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

Annotations cover the read-only, idempotent safety profile, and the description adds real context on top: the historical coverage boundaries, the caveat that first_seen may equal coverage start, the cost of $0.010 per call, and a free sample invocation. That is exactly the supplementary information an agent needs before spending credits.

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 purpose question is front-loaded, followed by coverage, interpretation signals, pricing, and the return shape. The field enumeration is long but each element is load-bearing because no output schema exists; only the sample-JSON fragment is mildly extraneous.

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 fully carries the return contract: the top-level fields, the event object structure with its type enum, and the coverage block. Combined with documented cost and coverage history, an agent has everything needed to call and interpret this correctly.

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%, and the schema already explains that only registered names directly under a TLD are valid (no subdomains, no ccTLDs). The description adds nothing about the domain parameter beyond that, so the 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?

The opening question names the exact resource and the exact events it reports: registry zone file appearance, disappearance, return, and nameserver changes. Nothing about it could be confused with siblings like domain_whois or domain_age, which cover registration and creation data rather than zone-file history.

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 clear usage context: coverage windows (daily since 2026-08-05, monthly 2013-2018), the investigative interpretation of a nameserver move (hijack/sale) or a brand-new name (phishing), and the per-call price. It stops short of explicitly routing to named alternatives such as domain_whois or domain_age, which keeps it out of 5 territory.

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