Bond Token (BT)
Bond Token (BT) is the fixed-rate principal token in Fira's yield tokenization system.
Overview
BT represents a fixed-rate claim on the underlying asset. Each BT is redeemable for 1 unit of underlying at maturity. Before maturity, BT trades at a discount in the AMM — the discount reflects the implied fixed rate.
BT and CT are always minted and burned in pairs. You cannot create BT without also creating CT, and vice versa. This ensures the total claims always match the underlying FW.
How it works
Minting
BT is created through the CouponToken's mintBC function. FW is deposited into the CT contract, and equal amounts of BT and CT are minted based on the current BC Index. The conversion rate ensures that BT amount + CT amount always equals the underlying value of the FW deposited.
Redemption
Pre-expiry: Both BT and CT must be provided. They are burned together and FW is returned.
Post-expiry: Only BT is required. The CT is not burned (it has zero economic value post-expiry). FW is returned at the current BC Index rate, with any post-expiry yield accrual going to the protocol treasury.
Trading
BT trades against FW in the Fira Market AMM. As expiry approaches, the BT/FW exchange rate converges toward 1:1 (at the BC Index rate), since BT becomes redeemable for full underlying value at maturity.
Key features
Fixed-rate principal — Predictable value at maturity
Strict access control — Only the paired CouponToken and the LiquidityInjector can mint or burn BT
Used as loan token — In fixed-rate lending markets, borrowers receive BT which is swapped to USDC via the router
Immutable references — FW address, factory, and expiry are set at creation and cannot be changed
Related contracts
BondToken.sol— Contract detailsCouponToken.sol— Paired CT contractYieldContractFactory.sol— Factory that deploys BT/CT pairs
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