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

Annotations already declare destructiveHint=true, idempotentHint=false and openWorldHint=true; the description adds real value on top with the '[write]' marker, the owner-account authorization requirement, the verified-address validation on forward_to, and the explicit return identifiers. It does not cover failure modes (duplicate mailbox, unverified zone) or irreversibility, which keeps it from a 5.

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 tight paragraph, front-loaded with the write marker and the verb, with each clause carrying distinct information. Slightly dense back-to-back parentheticals cost it the top score but 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 no-output-schema mutation tool, the description covers authorization, key validation, all inputs and the returned identifiers, which is close to sufficient. Missing only error/conflict behavior for a non-idempotent create.

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?

With 0% schema description coverage, the description must carry all six parameters, and it does: local_part, email_zone_id/email_zone_uuid, display_name, forward_to and signature each get meaning (sender name, copy destination, plain-text signature). The one gap is that it never explains whether the id and uuid forms are alternatives or mutually exclusive.

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 (create) and resource (mailbox) plus the exact resulting address shape `<local_part>@<zone domain>`, plus the scope constraint of a single connected e-mail zone. This clearly distinguishes it from list_email_accounts or send_email among the many siblings.

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?

Gives prerequisites ('Owner accounts only', forward_to must be a verified Email Routing address) which imply when the call can succeed, but never names alternatives or says when to create a new mailbox vs. reuse/forward an existing one. Usage is inferable rather than guided.

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