Undo.skill — File Edit Time Machine

> CRITICAL: After EVERY file modification (Write, Edit, MultiEdit, Shell commands that change files), you MUST run the snapshot script. This is non-negotiable — without snapshots, undo is impossible.

Base Directory

All scripts are located in this skill's directory. Use {baseDir} to reference them:

SCRIPTS="{baseDir}/scripts"

Quick Reference

Output Format

Every script outputs exactly one line of JSON. Parse it with your environment's JSON parser — do NOT read the human text.

init.js

{"action":"init","status":"success","initialized":true,"project":"/abs/path","storagePath":"~/.local/share/...","message":"Tracking initialized"}

{"action":"init","status":"success","initialized":false,"project":"/abs/path","storagePath":"~/.local/share/...","message":"Already tracked"}

snapshot.js

{"action":"snapshot","status":"success","committed":true,"project":"/abs/path","description":"Added error handling","hash":"abc1234","changedFiles":["src/index.js"],"fileCount":1}

{"action":"snapshot","status":"success","committed":false,"project":"/abs/path","description":"..."}

(committed=false means no changes since last snapshot)

list.js

{
  "action":"list",
  "status":"success",
  "project":"/abs/path",
  "count":3,
  "commits":[
    {
      "hash":"abc1234",
      "date":"2026-04-10 14:05:12 +0800",
      "timestamp":"2026-04-10T06:05:12.000Z",
      "description":"Added error handling",
      "files":[{"status":"M","path":"src/index.js"}],
      "isCheckpoint":false,
      "isInitial":false
    }
  ],
  "checkpoints":[{"name":"checkpoint/before-refactor","hash":"def5678"}]
}

undo.js

{"action":"undo","status":"success","undone":true,"mode":"steps","project":"/abs/path","steps":1,"targetHash":"abc1234","checkpointBranch":"checkpoint/undo-1712736312000"}

{"action":"undo","status":"error","error":"not_enough_history","mode":"steps","project":"/abs/path","requested":5}

checkpoint.js

{"action":"checkpoint","status":"success","project":"/abs/path","checkpoint":"before-refactor","branch":"checkpoint/before-refactor"}

watcher.js

On start:

{"action":"watcher-start","status":"success","project":"/abs/path","pid":12345,"debounceMs":5000,"pollMs":2000}

On each auto-snapshot:

{"action":"watcher-snapshot","status":"success","committed":true,"project":"/abs/path","hash":"abc1234","description":"[watcher] added: 1, modified: 2","timestamp":"2026-04-10T06:05:12.000Z","files":{"added":["new.js"],"changed":["index.js"],"deleted":[]}}

On stop:

{"action":"watcher-stop","status":"stopped","project":"/abs/path"}

Error Codes

First-Time Setup

After installing this skill, run init.js on the user's project before any other commands:

node "$SCRIPTS/init.js" /path/to/project

This attempts to auto-install git if missing, then initializes tracking. If git auto-install fails, it returns a structured error with platform details and install hints — use that info to guide manual installation.

Core Workflow

Rule 1: Snapshot After Every File Change

After ANY Write/Edit/Shell that modifies files, immediately run:

node "$SCRIPTS/snapshot.js" /path/to/project "Brief description of what changed"

Description guidelines:

• Specific: "Refactored auth middleware to use JWT" not "changed files"
• Mention key files
• Include intent
• Under 80 chars for the summary

Rule 2: Checkpoint Before Major Operations

node "$SCRIPTS/checkpoint.js" /path/to/project "before-refactor"

Rule 3: List History When Asked

node "$SCRIPTS/list.js" /path/to/project

Parse the commits array. Each entry has hash, timestamp, description, files, and isCheckpoint flag.

Rule 4: Undo on Request

# Undo last N changes
node "$SCRIPTS/undo.js" /path/to/project --steps 3

# Undo to named checkpoint
node "$SCRIPTS/undo.js" /path/to/project --to before-refactor

# Undo to specific time (ISO 8601)
node "$SCRIPTS/undo.js" /path/to/project --timestamp "2026-04-10T14:00"

After successful undo, inform the user that previous state is preserved on the branch from the JSON's checkpointBranch field.

Rule 5: Watcher for Long Sessions

node "$SCRIPTS/watcher.js" /path/to/project &

Auto-snapshots if you forget. PID in startup JSON. Stop with kill .

How It Works (Architecture)

~/.local/share/undo-skill/repos/
├── abc123def456/          # Bare git repo (MD5 hash of project path, 12 chars)
│   ├── HEAD
│   ├── refs/heads/main    # Latest snapshot
│   ├── refs/heads/checkpoint/undo-1712736312000
│   ├── refs/heads/checkpoint/before-refactor
│   └── _worktree/         # Temp working directory (auto-managed)
└── projects.json           # hash → absolute path mapping

• Storage: External bare git repo, indexed by MD5 of absolute project path
• Project isolation: The project's own .git is NEVER touched
• Branch strategy: main = current state; checkpoint/* = preserved states (never deleted)
• Undo: Creates checkpoint branch at HEAD → resets to target → force-pushes main

Important Notes

1. ALWAYS snapshot after writes — #1 rule, no exceptions
2. Output is JSON — parse the JSON line, ignore any other text
3. Don't double-snapshot — one change = one snapshot
4. Describe accurately — the description is how users and future AI understand history
5. Absolute paths only — always pass full absolute path
6. Undo preserves history — previous states live forever as checkpoint/* branches