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

Annotations already flag destructiveHint=true and idempotentHint=false; the description adds genuinely new context by characterizing the operation as 'best-effort' and explaining the mechanism ('pulls it from the send queue') and its failure mode ('not guaranteed if delivery already started'). That is exactly the kind of caveat annotations can't express. It doesn't say what the caller sees when the cancel fails to land.

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, zero filler, and the core action is front-loaded ahead of the caveat. Every clause carries information the 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 single-parameter mutation with a full annotation set and no output schema, the description covers what is being acted on, the best-effort semantics, and the failure condition. The only gap is what a successful or failed cancel returns to the caller, which is minor here.

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?

Only one parameter and schema coverage is 100%, with the 'id' field already documented as 'Email ID (em_xxx)'. The description adds no format or lookup guidance beyond that, so 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+resource ('cancel ... email') plus the eligible states (scheduled, accepted, attempted), which clearly separates it from siblings like cancel-campaign, cancel-automation-run, and retry-email. It doesn't explicitly name those siblings, but the resource noun alone disambiguates well.

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 a clear condition for use (email is scheduled, accepted, or attempted) and a clear boundary (not guaranteed once delivery has started), which is the key decision factor for an agent. It stops short of naming alternatives such as retry-email for already-failed sends, so it lacks explicit routing.

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