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

Annotations already declare destructiveHint=true, non-idempotent and open-world, so the safety profile is covered. The description adds genuinely new behavioral context: server-side header/recipient/quote handling and the sending-enabled plus allowlist gating. It stops short of describing failure modes or what a rejected send looks like, so not 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?

Three sentences, front-loaded with the action, then the gating conditions. The parenthetical about server-set headers is somewhat long but earns its place by clarifying what the caller need not supply.

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 operation with no output schema, the annotations carry the safety profile and the description carries preconditions and routing to reply_draft. The remaining gap (result/error reporting after a send attempt) is minor given there is no output schema to document.

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 description coverage is 89%, so the schema already documents uid, account, text, lang, quote, reply_all, attachments and folder. The description adds no parameter-level syntax or format detail beyond what the schema provides, making the baseline 3 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?

States a specific verb and resource ('Sends a reply to a message in the same thread') and explicitly distinguishes itself from the sibling reply_draft ('otherwise use reply_draft'). An agent can identify the operation and its alternative without opening either 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 explicit preconditions: only when the owner enabled sending AND every recipient matches send_allowlist, otherwise use reply_draft, and only when the owner says 'send'. It names the alternative and the exact conditions that select it, leaving little to inference.

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