JJ Con 2026 - Three Writes, Not One: Building Developer Tools on jj-lib - Lita Cho

If you are a Rust engineer comfortable with the jj CLI and want to build developer tooling on top of jj-lib — IDE integrations, code review tools, CI shims, custom GUIs — this talk is for you. Committing a transaction to the op log is not enough: your filesystem, your workspace state, and your git refs are three separate writes you have to coordinate yourself. This talk walks through the jj-lib architecture, how transactions interact with the op log and backend, and the gotchas along the way: stale workspaces, importing and exporting git refs, the half-finished async story, and what the jj CLI is quietly doing on every command that you will have to reimplement. You will leave knowing exactly which APIs to call, in which order, to avoid corrupting a workspace. #tech #opensource #jjvcs #jjcon