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

Annotations already cover the safety profile (readOnly, idempotent, open-world), so the bar is lower, and the description still adds real behavioral detail: data comes over RDAP with a WHOIS fallback for ccTLDs lacking RDAP. It also cautions that the age signal is weak, which helps the agent avoid over-weighting the result.

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 concrete return fields and data sources. The trailing clause about no published WarmInboxes study isolating the effect is brand-specific hedging that adds some interpretive value but slightly dilutes the crispness of the opening.

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 no output schema, the description carries the burden of describing return values and does so by listing registrar, creation date, expiry date and days remaining. The data-source fallback is also covered, leaving only minor gaps such as behavior when a domain is unregistered or the lookup fails entirely.

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 parameter with 100% schema description coverage, so the schema already explains the expected input including the 'acme.com' example. The description adds no format, normalization, or ccTLD input guidance beyond that, which is the baseline expectation when the schema does the work.

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 verb and resource ('Get a domain's registrar, creation date, expiry date and days remaining') and enumerates exactly what comes back. It also implicitly separates itself from sibling lookups by framing the result as context to be read alongside reputation and sending history rather than as a reputation verdict.

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?

It gives a clear usage context — 'useful context when diagnosing deliverability' — and points the agent toward companion checks (provider, reputation, sending history). What is missing is an explicit when-not or a named alternative for cases where the agent really wants reputation rather than registration data.

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