jj-merge-repos - SKILL.md Agent Skill

name: jj-merge-repos description: Merges two separate Jujutsu (jj) repositories into one. Use when combining repos, rebasing one repo onto another, or recovering from diverged histories.

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:

Delete one chain (if you just want the other repo's history)
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