Flash Loans

Veil supports native flash loans: borrow any amount up to the pool’s free liquidity, use the funds within the same Solana transaction, and return them — plus a fee — before the transaction ends. If repayment is missing or insufficient, the entire transaction reverts.

Fee: amount × flash_fee_bps / 10_000 (default 0.09%, configurable by admin)

Fee split: 90% to LPs (added to total_deposits) · 10% to protocol (accumulated_fees)

FlashBorrow and FlashRepay must appear in the same transaction. A FlashBorrow instruction without a FlashRepay in the same transaction will cause the transaction to revert on submission (the pool validator rejects state where flash_loan_amount > 0 at end-of-transaction through Solana’s atomic execution model).

FlashBorrow

Discriminator: 0x06

Transfer amount tokens from the pool vault to the borrower. Records the in-flight amount and fee in pool.flash_loan_amount. Rejects if another flash loan is already active on this pool.

Accounts

#	Name	Flags	Description
0	`borrower`	signer, writable	—
1	`borrower_token`	writable	Receives the borrowed tokens
2	`vault`	writable	Pool vault (source)
3	`pool`	writable	`flash_loan_amount` is set to `amount`
4	`pool_authority`	readonly	PDA that signs the vault transfer
5	`token_program`	readonly	—

Instruction Data

Offset	Size	Field	Description
0	1	discriminator	`0x06`
1	8	`amount`	Tokens to borrow (u64 LE)

Errors

FlashLoanActive — a flash loan is already in progress on this pool
InsufficientLiquidity — requested amount exceeds available liquidity
ZeroAmount — amount is zero
PoolPaused — pool is paused

FlashRepay

Discriminator: 0x07

Return the borrowed tokens plus the fee. Distributes the fee between LPs and the protocol treasury, then clears pool.flash_loan_amount.

Accounts

#	Name	Flags	Description
0	`borrower`	signer, writable	—
1	`borrower_token`	writable	Source token account (repays principal + fee)
2	`vault`	writable	Pool vault (destination)
3	`pool`	writable	—
4	`token_program`	readonly	—

Instruction Data

Offset	Size	Field	Description
0	1	discriminator	`0x07`
1	8	`amount`	Tokens to repay including fee (u64 LE)

The required repayment is pool.flash_loan_amount + fee. Any amount below this returns FlashLoanRepayInsufficient.

Errors

FlashLoanNotActive — no flash loan is in progress
FlashLoanRepayInsufficient — amount < principal + fee

Usage Pattern


import {
  flashBorrowIx,
  flashRepayIx,
} from './lib/veil/instructions';
 
const loanAmount = 1_000_000n;   // 1 USDC (6 decimals)
const fee = (loanAmount * 9n) / 10_000n;  // 0.09% = 900
const repayAmount = loanAmount + fee;
 
// Your custom instruction that uses the borrowed funds
const myInstruction = buildMyArbitrageInstruction(loanAmount);
 
const tx = new Transaction()
  .add(flashBorrowIx(borrower, borrowerToken, vault, pool, poolAuthority, loanAmount))
  .add(myInstruction)
  .add(flashRepayIx(borrower, borrowerToken, vault, pool, repayAmount));
 
await sendAndConfirmTransaction(connection, tx, [borrowerKeypair]);

Fee Calculation


function flashFee(amount: bigint, feeRateBps: bigint): bigint {
  return (amount * feeRateBps) / 10_000n;
}
 
function flashFeeSplit(fee: bigint): { lp: bigint; protocol: bigint } {
  const protocol = fee / 10n;
  return { lp: fee - protocol, protocol };
}