Flash Swaps



To understand Flash Swaps, make sure you understand Batch Swaps first.

Anyone who identifies a price discrepancy in two Balancer Pools can execute a Flash Swap. An arbitrageur who makes a flash swap does not need to hold any of the input tokens that one would normally need to make a swap. Instead, the arbitrageur identifies the imbalance, tells the Vault to make the swap, and is rewarded with the profit.


In this sample transaction (click here for logsopen in new window and click here for execution traceopen in new window), the sender identifies a price discrepancy between two pools that contain the same two tokens. They realize that if they were to sell 1,000,000 units of token 0xc256to pool 0x3a19 and then sell the proceeds to pool 0x32fc they would come out with more than 1,000,000 units of token 0xc256. Even though the sender doesn't hold any of the tokens, they make a call to batchSwap.

"kind": "0",
  "assets": [
  "limits": [


  • kind: GIVEN_IN (= 0) means that the swaps are formatted such that the amount represents the amount being sent to the pool
  • assets: the list of tokens used in the swap
  • limits: the number of tokens the swapper is willing to send to the Vault For a Flash Swap, these are set to zero (or less) since a Flash Swap doesn't require the swapper to send any tokens to the Vault)
"swaps": [
      "poolId": "0x3a19030ed746bd1c3f2b0f996ff9479af04c5f0a000200000000000000000004",
      "assetInIndex": "0",
      "assetOutIndex": "1",
      "amount": "1000000"
      "poolId": "0x32fc95287b14eaef3afa92cccc48c285ee3a280a000100000000000000000005",
      "assetInIndex": "1",
      "assetOutIndex": "0",
      "amount": "0"

The first swap, shows an exchange of 1000000 units of assets[0] for assets[1] in pool 0x3a19. The next swap shows the opposite swap of assets[1] for assets[0] in pool 0x32fc. You may notice in the second swap that the amount field is set to 0. Sending zero as the amount in a swap step simple means it uses the value output by the previous swap.

  "funds": {
    "sender": "0x44c42303d71fc693d553d71309c80461010b8457",
    "fromInternalBalance": false,
    "recipient": "0x44c42303d71fc693d553d71309c80461010b8457",
    "toInternalBalance": false
  "deadline": "999999999999999999"

Finally, the FundManagement struct and deadline are set the same way as any other batchSwap. The user defines the sender and recipient as themselves, and sets both internal balance flags to false. The net return after the two swaps is 2285700 units of token 0xc256, which is sent directly to the recipient.