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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=false, so the safety profile is covered. The description adds real value beyond that by disclosing exactly what is returned per mailbox (today's limit, ramp-up, domain DNS health) — important because there is no output schema. No mention of ordering or size limits, but for a zero-param read this is solid.

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 no filler. The opening 'List mailboxes.' echoes the name/title and is slightly redundant, but the second sentence immediately earns its place by describing the payload.

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-param, read-only listing with no output schema, the description supplies the essential return-value context an agent needs to decide whether to call it. Gaps are minor: no statement of ordering, pagination, or whether unconnected/disabled mailboxes appear in the result.

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?

Zero parameters, so there is nothing for the description to disambiguate. Baseline 4 applies; the description correctly implies no filtering input is expected, confirming the tool returns the whole mailbox set.

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 (list) plus resource (mailboxes), with a second sentence that defines what a mailbox record contains: connected sending mailboxes with today's limit, ramp-up, and domain DNS health. This is far more than a restatement of the name. It does not, however, explicitly distinguish itself from the nearby list_domains or add_mailbox siblings.

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?

Usage is only implied — an agent can infer 'call this to discover your connected sending mailboxes' but there is no explicit when-to-use, no mention of what to do if the list is empty, and no routing signal versus list_domains (which arguably overlaps via the DNS-health dimension).

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