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

Goes well beyond the annotations: discloses that the redemption is held and applied at the next creation/change/renewal, that it changes what the customer pays, what is returned (total discounted so far and currency), the failure mode (validation error on bad code/currency), the default per-call confirmation switch, and the API key requirement. That is unusually rich behavioral context for a mutation tool.

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?

Purpose and mechanism are front-loaded and most sentences earn their place, but the confirmation instruction is stated twice ('By default you confirm each call; you can switch that off' and 'confirm with the user before each call'), which is mild redundancy in an otherwise dense paragraph.

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?

With no output schema, the description correctly explains what is returned and covers auth, confirmation, and the error path. The main remaining gap is idempotency/repeat behavior (e.g., what happens if the coupon is already redeemed on the account), which an agent might need before retrying.

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 description coverage is 100%, so all three parameters (account_code, coupon_code, currency) are already documented in the schema. The description mentions currency only in the context of the return value and the validation error, adding no new syntax or selection semantics for the parameters. Baseline 3 is appropriate.

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 and resource ('attach a coupon to an account', 'redeems a coupon code for one account') and clarifies the mechanism (redemption held on the account, applied at next subscription creation/change/renewal). An agent can distinguish this from recurly_change_subscription or recurly_cancel_subscription without opening a schema.

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?

'Before or after a subscription' gives clear timing context for when this applies, and the confirmation guidance sets invocation expectations. However it never names an alternative path (e.g., applying a coupon at subscription creation vs. redeeming standalone), so there is no explicit when-not-to-use guidance.

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