Sui's Cut Package — How Sui Freezes Its Execution Layer

Sui upgrades its Move VM without breaking the chain. The secret? A small Rust CLI tool called cut that snapshots execution crates so old and new logic can live side by side.

The Problem: Upgrading a Live Blockchain

Imagine Sui wants to change how the Move VM works — fix a bug, optimize gas metering, or ship a whole new VM like Bella Ciao. But here's the catch:

Every validator on the network must produce identical results for every transaction — past and present. If you replay a transaction from 6 months ago, it must produce the exact same output it did originally. Otherwise validators disagree, and that's a consensus fork.

So you can't just update the code and move on. You need to keep the old code around, frozen exactly as it was.

The Solution: Versioned Snapshots

Sui solves this by keeping multiple frozen copies of its execution layer side by side:

When a transaction comes in, Sui checks which protocol version it belongs to, then routes it to the correct frozen version. Old transactions use old code. New transactions use new code. Everyone agrees.

What Is the Cut Package?

The cut package (sui-execution/cut/) is a small Rust CLI tool — not a smart contract, not a framework. It's a developer utility that automates creating those frozen snapshots.

In plain English: it copies crates, renames them, and rewires their dependencies so the old and new versions don't clash.

The Four Files

The entire tool is just 4 Rust files. No framework, no blockchain logic — pure file manipulation:

How It Works: Step by Step

When the Sui team is ready to freeze a new execution version, they run something like:

Here's what happens under the hood:

If anything goes wrong at any step, the tool rolls back — deletes the newly created directories and leaves the repo clean.

The Wrapper Script

In practice, devs don't call the cut binary directly. There's a Python script (scripts/execution_layer.py) that wraps it and handles extra housekeeping:

Why This Matters

Without this system, Sui would face an impossible choice on every upgrade:

Option A: Update the VM and break replay of historical transactions → consensus fork.

Option B: Never update the VM → no improvements, no bug fixes, no new features.

The cut tool gives them Option C: freeze the old version, ship the new one, and let them run side by side. This is how Sui went from v0 all the way to the Bella Ciao VM rewrite without a single consensus-breaking moment.