> For the complete documentation index, see [llms.txt](https://docs.fira.money/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fira.money/developers/protocol-contracts/yield-contracts/coupon-token.md). # Coupon Token {% hint style="info" %} **Source:** `src/fira_bonding/core/YieldContracts/CouponToken.sol`, `src/fira_bonding/core/YieldContracts/InterestManagerCT.sol` {% endhint %} ## Contract summary CT extends `InterestManagerCT` and `FiraERC20`. Like BT, the FW, BT, factory, expiry, and `doCacheIndexSameBlock` fields are all immutable. All external mutating functions use `nonReentrant`. ## Minting BT+CT `mintBC(receiverBT, receiverCT)` takes FW that has been pre-transferred to the CT contract, converts the FW amount to underlying asset units via the BC index, and mints equal BT and CT. The BC index is `max(FW.exchangeRate(), storedIndex)` — monotonically non-decreasing. `mintBCMulti` does the same for multiple receivers in one call. ## Redeeming BT+CT `redeemBC(receiver)` burns BT from `address(this)` (must be pre-transferred) and burns CT if not expired. FW is returned based on the current BC index. **Post-expiry:** The redeemable FW amount is calculated at the *current* BC index, but the excess between the post-expiry first index and the current index accrues as treasury interest. Post-expiry yield goes to the protocol, not to redeemers. ## Interest accounting `InterestManagerCT` tracks per-user interest via `UserInterest { index, accrued }`. On every CT transfer (`_beforeTokenTransfer`), interest is distributed to both sender and receiver: ``` interest = principal × (currentIndex - prevIndex) / (prevIndex × currentIndex) ``` Claims go through `_doTransferOutInterest`, which deducts `interestFeeRate` and sends the fee to treasury. ### Key design decisions * Interest is based on CT balance, not FW balance * Protocol fees are taken at claim time, not at accrual time * `_distributeInterestForTwo` skips `address(0)` and `address(this)` to avoid phantom accounting ## BC index caching If `doCacheIndexSameBlock` is true, the BC index is only updated once per block. This prevents sandwich attacks where the FW exchange rate is manipulated within a block to extract value from CT holders. ## Post-expiry state `_setPostExpiryData()` is called on the first interaction after expiry. It snapshots the BC index and reward indices. After this, the interest index is frozen to `postExpiry.firstBCIndex`, and all subsequent yield flows to treasury. `redeemInterestAndRewardsPostExpiryForTreasury()` sweeps all post-expiry interest to the treasury. ## Inherited contracts * `InterestManagerCT` — Per-user interest tracking * `FiraERC20` — ERC-20 with reentrancy guard --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.fira.money/developers/protocol-contracts/yield-contracts/coupon-token.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.