Sync is a new paradigm for building on transaction data that aims to provide developers with direct access to feeds of changes to transaction data, reducing integration time and streamlining how developers access and work with transaction information. The original Plaid Transactions API used a "local copy pattern" where customers kept their own copy of user transactions and merged it with updates from Plaid, but this approach had limitations such as dealing with edge cases and inconsistencies. To address these challenges, Plaid has introduced Sync, which uses a diff-based model to deliver transaction data, providing a simpler and more efficient way for developers to integrate with Plaid's data. The new API design prioritizes simplicity, efficiency, and semantics, while also decoupling webhook delivery from consistency, allowing customers to focus on building scalable and robust systems without worrying about edge cases. With Sync, developers can build transaction-driven applications faster and easier, and Plaid is leading the fintech industry by introducing a new mental model for financial data delivery that guides customer feedback and innovation.