Allocation and reassignment of unique identifiers for synchronization of content items

A client can allocate and reassociate unique identifiers to local content items associated with an account at a content management system, and use the unique identifiers to commit operations for the content items on the content management system. For example, a client can create a content item and determine the content item does not have an identifier from the content management system. The client obtains an identifier for the content item and asks the content management system to verify a uniqueness of the identifier. When the identifier is unique, the client adds a node corresponding to the content item to a local tree representing a state at the client of content items associated with the account, and uploads the content item with the identifier to the content management system. When the identifier is not unique, the client obtains a new identifier for the content item.