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The Ultimate SB3Utility Tutorial: Mastering Scratch File Modding & Asset Injection

If you’ve ever wanted to go beyond the limitations of the standard Scratch editor—whether to inject high-quality custom sounds, import HD vector assets, or fix corrupted project files—you’ve likely stumbled upon the need for SB3Utility. This powerful, open-source tool acts as a surgical scalpel for .sb3 files (Scratch 3.0 project files).

In this comprehensive tutorial, you will learn what SB3Utility is, how to install it, and how to perform advanced operations like batch asset replacement, JSON editing, and project repair.

6. Tutorial Part 2: Swapping Sounds Without Re-encoding

By default, Scratch re-encodes sounds to mono, 16-bit, 22kHz WAV. SB3Utility lets you inject true stereo MP3s or high-quality WAVs.

Basic Python example using sb3utils (or manual handling):

import zipfile
import json
import shutil
import os
def extract_sb3(sb3_path, output_dir):
with zipfile.ZipFile(sb3_path, 'r') as zip_ref:
zip_ref.extractall(output_dir)
print(f"Extracted to output_dir") sb3utility tutorial
def repack_sb3(source_dir, output_sb3):
with zipfile.ZipFile(output_sb3, 'w', zipfile.ZIP_DEFLATED) as zipf:
for root, dirs, files in os.walk(source_dir):
for file in files:
zipf.write(os.path.join(root, file),
arcname=os.path.relpath(os.path.join(root, file), source_dir))
print(f"Created output_sb3")
Basic Usage
4. Basic Workflow: Opening and Exploring a Project

Launch SB3Utility.
Click File → Open and select your .sb3 file.
The left panel will show a tree view: Stage and all Sprites.
Click on a sprite. The right panel will display its assets (costumes, sounds) with details like MD5 hash, file size, and data format.

Pro Tip: The MD5 hash is critical. Scratch uses these hashes to cache assets. If you replace an asset, SB3Utility automatically updates the hash in project.json.
Converting SB3 to JSON

Open SB3 Utility and select the "Convert" tab
Choose the SB3 file you want to convert
Select the output format (e.g., JSON)
Choose the output file location
Click "Convert"

Step 3: The Repack
Now came the magic. The tutorial instructed: Recompress the folder into a new .zip file, then rename the extension back to .sb3. Launch SB3Utility
Maya selected all the inner files (not the folder itself—the tutorial was specific about that), right-clicked, and chose "Compress." A new Archive.zip appeared. She renamed it to cosmic_rescue_FIXED.sb3.
Her finger hovered over the mouse button. If this failed, she had no backup.
She double-clicked.
The green flag appeared. The Scratch editor loaded. Sprites appeared. Gloop wiggled his antennae. The hyperdrive button was ready.
It worked.