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

Adds substantial behavior beyond the destructiveHint annotation: attachments are dropped and surfaced via `attachments_dropped`, long bodies are capped and flagged by `truncated`, and a Gmail send scope with reconnect-on-permission-error is required. It also warns to confirm the recipient before sending.

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?

Front-loaded with the action and alternates between prose and inline code for key semantics, with the critical attachment warning given prominence. It is dense and mostly earns its length, though the `note`/`include` restatements slightly duplicate the schema.

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 send-side mutation tool with no output schema, the description covers the surprising return fields (`attachments_dropped`, `truncated`), the auth requirement, and the send confirmation. Nothing an agent needs before invoking it 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?

With 100% schema description coverage the schema already documents all six parameters, and the description largely restates `include` and `note` semantics. It adds only marginal meaning (the default of 'latest' and note placement), so the baseline of 3 is appropriate.

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 ('Forward a Gmail thread') and immediately distinguishes itself from siblings like send_message/send_draft by describing the quoted forward header and the `note` placement. An agent can identify the operation without opening the 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?

Gives clear operational context: 'This actually sends — confirm the recipient with the user first' and flags the multi-account case, plus the 'include' default. It does not explicitly name alternatives (e.g., use create_draft to stage instead of sending), so routing guidance is strong but not exhaustive.

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