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

Annotations already flag destructiveHint=true, idempotentHint=false and readOnlyHint=false, so the description's job is to add context, and it does: the state gate on 'queued' explains when the tool will fail. It does not say what happens on a rejected cancellation (error vs silent no-op), which is the remaining behavioral gap.

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, both earning their place: the action/outcome comes first, then the eligibility constraint. No restatement of the title or the schema.

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?

Output schema and annotations carry the return shape and safety profile, so the description only needs purpose plus the state prerequisite, both present. Slightly incomplete on post-condition behavior when cancellation is rejected.

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 a single parameter at 100% schema description coverage, the schema already defines `id` fully; the description only reinforces scope ('one of the current user's queued emails') without adding format or sourcing detail beyond the schema. 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 (cancel) plus resource and scope (one of the current user's queued emails), and adds the outcome ('so it is never sent'), which separates it from send/reschedule/retry siblings by implication. It never names an alternative tool explicitly, so it stops short of a 5.

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 precondition for use: the email's status must be 'queued', and 'once it is sending, it is too late'. That is real when-to-use guidance, but it does not point at reschedule-email-tool or retry-email-tool for the cases where cancellation is not applicable.

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