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

Annotations already establish this as a non-read-only, non-idempotent, non-destructive write, so the description's job is to add operational context beyond that. It does: the domain defaults to the tenant default domain and ownership is always bound to the API key's owner — two facts an agent needs before calling. It stops short of richer lifecycle behavior, but expiry/recovery mechanics are partially covered by the schema.

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 sentences with no filler; the purpose is front-loaded and the second sentence compresses two important behavioral defaults efficiently. The only inefficiency is that the domain-default fact repeats the schema's parameter description, which keeps it from a 5.

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 an output schema present and 100% parameter coverage, the description only needs to supply tool-level linkage, which it does: purpose, default domain, and ownership. The 'temporary' label slightly undersells the tool since noExpiration can create permanent mailboxes, but the schema exposes that override, so nothing an agent needs to call it correctly is missing.

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 all five parameters are already documented in the input schema, landing this at the baseline of 3. The description's domain-default fact duplicates the schema's 'Defaults to the tenant default domain,' adding no new parameter-level meaning.

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 opens with a specific verb+resource pair ('Create a new temporary mailbox'), and the 'temporary' qualifier sharpens default behavior beyond the plain title. Sibling tools are all different operations (delete, get, list, delete_bulk), so confusion risk is low, but the description doesn't explicitly name alternatives, keeping it at a 4 rather than 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?

The core action statement implies when to use the tool (when a new mailbox is needed), and the domain/owner defaults subtly inform call decisions. However, it names no alternative tools and provides no explicit when-to-use or when-not-to-use guidance, so usage context is only implied.

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