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

With no annotations, the description carries the full burden. It does disclose a useful behavioral trait beyond a bare name: the result is scoped to mailboxes the caller can read, and the returned shape is address plus name. It says nothing about ordering, pagination, or what an empty list means, so it is adequate but not rich.

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 with no filler: the listing behavior comes first, the follow-on usage second. Every clause earns its place and the most important fact is front-loaded.

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?

There is no output schema and no annotations, so the description has to cover basic return information, which it does by naming the returned fields. For a zero-parameter read-only lookup this is nearly complete; only edge-case behavior (empty results, multiple accounts) is unaddressed.

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 schema declares zero parameters, so there is nothing for the description to disambiguate and the baseline is 4. The note about the `mailbox` argument refers to other tools' parameters, not this one's, so it adds no parameter semantics here.

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 specific resource ("e-mail mailboxes you can read") and explicit return fields (address and name). It also names the sibling family ("other mail_* tools"), so an agent can place it among mail_folders, mail_read, mail_search without opening any schema.

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 second sentence gives concrete downstream usage: "Pass the address as `mailbox` to the other mail_* tools." That is a clear cue for when to call this tool (first, as discovery). It lacks any explicit exclusion or comparison against a named alternative, which keeps it out of the 5 range.

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