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

Annotations already flag destructiveHint=true, and the description goes further by explaining billing behavior: the subscription remains active until term_end by default, and bill_date shifts the effective date to the next bill date. It also explicitly says 'This changes billing state,' adding useful behavioral context beyond the annotation.

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?

Three concise sentences with no filler. The core action and default behavior are front-loaded, the special-case parameter usage is explained in the second sentence, and the endpoint reference is a useful appendix. Every sentence earns its place.

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 simple two-parameter mutation tool with destructiveHint already annotated and full schema coverage, the description covers the essential behavior and effective-date semantics. It does not explain consequences like reactivation or whether cancellation can be undone, but those are not required for calling the tool 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 coverage is 100%, so the schema already documents both parameters. The description adds meaningful context by spelling out the default timeframe behavior and what bill_date does, but this largely restates the schema's enum description for timeframe. It does not introduce entirely new parameter semantics.

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 uses a specific verb and resource: 'Cancel a subscription.' It clearly distinguishes cancellation from the sibling recurly_pause_subscription by stating that cancellation changes billing state and takes effect either at term end or next bill date. The title also reinforces the write nature.

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?

The description clearly explains when the cancellation takes effect and how to choose between timeframe options, which is key usage context. However, it does not explicitly contrast this tool with recurly_pause_subscription or list when cancellation is preferred over pausing, so alternative selection is left partly to inference.

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