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docs(FLOW-16): Document precision loss in cross-VM amount conversions #37

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docs(FLOW-16): Document precision loss in cross-VM amount conversions #37

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11 changes: 10 additions & 1 deletion cadence/contracts/FlowYieldVaultsEVM.cdc
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Expand Up @@ -24,6 +24,13 @@ import "FlowEVMBridgeConfig"
/// 2. For each request, calls startProcessing() to mark as PROCESSING (deducts escrow for CREATE/DEPOSIT)
/// 3. Executes Cadence-side operation (create/deposit/withdraw/close YieldVault)
/// 4. Calls completeProcessing() to mark as COMPLETED or FAILED (refunds escrow for CREATE/DEPOSIT failures)
///
/// PRECISION NOTE:
/// EVM uses uint256 with 18 decimals (wei), while Cadence uses UFix64 with 8 decimals.
/// Converting between these formats truncates precision beyond 8 decimal places.
/// For example: 1.123456789012345678 FLOW (EVM) becomes 1.12345678 FLOW (Cadence).
/// This is not exploitable (users receive slightly less, not more) and the 1 FLOW
/// minimum deposit makes any dust loss negligible (~0.0000001% maximum).
access(all) contract FlowYieldVaultsEVM {

// ============================================
Expand Down Expand Up @@ -1869,9 +1876,11 @@ access(all) contract FlowYieldVaultsEVM {
/// @notice Converts a UInt256 amount from EVM to UFix64 for Cadence
/// @dev For native FLOW: Uses 18 decimals (attoflow to FLOW conversion)
/// For ERC20: Uses FlowEVMBridgeUtils to look up token decimals
/// PRECISION WARNING: UFix64 has only 8 decimal places vs uint256's 18.
/// Precision beyond 8 decimals is truncated (e.g., 1.123456789... → 1.12345678).
/// @param value The amount in wei/smallest unit (UInt256)
/// @param tokenAddress The token address to determine decimal conversion
/// @return The converted amount in UFix64 format
/// @return The converted amount in UFix64 format (truncated to 8 decimals)
access(self) fun ufix64FromUInt256(_ value: UInt256, tokenAddress: EVM.EVMAddress): UFix64 {
if tokenAddress.toString() == FlowYieldVaultsEVM.nativeFlowEVMAddress.toString() {
return FlowEVMBridgeUtils.uint256ToUFix64(value: value, decimals: 18)
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5 changes: 5 additions & 0 deletions solidity/src/FlowYieldVaultsRequests.sol
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Expand Up @@ -30,6 +30,11 @@ import {
* Processing uses atomic two-phase commit:
* - startProcessing(): Marks request as PROCESSING, deducts user balance
* - completeProcessing(): Marks as COMPLETED/FAILED, credits claimable refunds on failure
*
* PRECISION NOTE: EVM uses uint256 with 18 decimals, while Cadence uses UFix64 with 8 decimals.
* Amounts are truncated beyond 8 decimal places during cross-VM conversion.
* Example: 1.123456789012345678 FLOW → 1.12345678 FLOW (loss of ~9e-9 FLOW).
* This is not exploitable (truncation favors the protocol) and the minimum deposit mitigates dust.
*/
contract FlowYieldVaultsRequests is ReentrancyGuard, Ownable2Step {
using SafeERC20 for IERC20;
Expand Down