Where the application's mail comes from

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

Annotations already declare readOnlyHint=true and openWorldHint=false, so safety is covered. The description adds the non-obvious platform limitation that user-owned domains are not supported, but it does not describe the return shape, error behavior, or whether an app must be linked.

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 the core values and then the key constraint. The first sentence is a noun-phrase fragment rather than a full sentence, but it wastes no words and keeps the important limitation visible.

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 zero-parameter read-only lookup with no output schema, the description lists the three values the caller can expect and warns about the unsupported custom-domain path. It does not explicitly state the return structure or required app-linking context, but it is largely complete for this simple tool.

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?

The tool has zero parameters and an empty schema, so there are no parameter semantics to explain. Baseline 4 applies because the description does not need to compensate for missing parameter documentation.

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 the specific resource—the platform sending domain, the application's address under it, and the From value—so the agent knows what information is available. It lacks an explicit verb like 'returns' and does not distinguish itself from siblings such as email_overview or email_status, but the resource itself is clear.

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 includes a clear negative instruction: never send the user to add DNS records, because custom-domain sending is unsupported. However, it does not say when to call this tool rather than email_overview, email_status, or email_create, so tool-selection guidance is only implied.

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