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

The description adds security-relevant behavior beyond annotations: 'Credentials are never returned,' and it clarifies that only connected sending mailboxes are listed. This supplements the read-only and idempotent hints with operational context. No contradiction with annotations.

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 compact sentences accomplish three goals: defining the result set, providing a downstream use case, and stating a security guarantee. There is no redundant or extraneous content; every clause earns its place.

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 list tool with annotations covering safety, the description is complete: it specifies return fields, scope, a security boundary, and how to use the result. Missing pagination or ordering details are negligible 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?

There are no input parameters, so the description is not required to explain parameter semantics. It still adds useful information about the output fields (id, from address, active flag), but this is not parameter-related, so the baseline for 0 parameters 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 description clearly identifies the operation (list) and the resource (connected sending mailboxes), and enumerates the returned fields: id, from address, and active flag. It also scopes the result to SMTP, Gmail OAuth, and API providers, distinguishing this tool from siblings like test_mailbox.

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 description provides a concrete usage context: 'Use the id when creating a campaign,' which tells an agent when the output is needed. However, it does not explicitly mention alternatives or when not to use this tool, so it falls short of full exclusionary guidance.

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