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

Annotations already declare readOnlyHint=true, openWorldHint=true, and destructiveHint=false, so safety is covered. The description adds genuinely useful behavior beyond that: the tool costs 10 credits, and the returned fields vary by plan tier (paid plans add expiration date and last registry update), which an agent needs to know to interpret results.

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?

Two sentences, front-loaded with the core behavior and ending with the cost note, with essentially no filler. The opening 'Domain Age: Domain Age' restates the name/title redundantly, which is the only real waste.

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 read-only lookup with an output schema, the description covers what is returned, the tier-dependent variation in results, and the cost. Nothing an agent needs in order to call it correctly or interpret its output 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?

There is a single required parameter with 100% schema description coverage, so the schema already documents 'domain'. The description adds no format details, such as whether bare domains vs URLs vs subdomains are accepted, so the baseline of 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?

The description names a specific resource (domain registration date) and enumerates the returned facts: registration date, age in days/months/years, readable age, age bracket, and newly-registered flag. It is clearly distinct from a generic WHOIS lookup, though it never names sibling tools like DomainExpiration or WHOISLookup that could be confused with it.

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?

There is no when-to-use or when-not-to-use guidance and no reference to alternatives, despite DomainExpiration and WHOISLookup being close siblings. The implied use case (checking how old a domain is) is inferable from the purpose statement but never stated as guidance.

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