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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered. The description adds value beyond that by disclosing sort order ('newest first') and a derived output feature ('detected verification codes') that the agent would not otherwise know to expect. It does not mention pagination or default-limit behavior, so it is not fully exhaustive.

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 with zero filler. The resource and scope are front-loaded, and the two distinguishing modifiers (ordering, verification codes) follow immediately.

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 simple read-only list tool with full annotation coverage, a complete input schema, and no output schema, this covers what an agent needs to call it and know roughly what comes back. The one gap is routing context versus read_email and wait_for_email, which matters given the sibling set.

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%, so mailbox, limit, and unread_only are all documented in the schema itself. The description adds no syntax, format, or default information beyond that, 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?

States a specific verb and resource ('List emails in a mailbox') plus two distinguishing traits: newest-first ordering and surfacing of detected verification codes. It does not explicitly disambiguate from siblings like read_email or wait_for_email, which is the only thing keeping it off 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?

Usage is implied by 'list emails in a mailbox' — an agent can infer this is the bulk-listing call. However, no sibling is named as an alternative, and there is no guidance on when to prefer read_email (single message) or wait_for_email (blocking on arrival) over this list call.

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