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

Annotations already declare destructiveHint=true and readOnlyHint=false, so the agent knows it is a mutating operation. The description adds valuable context beyond the annotations: the 30-second cutoff, the cost refund, and the limitation on unsending after submission, which are not present in the structured metadata.

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?

The description is three short sentences with zero filler. The core action is stated first, followed by the critical timing rule and a limitation. Every sentence earns its place and the key information is front-loaded.

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 one-parameter tool with a full input schema, annotations, and an output schema, this description covers all essential behavioral rules: timing, refund, and the unsend limitation. Nothing an agent needs to call it correctly 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?

The single parameter 'id' has 100% schema description coverage, so the schema already documents it as a message identifier. The description does not need to add syntax or format details beyond the schema, and it does not introduce any parameter-specific semantics that are missing.

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 states a specific verb and resource ('Cancel scheduled message') and explains the action with a timing constraint and refund behavior. It is clearly distinct from sibling tools like send_sms, get_message, or delete_contact, so an agent can tell it apart without opening schemas.

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 gives explicit conditions for use: cancellation is only allowed at least 30 seconds before send_at, and it cannot unsend a message already submitted to the network. This provides a clear when and when-not, though it does not name alternative tools explicitly.

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