, so return values need not be spelled out; the description goes further and flags the semantically important field (parent_id). With annotations covering safety and the schema covering both parameters, nothing essential is missing for correct invocation.

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% with only 2 parameters, so the schema already documents 'query' and 'page'. The description repeats the accepted query forms without adding format or syntax detail beyond what the schema provides, and says nothing about the pagination parameter. Baseline 3 applies.

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 ('look up') and resource ('ICANN-accredited registrars') plus the three lookup keys (name, alias, IANA ID), and enumerates the returned fields. It also names related siblings (whois, available, expired), so an agent can locate it in the toolset without opening the schema.

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 gives clear context for when this tool applies (looking up a registrar by name/alias/IANA ID) and points at related tools, but it lists those siblings without stating the conditions under which one should be preferred over registrar. No explicit when-not guidance.

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