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

Annotations already declare destructiveHint=true and openWorldHint=true, so the safety profile is covered. The description adds non-obvious policy gating (per-account send enablement, send_allowlist recipient matching) that the annotations cannot express, plus the range of attachment sources. It stops short of describing delivery failure behavior or rate limits.

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, zero filler, with the action front-loaded and the escape hatch pushed to the end. Every clause carries load: mechanism, optionality, preconditions, alternative.

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 send tool whose annotations already flag it as destructive and open-world, the description covers the critical invocation gates and the correct alternative. It does not address what a rejected send returns or how partial-recipient failures behave, a minor gap against the complexity of an 11-parameter tool.

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 coverage is 64% (moderate), so the schema carries much of the burden already; the inline attachment-object description in the schema largely duplicates the description's mention of mailbox attachments, uploads, and inline content. The description adds little format-level detail for the 11 parameters beyond that.

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?

Opens with a specific verb+resource ("Sends an email via SMTP") and immediately scopes the capability around attachments. It also names the sibling it is not (create_draft), so an agent can distinguish it from the draft/reply family without opening a schema.

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?

States the two hard preconditions for use (owner enabled sending for the account AND every recipient matches send_allowlist) and gives the explicit fallback ("Otherwise use create_draft"). This is a clear when/when-not/alternative statement rather than implied guidance.

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