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

Annotations already declare readOnly, idempotent, non-destructive, and openWorld, so the safety profile is covered. The description adds that availability is checked live (.com/.net), which explains the openWorldHint network behavior, but it says nothing about rate limits, latency, or how many domains are returned.

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 short sentences, front-loaded with what the tool produces and why, followed by the input instruction. No redundant restating of the name or title.

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 tool with full schema coverage and a complete annotation set, the description covers purpose, rationale, and input form adequately. It would be slightly stronger if it hinted at the shape of the output (e.g., how many suggestions or what availability data looks like), but no output schema exists to carry that.

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%, so the single 'brand' parameter (business name or domain, max 80 chars) is already fully documented in the schema. The description restates the accepted input form but adds no syntax, format, or constraint detail beyond it — 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?

Specific verb+resource ('suggest lookalike sending domains') plus a stated goal ('so the main domain is never put at risk') and a scope qualifier ('with live .com/.net availability'). This is clearly distinguishable from siblings like check_* and generate_* which do DNS/record work.

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?

The cold-email use case is stated explicitly, giving the agent clear context for when this tool applies, and the input guidance ('Give a business name or its website') tells the agent how to invoke it. No alternatives or exclusion conditions are named, but the sibling set is non-overlapping so that gap is minor.

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