Cancel a subscription (now, or at the end of the current period)

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

Annotations already declare destructiveHint=true and idempotentHint=false, so the safety profile is covered structurally. The description adds genuinely new behavior — the two cancellation timings and the `409 subscription_canceled` outcome for repeated calls — but says nothing about reversibility, reactivation, proration, or what the call returns.

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 opening sentence is a verbatim duplicate of the title, which is pure waste, and the parenthetical restates the same idea a third time. The remaining content is compact and front-loads the mode choice correctly, but the redundancy costs it.

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 destructive mutation tool with no output schema, an agent would benefit from knowing what a successful cancellation returns (canceled object, effective date, whether the subscription becomes read-only). The description covers the two timing modes and the error case but leaves that return/effect picture incomplete.

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 coverage is 67%, and the description reinforces the meaning of `at_period_end: true` beyond the schema's one-line note. However, the `reason` parameter is undocumented in both the schema and the description, so the description does not fully compensate for the coverage gap.

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 names a specific verb and resource ('Cancel a subscription') and immediately distinguishes the two modes of cancellation. It is clearly separable from siblings like get_subscription, list_subscriptions, and create_subscription, though that separation comes from the verb itself rather than any explicit sibling differentiation.

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?

It gives real conditional guidance: use `at_period_end: true` to cancel at the end of the period versus immediately by default, and it documents the 409 error for re-canceling. It does not mention alternatives (e.g., plan changes via update_subscription) or when not to cancel, so usage context is implied rather than complete.

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