Proton, FEX and Lepton: How Steam Frame Runs Your Games

Steam Frame’s standalone games run through three layers, because its chip is not the kind PC games are built for. Valve’s documentation says the headset uses an Arm64 Snapdragon 8 Gen 3, and runs Windows games through Proton and x86 code through FEX. Android games run in a container called Lepton.

The Verified badge on the headset covers less than it may seem to. Valve says Steam Frame Standalone Verified covers only games running on the headset, not games streamed from a PC, which Valve calls the Frame’s primary use.

Three layers, and what each one does

Layer	What it handles	Valve’s description
Proton	Windows games	Wine plus translation of DirectX to Vulkan
FEX	x86 code, 32-bit and 64-bit	Translates x86 instructions to Arm64
Lepton	Android games	A container, to keep overhead down

A Windows game on the Frame goes through two of them. Proton handles the fact that it was written for Windows; FEX handles the fact that it was compiled for Intel and AMD chips rather than Arm. FEX sends graphics calls straight to the headset’s own Vulkan and OpenGL libraries rather than emulating them, which is where Valve says it saves overhead.

Lepton exists for a different case. Valve says it makes sense for VR developers who already built an Android Arm64 version of their game for other headsets to ship that version on the Frame. Launching any Android game from Steam starts a Lepton container; Valve says no Lepton container runs by default.

What Valve concedes

Valve does not claim translation is free. It says FEX uses code caching to minimise in-game stuttering “as much as possible”, which is not the same as removing it. Stutter from work done the first time it is needed is familiar on PCs too, where the usual cause is a game building shaders mid-play. Valve publishes no figure for how much speed FEX costs. It also calls the Frame primarily designed for streaming games from a PC, with standalone play as the extra.

What the Frame’s Verified badge promises

	Steam Deck	Steam Machine	Steam Frame, standalone
Minimum	30fps at 800p	30fps at 1080p	2D: 30fps at 1280x720; VR: 72fps at 1728x1728
Covers streamed games	Not applicable	Not applicable	No
Link to other devices	Independent of Machine	Deck Verified guarantees Machine Verified	Its own review

The badges are the same four as on the Deck: Verified, Playable, Unsupported and Unknown, which we set out in what the Steam Deck badges promise. A VR game rendering below 1440x1440 is marked Unsupported on the Frame outright. Valve’s Frame review names Proton problems as a cause of Unsupported; it does not mention FEX or Lepton failures, so how those are treated is not something Valve’s page says. A rating does not stop a game being sold for the Frame, Valve says; it changes how it is presented.

The Deck and Machine do not need any of this

Valve’s Deck and Machine documentation describes Proton only. Valve describes the Machine’s processor as a semi-custom AMD desktop-class CPU, and says the Machine runs the same software as the Deck. So FEX and Lepton appear only in Valve’s Frame documents. That is our reading of what Valve documents, not a statement Valve makes, but nothing in the Deck or Machine pages mentions either. If you are choosing between them, the other two are covered in the Steam Machine’s price and waitlist.

How much of the library uses Android

Valve publishes no count. VR.org’s analysis of Steam metadata on 11 September found 52 of 130 Frame-verified games pointing at the Lepton runtime; Ars Technica cited it as 52 of the first 120. Either way, a large share of what is verified to run on the headset today is an Android build rather than the PC version you may already own.

If you are still waiting on a Frame, the queue works the way we set out in Steam Frame is sold by lottery.