jj-hunk

Programmatic hunk selection for jj (Jujutsu).

Select specific diff hunks when splitting, committing, or squashing—without interactive UI. Designed for AI agents and automation.

Installation

1. Install the binary

cargo install jj-hunk

2. Configure jj

Add to ~/.jjconfig.toml:

[merge-tools.jj-hunk]
program = "jj-hunk"
edit-args = ["select", "$left", "$right"]

3. Verify

jj-hunk --help

Quick Start

# See what hunks exist in your changes
jj-hunk list

# Split changes: hunks 0,1 of foo.rs → first commit, rest → second
jj-hunk split '{"files": {"src/foo.rs": {"hunks": [0, 1]}}, "default": "reset"}' "first commit"

# Commit specific files, leave rest in working copy
jj-hunk commit '{"files": {"src/fix.rs": {"action": "keep"}}, "default": "reset"}' "bug fix"

# Squash specific changes into parent
jj-hunk squash '{"files": {"src/cleanup.rs": {"action": "keep"}}, "default": "reset"}'

Commands

Command	Description
jj-hunk list	List all hunks as JSON
jj-hunk split <spec> <message>	Split changes into two commits
jj-hunk commit <spec> <message>	Commit selected hunks
jj-hunk squash <spec>	Squash selected hunks into parent

Spec Format

{
  "files": {
    "path/to/file": {"hunks": [0, 2]},
    "path/to/other": {"action": "keep"},
    "path/to/another": {"action": "reset"}
  },
  "default": "reset"
}

• {"hunks": [indices]} — select specific hunks by index
• {"action": "keep"} — keep all changes in file
• {"action": "reset"} — discard all changes in file
• "default" — action for unlisted files ("keep" or "reset")

Example Output

$ jj-hunk list
{
  "src/lib.rs": [
    {"index": 0, "type": "replace", "removed": "old_fn()\n", "added": "new_fn()\n"},
    {"index": 1, "type": "insert", "removed": "", "added": "// new comment\n"}
  ],
  "src/main.rs": [
    {"index": 0, "type": "delete", "removed": "dead_code()\n", "added": ""}
  ]
}

How It Works

jj-hunk integrates with jj's --tool mechanism:

1. You run jj-hunk split/commit/squash with a JSON spec
2. jj-hunk writes the spec to a temp file and sets JJ_HUNK_SELECTION env var
3. jj invokes jj-hunk select $left $right as the diff tool
4. jj-hunk reads the spec and modifies $right to include only selected hunks
5. jj snapshots the result

For direct control:

echo '{"files": {"src/foo.rs": {"hunks": [0]}}}' > /tmp/spec.json
JJ_HUNK_SELECTION=/tmp/spec.json jj split -i --tool=jj-hunk -m "message"

Use Cases

AI Agents

The primary use case. AI agents can create clean, logical commits without interactive prompts. Instead of dumping all changes into one commit, an agent can:

1. Analyze changes with jj-hunk list
2. Group files by logical concern (schema, services, tests, etc.)
3. Split iteratively to create a narrative commit history

The JSON spec format is easy for LLMs to construct programmatically.

Clean History Workflow

Reorganize messy development history into reviewer-friendly commits. Squash everything, then split by concern:

jj squash --from 'all:trunk()..@-' --into @
jj edit @
jj-hunk split '{"files": {"src/db/schema.ts": {"action": "keep"}}, "default": "reset"}' "feat: add schema"
jj-hunk split '{"files": {"src/api/routes.ts": {"action": "keep"}}, "default": "reset"}' "feat: add routes"
jj describe -m "feat: add UI"

See .claude/commands/clean-history.md for a complete workflow.

CI/CD Automation

Script commit splitting in pipelines. Enforce commit hygiene rules, auto-split by file patterns, or validate that commits are properly scoped.

Partial Commits

Keep experimental code in working copy while committing only the finished parts:

jj-hunk commit '{"files": {"src/fix.rs": {"action": "keep"}}, "default": "reset"}' "fix: handle edge case"
# Experimental changes remain uncommitted

Claude Code Integration

This repo includes a Claude Code command for the clean history workflow:

/clean-history [bookmark-name]

The command guides through squashing, splitting, and creating a PR with narrative-quality commits.

License

MIT