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

Annotations already cover readOnly/idempotent/non-destructive, so the bar is lower; the description adds genuinely new behavior by noting that only policy-passed mail is returned (a silent filtering rule) and that a `read` capability is required. It does not describe pagination or default limit behavior, but the added filtering and auth disclosure is substantive.

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; the resource and scope are front-loaded, followed by the returned fields, then the constraint and auth requirement. No filler sentences.

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?

With no output schema, the description usefully lists the returned fields, and it discloses the auth requirement and the policy-passed filtering rule for a read tool whose annotations are already rich. Only pagination behavior is left unaddressed, which is minor given the cursor parameter's schema documentation.

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 description coverage is 100%, with each of the four parameters, including cursor and inbox scoping, already documented in the schema, so the baseline is 3. The description adds no format or syntax detail beyond the schema and merely echoes the returned-field 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?

States a specific verb+resource ('List delivered messages in a scoped inbox') and enumerates the returned fields, which is more than a restatement of the title. It implies a distinction from list_drafts/get_message but never names a sibling, so an agent must still infer the boundary.

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 gives a prerequisite ('Requires the `read` capability') and a scope constraint, which is useful context for deciding whether the call will succeed. However it never says when to prefer this over get_message, get_thread, or list_drafts, leaving the alternative selection to inference.

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