Vulkan

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Vulkan is a low-overhead, cross-platform API, open standard for 3D graphics and computing developed by Khronos Group, a successor to OpenGL.

Source exclusively uses Direct3D for rendering and does not support Vulkan. However, as of recent updates, some games include a built-in copy of DXVK, a library used to translate Direct3D calls to Vulkan. This is disabled by default, but can be enabled in supported games by passing -vulkan into the game's launch options.

Source 2 has implemented a native Vulkan renderer, used by default on Linux; it can also be enabled on Windows by passing -vulkan into the game's launch options. Launching the game with Workshop Tools (confirmed on Counter-Strike 2), will always enable Vulkan.

DXVK notes

Vulkan requires NVIDIA Kepler-based cards (GeForce 600 series) or AMD GCN 1.0 or later.
- Note:NVIDIA Kepler-based and AMD GCN 1.0 - 3.0 cards only support Vulkan 1.2, so some Source/Source 2 games that has newer version of DXVK, which uses Vulkan 1.3 may not work for these cards. Previous version of DXVK (1.10.3) are the last version to support Kepler-based and GCN 1.0 - 3.0 card.
- Vulkan 1.3 is supported on NVIDIA Maxwell-based cards (GeForce 900 series, aswell as 745, 750 and 750Ti) and AMD GCN 4.0 cards.
During first launch, the game may stutter. But after waiting for DXVK to fully cached, there would be less stuttering.
On Windows, Direct3D is supported natively, so in most cases there is no reason to enable DXVK and doing so may do more harm than good. It should only be used as a last resort to work around graphics driver bugs, or to slightly improve performance on systems which is CPU-limited or games that are poorly optimized. However, in Source 2, Vulkan will be always enabled when launching Workshop Tools (confirmed on Counter-Strike 2).
- Portal 2 in particular is known for having issues with projected textures on various graphics drivers, especially with newer version of graphics drivers: AMD drivers will fail to render all shadows cast by them, while Nvidia drivers will occasionally display static effect which appear on certain materials being hit by one. Enabling DXVK circumvents these issues.
- Using DXVK on Windows may break borderless window mode, causing it to behave identically to exclusive fullscreen. This "feature" appears to be a part of the graphics drivers themselves and not fixable by the game.
- Using DXVK while having DirectX level (mat_dxlevel) set lower than 90, such as 80 or 81, will cause graphical glitches and the game may crash.
- Importantly, for people with epilepsy, using DXVK while building cubemaps will also causes the screen to flash black and white repeatedly instead of showing the captured cubemap image.
On Linux, where Direct3D is not supported, the Source Engine defaults to using Valve's in-house library ToGL to translate Direct3D calls to OpenGL. Using DXVK on Linux is usually recommended if the game supports it, as ToGL is known for having somewhat poor performance and causing some graphical issues.
On macOS, DXVK is not supported, due to Vulkan itself not being supported by Apple. Source Engine games on Mac include ToGL as their only rendering mode.
In all games by Valve that support the -gamepadui launch argument, using it will also enable DXVK automatically, including on Windows. Use -dx9 if you don't want this.

Games that support Vulkan

Source (DXVK)

Half-Life 2 (added since 2022 Steam Deck update)
Portal (added since 2022 Steam Deck update)
- Portal with RTX (Always used as part of the RTX Remix runtime, then converted to DX12; Source's copy of DXVK is also included, but attempting to enable it will break rendering.)
Left 4 Dead 2 (since the June 15, 2021 update)
Portal 2 (added in the February 2021 update)
Counter-Strike: Global Offensive (Linux only)
Strata Source (Linux only, enabled by default)
Black Mesa ( Xengine), (added since Necro Patch/April 16, 2024 update.)
Bug:Lens flares have hard edges.
Bug:Anti-aliasing have issues.
Note:Temporarily unavailable and disabled with the Necro Update hotfix released in April 20, 2024.
Team Fortress 2 (added in April 19, 2024, with the release of 64-bit update)