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

Goes well beyond the annotations (which only say non-readonly, non-destructive, non-idempotent) by disclosing hard limits — plain text only, no attachments, 100 kB, 50 recipients — and the side effect that a reply draft gets a "Re:" subject when none is given. That is exactly the operational context an agent needs before writing.

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?

Three sentences, front-loaded with the send/no-send distinction, then constraints, then the optional threading parameter. No filler and nothing redundant with the schema field descriptions.

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?

Mutation tool with annotations and an output schema, so return values need no explanation; constraints, non-delivery semantics, and threading behavior are all covered. Nothing material is missing for a correct call.

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 100%, so baseline is 3; the description earns above that by explaining reply_to_message_id's threading behavior and subject mutation and by pointing to search_messages as the id source, both of which are absent from the schema text.

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 (save a message or reply into Drafts) and immediately scopes it against sending. An agent can distinguish this from send_message 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?

"It is not sent: the person opens their mail client and sends it themselves" gives clear when-to-use context and implicitly steers the agent to send_message for actual delivery. It also names prerequisites for reply_to_message_id (an id from search_messages), but never explicitly names the sibling tool to use instead.

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