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

Annotations already provide readOnlyHint, idempotentHint, destructiveHint. The description adds behavioral detail about pending DNS verification and next actions, which is not covered by annotations. No contradiction.

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 a question that captures the tool's purpose. Every sentence is meaningful and contributes to understanding. No unnecessary words.

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 parameters, clear annotations, and an output schema, the description fully equips the agent to decide when to call the tool. It explains the timing relative to sendEmail and the error context, which is sufficient for correct invocation.

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?

Tool has no parameters, and schema coverage is 100%. Description does not need to explain parameter semantics. The baseline for 0-param tools is 4, and the description provides no extraneous info.

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 starts with a clear question 'What can I send FROM?' and explicitly states it lists verified sendable domains and pending verification domains, with next actions. It distinguishes itself from sibling tools like 'listDomains' by focusing on outbound sendability.

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?

Explicitly tells the agent to call this BEFORE sendEmail to pick a valid from domain, and warns that the account email may not be sendable. Also mentions the error message that echoes this list, helping the agent understand when to use it to avoid errors.

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