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

The annotations already establish readOnlyHint=true and openWorldHint=false, and the description adds genuinely useful context beyond them: results are headers rather than full messages, and the default ordering is newest first. It stops short of describing pagination behavior or the exact header fields returned.

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?

A single sentence that front-loads the verb and resource, then tacks on the two most decision-relevant facts (default ordering, folder filter). Nothing is wasted.

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 read-only list tool with zero required parameters, full schema coverage, and annotations covering safety, the description covers what an agent needs to call it correctly. The absence of an output schema means the return shape would ideally be sketched out in more detail than 'message headers'.

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?

Schema coverage is 100%, so all four parameters are already documented, including mailboxId's 'Folder ID from mailbox_list. Omit for all mail.' The description only adds the default sort semantics ('newest first by default'), which is a minor supplement rather than new depth, so the baseline 3 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 gives a specific verb ('Lists') and resource ('message headers in the mailbox'), and the word 'headers' implicitly separates it from mail_get, which returns full messages. It does not name any sibling explicitly, so it falls just short of a 5.

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?

'Filter by folder with mailboxId' and the default newest-first ordering imply the main usage context, but there is no explicit when-to-use guidance, no mention of when to reach for mail_get or mailbox_list instead, and no stated prerequisites.

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