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Steam Linux Runtime 4 initially stopped some .NET games from starting because it omitted the ICU libraries required by CoreCLR. Valve promoted build 4.0.20261021.265997, including ICU v76, to stable on October 22, 2026, and closed the tracking issue. The original “fix has no date” wording described the situation reported on October 19; it is no longer current. Updating Steam Linux Runtime 4 is now the primary remedy, although a title can still have separate compatibility problems.
What broke in Steam Linux Runtime 4?
The first Runtime 4 builds removed libicuuc, libicui18n and libicudata. CoreCLR, the .NET runtime used by .NET 5 and later, requires ICU for normal globalization functions unless an application was built in invariant globalization mode. Without those libraries, affected games stopped during startup with an error telling the player to install libicu or icu-libs. The same game build could run under Runtime 3, which still supplied ICU. Valve’s issue tracker documents the missing-library failure and its resolution at issue #860.
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Steam Linux Runtime is a pressure-vessel container paired with a library-providing runtime. Runtime 3, known as “sniper,” is used by Proton 8.0 through 10.0, while Runtime 4 is the environment for Proton 11 and newer, according to Valve’s runtime documentation.
Simon McVittie, a Steam Runtime contributor, described the design goal as: “The general design principle is that each new runtime is as small as we can reasonably make it.” Removing ICU reduced the base runtime, but it also exposed applications that assumed those libraries would always be present.
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Which games and projects were documented as affected?
The issue tracker lists several concrete examples. They show the range of software touched by the omission, not a complete inventory of every .NET title that could fail.
Resonite
Resonite’s bootstrapper uses the Linux runtime for its core while Proton handles its renderer. The missing ICU libraries prevented that combination from using newer Proton builds tied to Runtime 4.
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The newer RimWorld engine runs on .NET 10. Its developers reported that, without libicu, the engine was effectively limited to Runtime 2 or Runtime 1, complicating use of newer glibc on older Linux distributions.
Unity 7.0 Alpha
A Unity 7.0 Alpha test executed inside Runtime 4 crashed with the missing-ICU message, providing a reproducible example outside a released game.
Battle for Wesnoth
The issue records that the ICU gap had been discussed with the Battle for Wesnoth project as an early Runtime 4 adopter.
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When did Valve release the fix?
The public timeline changed within days of the undated-fix report:
| Date | Event |
|---|---|
| October 14, 2026 | ValveSoftware issue #860 was opened. |
| October 16, 2026 | Simon McVittie explained the intentional runtime reduction and the compatibility trade-off. |
| October 19, 2026 | A report described a future ICU inclusion without a published version, build or release date. |
| October 22, 2026 | Runtime 4 build 4.0.20261021.265997, carrying ICU v76, was promoted to stable and issue #860 was closed as completed. |
Runtime 3 carried ICU 67. The Runtime 4 solution uses ICU 76, roughly four years newer, and the versions have an incompatible ABI. That is why simply copying a Runtime 3 library into the new container is not a reliable repair; the runtime must provide the matching library set, or the application must ship a compatible copy.
How do Runtime 3 and Runtime 4 compare?
| Runtime branch | Proton range | ICU availability | Current status |
|---|---|---|---|
| Runtime 3 (“sniper”) | Proton 8.0–10.0 | ICU 67 supplied by the runtime | Established branch for those Proton releases |
| Runtime 4 (“steamrt4”) | Proton 11 or newer | ICU v76 in stable build 4.0.20261021.265997 |
Stable after the October 22, 2026 promotion |
The branches are not interchangeable library pools. A Proton version selects its corresponding runtime generation, so changing Proton can change the container and the libraries visible to a game.
What should players do if a .NET game still fails?
- Update the runtime and Proton. Let Steam install the current stable Steam Linux Runtime 4 and use Proton 11 or a later release when the game supports it.
- Restart Steam completely. Valve’s known-issues page notes that a full Steam restart may be necessary after switching runtime branches.
