edmundmiller/jj-merge-repos

Merging JJ Repositories

Guide for merging two separate Jujutsu repositories into a single unified repo.

When to Use This Skill

• Combining two unrelated repositories into one
• Recovering from diverged histories (local vs remote rewrites)
• Splicing commit histories together
• Rebasing an entire repository onto another

How It Works

All JJ repos share a common empty root commit. When you add a remote from another repo and fetch, you get two diverging chains from that root. You can then:

1. Delete one chain (if you just want the other repo's history)
2. Rebase one chain onto the other (splice them together)

Quick Reference

Add and Fetch Another Repository

# Add the other repo as a remote
jj git remote add other-repo <url-or-path>

# Fetch all commits from it
jj git fetch --remote other-repo

View Both Histories

After fetching, you'll see two separate commit chains:

jj log

Both chains start from the shared empty root commit.

Option 1: Keep Only One History

Delete the unwanted chain of commits:

# Abandon all commits from the old/unwanted branch
jj abandon <commit-range>

Option 2: Splice Repositories Together

Rebase one repo's history onto the other:

# Move all commits from one chain atop the other
jj rebase --source <root-of-chain-to-move> --destination <tip-of-other-chain>

Example: Recovering from Diverged Histories

When local and remote diverged due to history rewriting:

# Add the remote (source of truth)
jj git remote add origin <url>

# Fetch remote commits
jj git fetch

# You now have two chains from root
# Delete the local chain you don't want
jj abandon <local-commits>

# Or rebase local work onto remote
jj rebase --source <local-root> --destination <remote-tip>

Example: Combining Two Unrelated Projects

# Start in repo A
cd repo-a

# Add repo B as a remote
jj git remote add repo-b /path/to/repo-b

# Fetch repo B's history
jj git fetch --remote repo-b

# Rebase repo B's commits onto repo A's main
jj rebase --source <repo-b-root> --destination main

Caveats

• Merge conflicts are likely when combining unrelated repos with overlapping files
• Resolve conflicts during the rebase as needed
• Consider whether you actually want the combined history or just the files

Tips

• Use jj log --all to see all commits including from other remotes
• The root commit (empty) is always shared between all chains
• jj abandon removes commits without affecting working copy
• After cleanup, run jj git push to update remotes if needed