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

Annotations already declare readOnly, idempotent, open-world and non-destructive, so the safety profile is covered and the description doesn't need to repeat it. The description adds that classification comes from MX lookups and names the four result categories, which is useful. However, with no output schema there is no explanation of the per-domain result shape, handling of domains without MX records, or lookup failure behavior.

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 action and mechanism. The second sentence carries the business context for the split and its consumer, plan_from_list, so it earns its place, but it is the softer half of the definition.

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 bulk classification tool with annotations covering safety, this is nearly complete: what it does, how it decides, what categories come out, and who consumes the output. The remaining gap is that, without an output schema, the description doesn't sketch the return structure or error behavior for unresolvable domains.

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?

Only one parameter, and schema coverage is 100%, so the schema already documents `domains` as an array of strings. The description reinforces batch semantics ('a list of domains') but adds no format, normalization, or size-limit guidance beyond the schema.

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 ('split a list of domains by the email provider'), names the mechanism ('from their MX records'), and enumerates the output buckets (Google, Microsoft, security gateway, other). The 'bulk' framing plus the plan_from_list reference clearly separates it from single-domain siblings like lookup_mx and check_google_domain.

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?

Implies when to use it through the routing context ('WarmInboxes routes Google-hosted recipients...') and names plan_from_list as the follow-on for sizing. It doesn't explicitly exclude lookup_mx/check_google_domain for single-domain cases, so it stops short of a full when/when-not statement.

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