jj

name: jj description: Reference guide for jj (Jujutsu) version control operations

jj Reference Guide

Reference for common jj operations, conventions, and patterns used in this codebase.

Commit Description Format

All commit descriptions MUST be a single line following this format:

<project-prefix>: <short description>

IMPORTANT: Always use a one-line commit message. Never use multi-line descriptions or bullet points.

Project prefix is the descriptive folder path identifying the subsystem (e.g., src/auth, lib/utils, services/api).

Description starts with a lowercase verb: add, update, fix, refactor, remove, etc.

Examples

src/auth: add JWT token refresh logic
lib/utils: fix date parsing for ISO formats
services/api: add rate limiting middleware
db/migrations: add users table
docs: update installation instructions
tests/integration: add checkout flow tests

Analyzing Changes

Overview Commands

# Files changed with change type (modified/added/deleted)
jj diff --summary
jj show <rev> --summary

# Lines changed per file histogram
jj diff --stat
jj diff -r <rev> --stat

# Full diff
jj diff
jj diff -r <rev>

Size-Based Approach

• Small changes (≤5 files, ≤200 lines): read the diff directly
• Large changes (>5 files or >200 lines): use /jj-context to get a structured summary

Common Operations

Creating a Commit (jj commit)

Creates a new commit from working copy changes:

jj commit -m "<project-prefix>: <description>"

After committing, @ becomes an empty working copy on top of the new commit.

Describing a Commit (jj describe)

Sets or updates the description of an existing commit:

# Describe parent commit (most common)
jj describe @- -m "<project-prefix>: <description>"

# Describe any commit
jj describe <rev> -m "<description>"

Creating a Bookmark (jj bookmark)

Bookmarks use john/<descriptive-name> format with kebab-case:

# On parent commit (if @ is empty)
jj bookmark create john/<name> -r @-

# On current commit
jj bookmark create john/<name>

# List bookmarks
jj bookmark list

Naming: Use 2-4 word kebab-case names describing the work:

• john/s3-inventory-download
• john/fix-auth-refresh
• john/refactor-poller-config

Splitting a Commit (jj split)

Splits a commit into multiple commits by file:

# Split specific files into a new commit
jj split -r <rev> -m "<description>" path/to/file1 path/to/file2

# Last group: just describe the remainder
jj describe -r @- -m "<description>"

Note: After each split, the revision ID changes. Use @- to refer to the result.

Grouping suggestions:

• Core feature + its tests together
• Configuration/infrastructure separate
• Database migrations separate
• Refactoring/cleanup separate

Squashing Changes (jj squash)

Moves changes from working copy into ancestor commits.

Without options, squashes all changes from @ into its parent (@-):

jj squash  # @ → @-

With --into (or -t), squash into a specific commit:

jj squash --into <change-id> path/to/file.py  # specific files
jj squash --into <change-id> "src/**/*.py"    # glob pattern
jj squash --into @--                           # grandparent

For sub-file chunks (specific hunks, not whole files):

1. Save original @ change ID: jj log -r @ --no-graph -T 'change_id'
2. Create intermediate: jj new --insert-before @
3. Write only the hunks for target commit
4. Squash: jj squash --into <target-change-id>
5. Return to rebased @: jj edit <rebased-@-change-id>
6. Remove duplicated hunks
7. Restore working copy: jj new

Always use change IDs (e.g., ksrmwuon) not commit IDs - they're stable across rewrites.

Revision Syntax

Working Copy Behavior

• In jj, @ (working copy) is often an empty commit on top of your actual work
• Check jj log for "(empty)" to know if @ has changes
• jj commit creates commit and leaves @ empty
• jj describe modifies existing commit, doesn't change @