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

Annotations already establish that this is a non-idempotent, non-destructive write operation. The description adds meaningful context beyond them: the per-call cost, that failures are not charged, the mailbox-identity constraint, and the reply-threading behavior.

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?

The description is short and front-loads the core action and reply behavior. The cost is slightly redundant — '$0.001.' followed by 'Costs $0.001 per call' — which is minor waste but not confusing.

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 write tool with no output schema, the description covers the key behavioral facts an agent needs: cost, failure charging, mailbox scope, and reply threading. It stops short of stating what the call returns or what error conditions look like, but annotations already cover the safety profile.

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 only 17% schema description coverage, the description does real work by explaining body as a text body, the roles of to/cc/subject, and especially the reply_to_message_id threading semantics. However, it does not clarify the array constraints for to/cc, the mailbox parameter's optionality, or subject limits, leaving gaps at low schema coverage.

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: 'Send an email as one of the company's mailboxes,' then lists the core fields. This clearly distinguishes it from google_mail_read and google_mail_search, though it does not explicitly contrast with sibling senders like email_send or m365_mail_send.

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 reply_to_message_id sentence gives useful conditional guidance for threading, but there is no explicit when-not-to-use or comparison to alternative send tools. Prerequisites such as a connected Google account are implied rather than stated.

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