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XCM V5

The latest iteration of XCM is version 5. The main RFCs defining the changes in version 5 are the following:

What are Legacy RFCs?

The first two RFCs are legacy ones from when XCM RFCs were separate from the Polkadot Technical Fellowship RFCs. As part of the XCM V5 specification, these two were combined since the Polkadot Technical Fellowship Manifesto specifies XCM as part of its mandate.

Version 5 addresses key pain points that developers faced in previous versions:

  • Before XCM V5
    • Cross-chain transfers were limited to only one transfer type, requiring multiple different XCMs to send the entirety of the tokens.
    • Fee management across multiple hops was extremely complicated to get right, particularly because of delivery fees.
    • Cross-chain transfers cleared the origin, making it impossible to Transact in the same XCM as a transfer.
    • Claiming trapped assets when transfers failed was troublesome, most of the time requiring governance to intervene.
    • Having to specify the weight for Transact was brittle.
  • After XCM V5
    • A unified cross-chain transfer instruction handles all possible transfer types and allows multiple ones in the same XCM.
    • The fee payment mechanism has been standardized to handle multiple types of fees, making fees much more predictable.
    • It's now possible to preserve the origin during a cross-chain transfer, enabling the use of Transact in the same XCM as the transfer.
    • A custom asset claimer can be set, reducing the need for governance to intervene for claiming trapped assets.
    • Transact no longer needs to specify the weight of the call, reducing the errors in cross-chain interactions.

This section aims to give you all the changes in this new version you need to make the most of the latest and greatest in cross-chain interactions.

In This Section

Migration Guide (XCM V4 → XCM V5)

Step-by-step guide for migrating from XCM V4 to V5, covering breaking changes, new features, and best practices for upgrading your cross-chain applications.

Writing XCM Programs

Learn how to write XCM programs in version 5, including new execution patterns, instruction syntax, and best practices for cross-chain communication.

Fees

Explore the key differences in fee handling between XCM V4 and V5, including new payment mechanisms, delivery fees, and improved predictability for cross-chain transactions.

Transfers

Explore the key differences in transfer handling between XCM V4 and V5, including unified transfer instructions, multiple asset types, and improved cross-chain transfer capabilities.

Transact

Explore the key changes to the Transact instruction in XCM V5, including automatic weight calculation, improved reliability, and simplified cross-chain execution patterns.

Asset claimer

Learn how the AssetClaimer hint improves trapped asset recovery in XCM V5, enabling automated recovery and reducing the need for governance intervention.