NotProton is a new open-source app that brings Valve’s Steam Play to the macOS Steam client. It lets Mac users launch Windows games straight from their Steam library, using a feature Steam for Mac already contains but never switches on.
According to the project’s GitHub page, NotProton forces Steam Play on and ports parts of Valve’s Proton to macOS. Windows games then run through CrossOver. The developer publishes under the NotProtonNot account and says the documentation is still thin.
Mac gamers have a growing list of ways to run Windows titles, from GameToMac to hand-tuned setups like the one used for Palworld on Mac with DirectX and mods. NotProton takes a different route by working inside the Steam client itself.
NotProton switches on Steam Play inside the macOS Steam client
Steam Play is the feature that lets the Linux Steam client run Windows games through Proton. The macOS Steam client carries the same functionality, but it sits inactive. NotProton enables it by force, then adds the missing pieces so it can work on a Mac.
The actual game launching is handled by CrossOver. The app creates or places a copy of CrossOver in the ~/Library/Application Support/notproton/runners/ folder and patches it so Valve’s Steam client library gets loaded.
The README lists the exact builds NotProton is meant to work with:
| Component | Supported versions |
|---|---|
| Steam client | 1788652215 or 1790121765 |
| CrossOver | CrossOver 26.3 |
| CrossOver Preview | 20261006 or 2026082 |
| Builds | FEX and Rosetta |
Both the FEX and Rosetta builds are supported. Rosetta is Apple’s translation layer for x86 code, while FEX is an open-source x86 emulator. The developer recommends the FEX version of CrossOver Preview 20261006, because that preview includes both the FEX and Rosetta versions.
The project is split into five parts that port or patch Valve’s Proton pieces
The repository is organized by function, each folder covers one part of getting Steam Play to run on macOS:
- app is the macOS app itself.
- dylib holds the core logic.
- lsteamclient is a macOS port of Valve’s lsteamclient.
- steam-shim is a port of the steam-helper from Proton 9.
- ntdll-patch patches the CrossOver copy so lsteamclient loads.
The licensing is a mix, and the NOTICE file spells it out. Most of the project is GPL-3.0, while the parts based on Valve code keep the license Valve released them under:
| Part | License |
|---|---|
| Bulk of NotProton | GPL-3.0 |
| lsteamclient | Steamworks SDK License |
| steam-shim | BSD-3-Clause (ported from Proton 9) |
| Sparkle updater | MIT |
The documentation is sparse and the developer calls it early
The README says the release arrived several days later than planned, so the documentation is quite sparse. The developer promises to improve it, asks for issue reports on GitHub, and welcomes pull requests. A contributions policy is listed as coming soon.
The README does not include a game compatibility list, so there is no official word yet on which Steam titles run well. This is also an unofficial tool that patches a copy of CrossOver and force-enables a Steam feature, which is worth keeping in mind before pointing it at your main Steam library.
Anyone planning to try it should check the Steam client number first. If your build does not match 1788652215 or 1790121765, the project is not designed for it. For background on the Mac Steam client, see our earlier post on Steam for macOS getting hardware acceleration.
The README credits three earlier Mac Steam Play projects
The developer thanks people who worked on similar ideas before NotProton. Nat Brown made Kaon, which has similar goals. The Neutron project from the GitHub user mont127 takes a different approach. Gio was working on enabling Steam Play inside Steam on macOS before NotProton was released. The developer says NotProton was not based on any of their work.
Our Take
I think the surprising part is how little Valve would need to do to enable this officially on Mac in their Steam app. The macOS Steam client already contains Steam Play code, and one developer has now shown it can be woken up with some ported Proton components and a patched CrossOver.
Apple Silicon Macs have the horsepower for this. We saw it when we covered the base M6 Mac mini tested with seven AAA games, where most hit 60 FPS. What has been missing is convenient access to a Windows library, and a Steam-native approach would cover that.
For now, I would treat NotProton as a project for tinkerers. With no compatibility list and sparse docs, the first real test will be how many games people report working once more users try it.
