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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuine behavioral context beyond that: the underlying data sources (RDAP or WHOIS), the cost of 10 credits, and the fact that exact registration timestamps require a paid plan. It does not discuss failure behavior for unregistered or malformed domains.

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?

One long sentence conveys everything but opens with a redundant 'Domain Expiration: Domain Expiration is a domain expiration checker API' that restates the name and title twice. The substantive content (sources, return fields, cost) is front-loaded and useful, but the lead-in wastes space.

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 single-parameter lookup with an output schema, the definition covers the important extras: data source, returned fields, plan-gated fields, and credit cost. Only edge-case behavior (invalid or unregistered domains) is absent, which is a minor gap given the output schema exists.

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 defines 'domain'. The description only reiterates 'pass any domain' with no format, TLD, or normalization guidance, so it adds essentially nothing over the schema. 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 and resource: it reads a registry record over RDAP/WHOIS and returns days remaining, an expiration status, and domain age. That is concrete and distinguishable from generic lookups, but it never names the close siblings DomainAge, DomainAvailability, or WHOISLookup, so an agent must infer which one to pick.

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?

'Pass any domain' implies the input and the description implies the use case (checking how long before expiry), but there is no explicit when-to-use guidance and no mention of when to prefer DomainAge, DomainAvailability, or WHOISLookup instead. Usage is inferable rather than stated.

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