CMS21 Save Editor

Learning project — This was built as a hands-on exercise to understand how binary file formats work: reading/writing little-endian integers, IEEE-754 floats, length-prefixed strings, and locating data by byte-pattern signatures rather than hardcoded offsets. The .cms21b save format made for a good real-world target since it's complex enough to be interesting but small enough to reverse-engineer without tooling.

A save-file editor for Car Mechanic Simulator 2021 that runs entirely in your browser — no installation, no Python, no server.

Always back up your save files before editing. Default save location on Windows: %USERPROFILE%\AppData\LocalLow\Red Dot Games\Car Mechanic Simulator 2021\Saves\

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Using the hosted version

The easiest way — just open the link, drag in your save, make edits, and download.

Online save editor

Your save file never leaves your computer. Everything is processed locally in the browser.

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Features

Category	What you can edit
Player stats	Money (credits), level, XP, available skill points
Skills	Individual skill purchased/unlocked state, per-tier flags
Garage upgrades	Lock / unlock any of the 23 garage items (dyno, paint shop, OBD scanner, lifter, etc.)
Parts	Per-part condition (0–100%), bulk repair all worn parts

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How to use

1. Open the editor in your browser.
2. Drag and drop your .cms21b save file onto the drop zone, or click Browse file… to pick it.
3. Edit what you want across the Overview, Garage, and Parts tabs.
4. Click ↓ Download .cms21b to save the modified file.
5. Copy the downloaded file back to your saves folder, replacing the original.

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Save file location

Platform	Path
Windows (Steam/Epic)	%USERPROFILE%\AppData\LocalLow\Red Dot Games\Car Mechanic Simulator 2021\Saves\
Windows (Xbox Game Pass)	%LOCALAPPDATA%\Packages\RedDotGames.CarMechanicSimulator2021_h6vh6...\SystemAppData\wgs\
Linux (Steam)	~/.steam/steam/userdata/<id>/1190000/remote/

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Xbox Game Pass (PC) Specifics

The Game Pass version uses the Windows Game Save (WGS) system, which is different from the standard Steam version:

1. Hash Filenames: Files in the wgs folder have no extensions and use long hexadecimal names (e.g., 551DBD...).
2. Identification: To find your save, look for the largest file (typically 8KB–30KB) in the most recently modified subfolder.
3. Cloud Sync Override: Xbox will often overwrite your modified file with the cloud version. To prevent this:
   • Disconnect from the Internet.
   • Replace the original hash file with your edited one (Keep the original hash name!).
   • Start the game while offline.
   • Once at the main menu, Reconnect to the Internet.
   • Make a small ingame change (e.g., move a car) to trigger a sync.
   • If prompted, choose "Use Local Save".

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Profiles are named profile0.cms21b, profile1.cms21b, etc.

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Running locally (developers)

You only need Node.js 18+. No Python required.

git clone https://github.com/your-username/cms2021-save-editor.git
cd cms2021-save-editor/web
npm install
npm run dev

Then open http://localhost:5173 in your browser.

Building for production

cd web
npm run build

The web/dist/ folder is a self-contained static site — host it anywhere (GitHub Pages, Netlify, Cloudflare Pages, etc.).

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Deploying to GitHub Pages

1. Push the repository to GitHub.
2. Go to Settings → Pages and set the source to GitHub Actions.
3. Create .github/workflows/deploy.yml:

name: Deploy to GitHub Pages
on:
  push:
    branches: [main]
jobs:
  deploy:
    runs-on: ubuntu-latest
    permissions:
      contents: read
      pages: write
      id-token: write
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20
      - run: npm install
        working-directory: web
      - run: npm run build
        working-directory: web
      - uses: actions/upload-pages-artifact@v3
        with:
          path: web/dist
      - uses: actions/deploy-pages@v4

Every push to main will automatically rebuild and publish the site.

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Project structure

cms2021-save-editor/
└── web/                        # The entire application (static React + Vite)
    ├── src/
    │   ├── codec.js            # Binary decoder / encoder (pure JS, no dependencies)
    │   ├── App.jsx             # Main app — state, file handling, download
    │   ├── App.css
    │   └── components/
    │       ├── DropZone.jsx
    │       ├── Overview.jsx
    │       ├── Garage.jsx
    │       └── Parts.jsx
    ├── index.html
    ├── vite.config.js
    └── package.json

decode.py and editor.py in the repo root are a legacy Python CLI tool for power users who prefer the command line. They are not needed to run the web app.

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Binary format notes

The .cms21b format uses little-endian integers, IEEE-754 floats, and length-prefixed UTF-8 strings (1-byte length + data). See web/src/codec.js for the full annotated implementation.

File structure:

Magic         8 bytes   "PJOOOTER"
Header block  variable  profile name, date, version, 0xff padding
Parts section variable  per-part binary records (47 / 63 / 64 bytes each)
Tail section  variable  skills, garage state, car records, player stats

Key offsets in the tail section (all dynamic — located by byte-pattern search):

Field	How it's found
Stats block (money/level/XP)	Last occurrence of 5c fe ff ff 00 00 f0 41, then +17 bytes
Money	stats_offset + 0, uint32 LE
Level	stats_offset + 4, uint32 LE, stored as level - 1
XP	stats_offset + 8, uint32 LE
Skill points	garage_offset − 4, uint32 LE
Garage section	First occurrence of 17 00 00 00 09 paintshop; 23 items × 8 bytes; byte[0] = tier/purchase state
Skills section	First occurrence of fast_movement, back up to the preceding uint32 count

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Contributing

Pull requests are welcome. If you discover new fields or format details, please document them in the PR description.

To run a round-trip sanity check in the browser console:

import { decode, encode } from './src/codec.js'

const buf = await fetch('profile0.cms21b').then(r => r.arrayBuffer())
const save = decode(buf)
const reencoded = encode(save)
console.assert(
  new Uint8Array(buf).every((b, i) => b === reencoded[i]),
  'Round-trip mismatch!'
)
console.log('Round-trip OK')

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License

MIT — see LICENSE file.