F# 13.9.201

Microsoft has released F# 13.9.201, the latest version of the functional-first programming language for the .NET ecosystem. The update ships as part of the .NET 9.0.2 servicing release and focuses on compiler fixes, thread safety improvements, and tooling enhancements.

What Changed

F# 13.9.201 is a maintenance release that addresses several issues reported by the community. The compiler team has focused on reliability and developer experience, with notable improvements to computation expression error reporting and F# Interactive session isolation.

Key Fixes

• Thread-safe type-checking caches — The frameworkTcImportsCache is now properly synchronized, eliminating a race condition that could cause intermittent compilation failures in multi-threaded environments such as IDEs and build servers
• Better computation expression errors — Error reporting for let!, use!, and do! in computation expressions now includes accurate source ranges, making it significantly easier to locate and fix issues
• FSI session isolation — The F# Interactive scripting system now uses a per-session temporary directory instead of a global one, preventing file conflicts when running multiple FSI instances
• Attribute usage fix — A regression that restricted certain attribute usage patterns in F# 9.0 has been resolved, restoring full attribute compatibility
• Test infrastructure migration — The team completed the migration from NUnit to xUnit across the entire test suite, improving CI reliability and modernizing the testing pipeline
• Extension method resolution — System.Private.CoreLib is now correctly recognized as a system library during extension method resolution, fixing edge cases in code that extends core .NET types

Upgrade Path

F# 13.9.201 ships with the .NET 9.0.2 SDK. To upgrade, install the latest .NET 9 SDK from dotnet.microsoft.com. Existing F# projects targeting .NET 9 will automatically use the updated compiler on next build — no manual package changes required.

For projects that explicitly reference F# compiler packages, update the version to 13.9.201 in your project file or directory.packages.props.

Looking Ahead

The F# team continues development on F# 14, which will ship with .NET 10. Preview builds are already available, bringing further improvements to type inference, code generation, and tooling integration.