CS2 Net-Snap Fix (Apple Silicon / Rosetta / CrossOver)

👉 Recommended: use alien-agent/cs2-macos-patcher instead

That project fixes all the known macOS/CrossOver problems in Cities: Skylines II — not just this one. Alongside the elevated-network snap bug (which it fixes with the same approach as this repo, and credits this repo for finding the root cause), it also fixes Paradox Mods downloads, mod loading/installation, the pause-menu crash and spurious error dialogs. If you just want a fully working game on Apple Silicon, use theirs — it's a superset of this fix.

This repository stays up as the focused, standalone fix for the snap bug only, and as the place where the root cause was originally tracked down.

Fixes the bug where elevated networks (bridges, power lines, water pipes) snap down onto the structure below them in Cities: Skylines II — because the vertical (Y / height) check is ignored (it behaves purely 2D). Affects CS2 running on Apple Silicon Macs via CrossOver/Rosetta 2.

This patch keeps the game's performance: it only disables Burst for the small set of network systems while the Net tool is active, so the heavy simulation keeps running on Burst.

The bug

CS2 ships an x86-64 Unity Burst library (lib_burst_generated.dll) with SIMD (SSE) code for its ECS jobs. On Apple Silicon the game runs through Rosetta 2, which mistranslates one SIMD routine in the network height/geometry check and drops the Y lane. Result: an elevated network is treated as being at the same height as whatever is below it, so it connects/snaps down to the ground structure regardless of the height difference (e.g. a bridge 50 m up still snaps to a road below).

• The exact same binary is correct on native Windows.
• The game's managed C# path (Mono) computes it correctly.

A known sledgehammer workaround is to rename lib_burst_generated.dll so the whole game falls back to the managed path — this fixes the bug but costs ~50% FPS permanently (the entire simulation then runs without Burst).

What this fix does

Instead of disabling Burst permanently, it patches the managed assembly Game.dll so that — only while the Net tool is active — just the network systems run managed, while everything else (traffic, pathfinding, citizens, rendering, the Unity engine systems) keeps running on Burst.

Technically:

• A static flag NetToolSystem.s_ForceManaged is set to true in OnStartRunning and back to false in a newly added OnStopRunning — it simply marks "the Net tool is active".
• Each targeted network system gets its OnUpdate wrapped so that, while the flag is set, it turns BurstCompiler.Options.EnableBurstCompilation off for the duration of that update, schedules its jobs as usual (in parallel), then forces them to run via JobHandle.Complete() while Burst is still off, and finally restores the previous flag state. By default only the three systems that actually compute the snap/height math are wrapped — NetToolSystem, CourseSplitSystem and ValidationSystem (the confirmed minimal set; NetToolSystem alone is not enough in-game). Wrapping just these three, rather than every Game.Net/Game.Tools/Game.Objects system, avoids dozens of forced main-thread completes per frame while building. Set CS2_NETSNAP_EXTRA="Type,..." to add systems, or CS2_NETSNAP_FULL=1 to wrap them all.

That last step is the key. EnableBurstCompilation only affects a job when it actually executes, and network jobs execute asynchronously — after the update returns. Just toggling the flag around the update (an earlier attempt) didn't work: the jobs ran later, back on Burst, and the bug returned. And forcing each job to run synchronously/single-threaded crashes parallel jobs. Completing them inside the finally runs them right then, parallel-managed — correct, and crash-free — before Burst is switched back on. Tool systems are completed via their returned JobHandle; other systems via this.Dependency.

So while the Net tool is open, only the network jobs run managed (that's where the faulty height logic lives → bug fixed); simulation, rendering and the Unity engine systems stay on Burst → FPS stays good. When the tool is not active it's a no-op → zero performance impact. lib_burst_generated.dll stays enabled.

A simpler smart fallback (./patch.sh smart) is also available: it just flips the global Burst flag while the tool is selected. It fixes the bug too, but makes the whole game run managed while the tool is open (~50% FPS), so use it only if the default misbehaves on your setup.

The actual mistranslated SSE instruction was not isolated (not needed for the fix — the managed path is provably correct). The culprit is a SIMD routine in the network height/geometry check that drops the Y lane, reached from one of the parallel network-build jobs.

Requirements

• macOS on Apple Silicon, CS2 via CrossOver (Mono scripting backend — the default for CS2).
• .NET 9 SDK (dotnet) and Mono (monodis) to build/run the patcher:
  • brew install --cask dotnet-sdk
  • brew install mono

No copyrighted game files are distributed in this repo. You build the patch from your own Game.dll. The patcher only rewrites a few method bodies; it does not redistribute Colossal Order code.

Install

git clone https://github.com/icetear/cs2-net-snap-fix.git
cd cs2-net-snap-fix
./patch.sh        # builds Game.dll.patched from your original Game.dll
./install.sh      # close the game first; backs up Game.dll -> Game.dll.orig, installs the patch

Launch CS2, pick a network, raise its elevation, and drag the end over an existing ground road — it should no longer snap down.

If your game is in a non-standard location, set CS2_DATA:

CS2_DATA="/path/to/Cities Skylines II/Cities2_Data" ./patch.sh
CS2_DATA="/path/to/Cities Skylines II/Cities2_Data" ./install.sh

Uninstall

./uninstall.sh    # restores the original Game.dll

After a game update

Steam overwrites Game.dll on updates and on "Verify integrity of game files". Just re-apply:

./patch.sh        # re-patch the fresh original
./install.sh      # refreshes the Game.dll.orig backup to the new original

patch.sh/install.sh auto-detect — via an IL marker in the assembly, not a byte compare — whether the installed Game.dll is a fresh original or already our patch, and pull the source/backup accordingly. This is robust even when you switch between the default and smart modes, and a stale or corrupt Game.dll.orig can no longer poison the build (a backup that is itself a patch is refused). install.sh also refuses to install a patch built from a different game version (it records the source hash in Game.dll.patched.srchash). By default patch.sh targets the fixed minimal system list, which stays valid across versions as long as those three types still exist; if a future version moves the snap math elsewhere, use CS2_NETSNAP_EXTRA=... or CS2_NETSNAP_FULL=1 (which re-derives the full list from your current Game.dll).

How it's built (for the curious)

patcher/ is a small Mono.Cecil tool (.NET 9). By default patch.sh passes the three snap systems (NetToolSystem, CourseSplitSystem, ValidationSystem) — extendable via CS2_NETSNAP_EXTRA, or CS2_NETSNAP_FULL=1 to enumerate every Game.Net/Game.Tools/Game.Objects system with monodis as before — and asks the patcher's gated mode to: add the s_ForceManaged flag plus the OnStartRunning/OnStopRunning toggle on NetToolSystem, and wrap each network system's job-scheduling OnUpdate with the flag-gated "disable Burst → schedule → complete-while-off → restore" logic described above (tool systems via their returned JobHandle, others via this.Dependency). Systems whose OnUpdate has its own try/catch are skipped (a nested try/finally would corrupt the IL). The smart fallback instead re-enables Burst on every non-network system.

Caveats

• Re-apply after every game update / Steam file verification.
• Mod compatibility: only a runtime flag is toggled around system updates (no game logic changed), so risk is low; untested with mods that touch the same systems or BurstCompiler.Options.
• Long-term stability testing is still ongoing.
• Use at your own risk. This modifies a game file; always keep the Game.dll.orig backup.

License

MIT (see LICENSE) — applies to the tooling in this repo only, not to any Colossal Order game files.