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

Annotations declare destructiveHint=true and readOnlyHint=false, so the safety profile is partly covered. The description adds genuinely useful context beyond that: the message is not deleted but reverts to an ordinary draft, and the call fails after the send begins. That corrects the potentially misleading destructive framing and discloses the failure mode, though it says nothing about permissions or idempotency.

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 short sentences, front-loaded with the action, followed by the outcome and the failure condition. No filler and every sentence carries information an agent needs.

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 two-parameter mutation with full schema coverage and annotations covering safety, the description supplies the two things structured fields cannot: the resulting state of the message and the failure boundary. Nothing needed to invoke it correctly is missing, and no output schema exists to require return-value documentation.

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 100% and both parameters are documented there, including where the scheduled send id comes from (list_scheduled, send_message, send_draft). The description adds no parameter detail, so the baseline 3 applies.

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 (stop/cancel) and resource (scheduled send) with a clear scope: before the send goes out. An agent can distinguish this from send_message, send_draft, and list_scheduled without opening any 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?

The description defines the operative window ('before it goes out') and states the failure condition ('once the send has started or finished'), which tells the agent when the tool will not work. It does not name an alternative tool for already-sent messages, but the timing precondition is explicit.

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