Fees (XCM V4 → XCM V5)

XCM V5 introduces a new fee payment mechanism that simplifies and unifies the handling of fees across various XCM operations.

Key Changes from V4

XCM V5 replaces the BuyExecution instruction with a more predictable PayFees approach that handles both execution and delivery fees.

BuyExecution vs. PayFees

• XCM V4 Approach:

  • Used BuyExecution instruction that handles only execution fees.
  • Leftover assets after buying execution are returned to holding.
  • Required explicit specification of execution weight limits.
  • Delivery fees may not be found, and an error may occur.

• XCM V5 Approach:

  • Introduces PayFees instruction for unified fee handling.
  • All assets passed to PayFees are kept in a special fees register; they are NOT returned to holding.
  • No need to specify weights, only assets.
  • More predictable.

PayFees Instruction

The new PayFees instruction provides a cleaner interface:

// XCM V5 - New PayFees instruction
XcmV5Instruction.PayFees({
  asset: {
    id: // Asset id for fee payment
    fun: // Amount to dedicate for fees (both execution + delivery)
  },
})

This replaces the more complex BuyExecution pattern:

// XCM V4 - BuyExecution instruction
XcmV4Instruction.BuyExecution({
  fees: {
    id: // Asset id
    fun: // Fee amount (only execution will be charged, rest is returned to holding)
  },
  weight_limit: // Explicit weight limit
})

Backward Compatibility

XCM V5 maintains backward compatibility with BuyExecution for existing implementations. Both instructions are supported, allowing gradual migration:

• BuyExecution: Still supported for compatibility with existing XCM programs.
• PayFees: Recommended for new development and simplified fee management.

Migration Considerations

When migrating from XCM V4 to XCM V5:

• BuyExecution can remain in existing programs.
• New programs should use PayFees for better maintainability.
• Fee estimation becomes more straightforward with the new approach.
• No breaking changes to existing functionality.

When using PayFees, keep in mind that ALL assets passed to the instruction will be entirely dedicated to fees, not returned to holding. That's why it's more important than before to to estimate XCM fees properly.

RefundSurplus Instruction

When you overestimate fees with PayFees, you can recover unused funds using the RefundSurplus instruction.

You can use RefundSurplus to put the leftover fees back into holding. This is useful when you've overestimated the fees needed for your XCM program. You can then deposit them into some account with DepositAsset.

// After all instructions that send messages.
XcmV5Instruction.RefundSurplus(),
XcmV5Instruction.DepositAsset({
  assets: XcmV5AssetFilter.Wild(XcmV5WildAsset.All()),
  beneficiary: {
    parents: 0,
    interior: XcmV5Junctions.X1(
      XcmV5Junction.AccountId32({
        id: ACCOUNT,
        network: undefined,
      }),
    ),
  }
})

The RefundSurplus instruction:

• Takes any unused fees from the fees register.
• Moves them back to the holding register.
• Allows you to use or return the surplus assets with DepositAsset.

This is especially important with PayFees since all assets passed to it are dedicated to fees, unlike BuyExecution which automatically returned unused assets to holding.

Last update: August 14, 2025
| Created: August 14, 2025