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

Annotations already indicate destructiveHint=true. The description adds behavioral details beyond annotations: it explains the outcome (terminal status 'canceled'), the side effect (idempotency key becomes reusable), and an error condition (conflict if already dispatching). This provides useful context that annotations alone do not convey.

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, front-loaded with the core purpose. No redundant information. The description is concise and each sentence serves a purpose: first sentence states the action and outcome, second sentence provides parameter source and error condition. It is appropriately sized for the simple operation.

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?

Given the tool's simplicity (one parameter, output schema present), the description covers key aspects: what it does, when to use it, how to get the input, and an error scenario. It does not explain return values, but the output schema handles that. It is complete enough for effective use.

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 coverage, the baseline is 3. The description adds value by explaining how to obtain the 'id' parameter: from a sendEmail response that used scheduled_at, or from listSentEmails. This clarifies the source and context, making it more helpful than the schema description alone.

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?

The description clearly states the verb 'cancel' and the resource 'scheduled send'. It distinguishes this tool from siblings by specifying it applies only to still-scheduled sends, and mentions the terminal status 'canceled' and idempotency key reuse, which uniquely identifies its purpose.

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 provides explicit context on when to use the tool: pass the sent email id from a sendEmail response that used scheduled_at, or from listSentEmails. It also warns about a conflict error if the send has already started dispatching. No explicit when-not or alternatives are given, but the usage context is clear.

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