Swift 6.4 Ships as Stable Release, Making Swift Build the Default Package Manager Backend

Overview

Swift 6.4 is now generally available, according to a Swift.org blog post published September 15, 2026 by Joe Heck and Holly Borla. The release’s headline change is that Swift Build is now the default backend in Swift Package Manager, meaning, per the post, that “your projects build the same way on Linux, macOS, and Windows.” The GitHub repository for the Swift compiler carries the corresponding swift-6.4.0-RELEASE tag, according to swiftlang/swift’s release listing.

Joe Heck, who according to the post “works on Swift as part of the Open Source Program Office at Apple,” and Holly Borla, described in the post as “a member of the Swift Core Team and Language Steering Group, and the engineering manager of the Swift language team at Apple,” co-authored the announcement.

What We Know

Swift Package Manager “now uses Swift Build as its default build platform,” unifying the build process that previously diverged across platforms, according to the Swift.org post. The same release also adds Software Bill of Materials (SBOM) generation for Swift Package Manager under proposal SE-0509, “providing support for generating SBOM documents in either SPDX or CycloneDX format.”
The Subprocess library, originally introduced as proposal SF-0007 and released as an initial 0.1 version in 2025, “has reached 1.0,” providing what the post calls “a stable, cross-platform way to run and interact with other programs from Swift, from command-line tools to streaming processes.”
Interoperability was extended in two directions: Swift’s Span type now bridges directly with C++20’s std::span, letting developers “pass a Span to a C++ API that expects a std::span, and receive a std::span back as a Span, without writing manual conversion code at the boundary,” and the Swift/Java interop project — which lets developers call Swift from Java and Kotlin — “extends its support for calling async and throwing functions to protocol and callback wrappers, adds automatic Runnable mapping for closures, variadic parameter import, and support for Java record types.”
WebAssembly performance improved as well: JavaScriptKit, the library used to bridge Swift to JavaScript in the browser, now offers “safe bridging up to 40 times faster than earlier dynamic bridging,” and the Wasm SDK is “available from the Install Swift page of Swift.org, so compiling Swift for the browser requires no extra setup beyond adding the SDK,” per the release post.
On Android, the Swift SDK “is built with the new LTS NDK 30, which provides Android availability attributes both in the Swift runtime libraries and for your Swift packages using the default NDK,” and “Swift Build now supports Android in SwiftPM as well, removing the need for a post-install script,” according to the post.
Several language ergonomics changes landed: optional some and any types no longer require parentheses — “instead of (some Rocket)?, you can simply write some Rocket?” under proposal SE-0521 — and a new @diagnose attribute (SE-0522) lets developers “define the warning behavior directly in your code.” A new module-selector syntax using :: (SE-0491) lets developers disambiguate identically named types from different imported modules. Any asynchronous code written inside a defer block “is awaited and runs to completion before it exits” under a new proposal, SE-0493, and a new withTaskCancellationShield API (SE-0504) lets a closure run “shielded from the enclosing task’s cancellation.”
New non-copyable data structures target performance without sacrificing memory safety: UniqueBox (SE-0517) is described as giving “a smart pointer that uniquely owns a heap value, including non-copyable values, without reference counting,” while UniqueArray (SE-0527) “stores non-copyable elements without the copy-on-write allocations you would see when using Array.” A new Iterable protocol (SE-0516) lets developers “loop over elements and borrow them with the Iterable protocol, instead of copying each value.”
Embedded Swift, the compiler mode for microcontrollers, gains “support for existential types and richer error handling for microcontroller-class targets” in the 6.4 release, according to the post, which cross-references an earlier Embedded Swift post covering those improvements “in more depth.” The Embedded Swift additions to full existential-type support and untyped throws that were previously reported as available only in development snapshots are now part of this stable release.
Debugging also improved: Swift 6.4 “completes a multi-release overhaul of how the compiler tracks Swift modules in debug info,” with LLDB now importing modules “through precise dependency tracking instead of ambiguous by-name lookups,” and debug builds on Linux and Windows — along with dSYM bundles on Darwin — “shrink significantly since binary Swift modules are no longer embedded in them,” per the release post.
The VS Code extension for Swift is now available on the Open VSX Registry, meaning it “works not only in VS Code, but also Cursor, Antigravity, Kiro, and other development tools,” according to the post. Swift also gained a new documentation site, and “the documentation content for the standard library is now open source,” per the post.

What We Don’t Know

The release post does not give a specific date for when Swift 6.4 features that shipped only in development snapshots — such as some of the finer Embedded Swift diagnostics — will reach all supported platforms uniformly, nor does it break out adoption or download figures for the new Swift Build default. The post also does not quantify how much smaller debug builds become under the new module-tracking scheme beyond describing the reduction as significant.

Background

Swift 6.4’s feature set was previewed in stages. Async defer blocks and a borrowing Iterable protocol were among the features reported at WWDC 2026 in June, months ahead of the stable release. Embedded Swift’s expanded existential-type and untyped-throws support, covered in development snapshots in August, has now shipped as part of this general-availability release.