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

No annotations, so the description carries the full burden. It discloses the key behavioral constraint that cancellation only works on scheduled messages and is unavailable once sending starts, which is the most important trap for an agent. It stops short of saying what happens on a failed cancel (error vs. silent no-op) or whether the operation is idempotent.

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 short sentences, front-loaded with the action and then the constraint and fallback. Every clause earns its place; nothing is repeated from the schema or title.

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 single-parameter mutation tool with no annotations and no output schema, the description covers the critical precondition and the alternative path. It is slightly incomplete on the outcome of a successful call and how failures surface, but an agent has enough to call it correctly.

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 0%, but there is only one parameter, message_id, whose meaning is self-evident from its name and from the sentence context. The description adds no format, source, or constraint detail about the identifier (e.g., where to obtain it from list_outbox).

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 ('Cancel a message') and immediately names the sibling it must not be confused with ('delete_message'). An agent can distinguish it from delete_message and send_message 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 both the enabling condition ('still scheduled') and the exclusion ('already sent or sending can't be canceled'), then routes the agent to the correct alternative ('use delete_message'). This is explicit when-to-use and when-not-to-use guidance naming a real sibling.

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