- Try a Runtime 3-compatible Proton as a temporary workaround. If the title runs under an older Proton release, that environment may still provide ICU 67. This is a compatibility fallback, not a substitute for updating the affected game and runtime.
- Capture diagnostic details. Record the selected Proton version, runtime branch, Linux distribution, game build and the exact startup error.
- Enable logs when reporting the problem. Steam Runtime Diagnostics and
STEAM_LINUX_RUNTIME_LOG=1provide container details. For Proton games, addPROTON_LOG=1to the game’s launch options, as described in Valve’s bug-reporting guide.
Do not treat a registry edit, random DLL download, system-cleaning utility or hardware upgrade as a fix for this specific missing-library error. Those changes do not add ICU to the Steam Linux Runtime container.
What do developers targeting Runtime 4 need to change?
An application can avoid the dependency by deliberately using .NET’s invariant globalization mode, but that removes culture-aware data and behavior that many applications need. For applications that require full globalization, developers can bundle a compatible ICU build when licensing, size and maintenance requirements permit. The issue discussion gives ICU 76.1 from Debian 13 as an example of a suitable version.
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Developers should test the Linux bootstrapper or native component inside Runtime 4 rather than testing only the Proton-rendered portion. A game that launches its core in the container can fail before Proton has an opportunity to start the renderer.
Why was ICU removed in the first place?
Valve’s stated policy is to keep each new runtime as small as reasonably possible. ICU is a substantial, versioned dependency, and carrying it forward creates maintenance and ABI commitments. McVittie also cautioned in the issue that the policy decision belongs to Valve rather than to an individual contributor. The October 22 stable build shows that compatibility evidence led Valve to add ICU back to Runtime 4, while retaining the newer ICU 76 generation instead of copying Runtime 3’s ICU 67.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Frequently asked questions
What does ICU do in a .NET game?
ICU supplies internationalization data and algorithms used for culture-sensitive operations such as number and date formatting, text comparison and sorting. CoreCLR can run in invariant mode without it, but that mode intentionally provides limited, culture-independent behavior.
Does this affect every Linux game that uses Proton?
No. The documented failure concerns programs whose Linux-side .NET/CoreCLR component needs ICU. Native games, games using different runtimes and titles that bundle their own compatible ICU copy are not automatically affected.
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Can a developer ship ICU inside the game?
Yes, bundling a compatible copy is a possible design, subject to the project’s licensing and update obligations. Valve’s issue discussion cites ICU 76.1 from Debian 13 as an example rather than a universal packaging requirement.
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Where should a remaining Runtime 4 regression be reported?
Use Valve’s Steam Runtime bug-reporting instructions and include the runtime branch, Proton version, distribution, game build, exact error and diagnostic logs. Those details let maintainers distinguish a missing-ICU problem from an unrelated container or game regression.
Frequently Asked Questions
What does ICU do in a .NET game?
ICU supplies internationalization data and algorithms used for culture-sensitive operations such as number and date formatting, text comparison and sorting. CoreCLR can run in invariant mode without it, but that mode intentionally provides limited, culture-independent behavior.
Does this affect every Linux game that uses Proton?
No. The documented failure concerns programs whose Linux-side .NET/CoreCLR component needs ICU. Native games, games using different runtimes and titles that bundle their own compatible ICU copy are not automatically affected.
Can a developer ship ICU inside the game?
Yes, bundling a compatible copy is a possible design, subject to the project’s licensing and update obligations. Valve’s issue discussion cites ICU 76.1 from Debian 13 as an example rather than a universal packaging requirement.
Where should a remaining Runtime 4 regression be reported?
Use Valve’s Steam Runtime bug-reporting instructions and include the runtime branch, Proton version, distribution, game build, exact error and diagnostic logs. Those details let maintainers distinguish a missing-ICU problem from an unrelated container or game regression.
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