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

Annotations already mark it destructive, idempotent, and mutating, and the description adds substance beyond that: it is irreversible, it stops any further attempt to the number for this call, billing is limited to time already spoken, and reserved credit is released. It also reconciles with idempotentHint=true by noting that re-cancelling is a no-op rather than implying reversibility.

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 core action and scope lead the description, and the when-to-use and precondition sentences are short and purposeful. The billing point is stated twice ("nothing is charged beyond the time already spoken" and again in the live-call sentence), and the "Compliance:" fragment is slightly clipped, but neither obscures meaning.

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?

An output schema exists, so return values need no prose, and the description still signals that cancelled status comes back. For a one-parameter mutation it covers irreversibility, per-state behavior (live vs. scheduled), idempotency, and billing/compliance consequences, leaving nothing an agent needs before calling it.

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 call_id parameter is fully documented in the schema (100% coverage), so the schema carries the weight. The description adds only that the identifier may refer to a scheduled or live call, which is marginal over the field description already present.

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) and resource (a scheduled or live call), and explicitly enumerates what counts as cancellable: scheduled calls, in-progress calls, scheduled callbacks, and automatic retries. No sibling tool performs cancellation, so no disambiguation from siblings is required, and the note that the cancelled status is returned rounds out a complete picture of the operation.

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 positive trigger ("Use it when the user changes their mind or the task is no longer needed") as well as explicit exclusions: cancelling an already-cancelled call has no further effect, and a call that already ended cannot be cancelled. There is no sibling alternative that competes with this tool, so the missing "use X instead" clause is not a real gap.

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