Build-tool plugin executable on Windows uses ABI-incompatible symbols when built with WindowsExperimental.sdk

[tothambrus11]

A trivial executable target that consumes generated source from a .buildTool() plugin fails during compilation when launching the build plugin when using -static-stdlib SDKROOT=...WindowsExperimental.sdk.

CI matrix:

arch	link	result
x86_64	dynamic	✅
arm64	dynamic	✅
x86_64	static (-static-stdlib, SDKROOT=…WindowsExperimental.sdk)	❌
arm64	static (same)	❌

Minimal reproducer: https://github.com/tothambrus11/spm-plugin-windows-static

Failed run: https://github.com/tothambrus11/spm-plugin-windows-static/actions/runs/25639157545

Inaccurate (AI) interpretation

Summary

When SDKROOT points at WindowsExperimental.sdk, building any SwiftPM target that uses a .buildTool() plugin produces a plugin executable that fails to start with STATUS_DLL_NOT_FOUND (0xC0000135) because the Windows loader cannot resolve the PackagePlugin.dll it depends on. SwiftPM swallows the loader failure and surfaces only:

error: build planning stopped due to build-tool plugin failures

— the diagnostic-quality side of which is the subject of a companion issue.

Environment

• Toolchain: swift-DEVELOPMENT-SNAPSHOT-2026-04-30-a (Windows x86_64 and arm64), installed via compnerd/gha-setup-swift
• SDK: WindowsExperimental.sdk (via SDKROOT=…\Windows.platform\Developer\SDKs\WindowsExperimental.sdk)
• Build driver: default ("Swift Build") - EDIT: --build-system native doesn't fix this either.
• Reproduces with both swift build and swift build -Xswiftc -static-stdlib

Reproducer

Public minimal repository: https://github.com/tothambrus11/spm-plugin-windows-static

A trivial executable target consumes generated source from a .buildTool() plugin. CI matrix:

arch	link	result
x86_64	dynamic	✅
arm64	dynamic	✅
x86_64	static (-static-stdlib, SDKROOT=…WindowsExperimental.sdk)	❌
arm64	static (same)	❌

Failed run: https://github.com/tothambrus11/spm-plugin-windows-static/actions/runs/25639157545

Observed

The "Inspect plugin executable after failure" step in CI shows, on the produced .build\plugins\cache\BuildPlugin.exe:

ExitCode: -1073741515        ← 0xC0000135 STATUS_DLL_NOT_FOUND
--- stdout ---               (empty)
--- stderr ---               (empty)

llvm-readobj --coff-imports enumerates the imported DLLs and resolves each against the PATH the runner has (the same PATH SwiftPM uses to invoke the plugin):

[FOUND]   FoundationEssentials.dll  -> …\Swift\Runtimes\0.0.0\usr\bin\FoundationEssentials.dll
[FOUND]   KERNEL32.dll              -> C:\Windows\system32\kernel32.dll
[MISSING] PackagePlugin.dll                              ← root cause
[FOUND]   swiftCore.dll             -> …\Swift\Runtimes\0.0.0\usr\bin\swiftCore.dll
[FOUND]   swift_Concurrency.dll     -> …\Swift\Runtimes\0.0.0\usr\bin\swift_Concurrency.dll
[FOUND]   VCRUNTIME140.dll          -> …\vcruntime140.dll
[FOUND]   api-ms-win-crt-*-l1-1-0.dll  …

The toolchain does ship PackagePlugin.dll, but at usr\lib\swift\pm\PluginAPI\PackagePlugin.dll — a directory not on PATH and not in the loader's standard search.

Reasoning

1. The plugin executable's exit code is 0xC0000135 and both stdout and stderr are empty. By Windows semantics this can only originate from the loader, i.e. before any user main() or PackagePlugin runtime entry point runs. So the failure is not a plugin-protocol error or a SwiftPM/plugin handshake problem.
2. llvm-readobj enumerates the exact DLL imports of the produced binary, and Get-Command resolves each against the live PATH. Exactly one entry is missing — PackagePlugin.dll — and it is the only Swift runtime DLL the plugin imports that is not under …\Runtimes\0.0.0\usr\bin.
3. The matrix isolates the trigger: same package, same toolchain, same plugin target builds and runs successfully in the dynamic cells (no WindowsExperimental.sdk redirect) and fails in the static cells (with SDKROOT repointed). The only variable is the SDK.

Expected behavior

Either:

• (a) the build-tool plugin executable produced when targeting WindowsExperimental.sdk should not depend on a PackagePlugin.dll whose location is not in the loader search path (e.g. by linking PackagePlugin statically into plugin executables, or by emitting a manifest / rpath equivalent), or
• (b) SwiftPM should prepend the toolchain usr\lib\swift\pm\PluginAPI directory to the plugin process's PATH (or use AddDllDirectory / SetDefaultDllDirectories) before invoking the plugin.

(b) appears to be the smaller change and would also benefit any future SDK that ships a similarly placed PackagePlugin.lib import library.

[tothambrus11]

@compnerd

[compnerd]

Please don’t feed the raw output of AI tools as a report - that is not helpful to understand as it is excessively verbose with little substance. Without any additional evidence (i.e. deep analysis with code pointers), I’d say the only valuable bit here is: package plugins don’t work.

(a) the build-tool plugin executable produced when targeting WindowsExperimental.sdk should not depend on a PackagePlugin.dll whose location is not in the loader search path (e.g. by linking PackagePlugin statically into plugin executables, or by emitting a manifest / rpath equivalent), or

That is AI (and user - for not reading/understanding the output) failure. The experimental SDK is not static only, there is absolutely no reason to not depend on PackagePlugin.dll. In fact, it is impossible to not: SPM doesn’t support statically linking the manifest and plugins, so the dynamic linking is mandatory. I’d proposed enabling a statically linked manifest build but the idea was rejected. Unfortunately, I don’t remember the reasoning for the rejection.

(b) SwiftPM should prepend the toolchain usr\lib\swift\pm\PluginAPI directory to the plugin process's PATH (or use AddDllDirectory / SetDefaultDllDirectories) before invoking the plugin.

This again feels wrong. SPM already adjusts the path prior to invocation. Using AddDllDirectory would be an injection attack surface and would change the environment for the application which is an issue as it impacts user expectations. Furthermore, it requires modifying the user code. Likewise, SetDefaultDllDirextories is even worse. Not only does it have all the downsides of AddDllDirectory, it also will allow the user to override that and wipe it out potentially.

I suspect that you didn’t actually build the plugin in static mode, but dynamic mode - and the new SDK explicitly is designed to permit this. One SDK, but the user selects the linking mode. However, one of the things that isn’t being accounted for is that there is no ABI stability on Windows, and the two SDKs are incompatible. The runtimes are conflicting and the a dependency is failing to load.

[dschaefer2]

I'm pretty sure the plugin build doesn't take SDKROOT into account. It's not built with a build system. It's build just like the manifest is. Could be done though.

[tothambrus11]

Sorry for the AI report, I won't do that again.

However, one of the things that isn’t being accounted for is that there is no ABI stability on Windows, and the two SDKs are incompatible. The runtimes are conflicting and the a dependency is failing to load.

I installed yesterday's TheBrowserCompany build of the Swift development toolchain locally and confirmed that this is the case. When running BuildTool.exe manually, the initial issue is that PackagePlugin.dll is not found (exit with DLL_NOT_FOUND). If we adjust the Path like SPM before invoking BuildTool.exe, we get the ENTRYPOINT_NOT_FOUND error, indicating ABI incompatibility.

$env:PATH = "C:\Users\<USER>\AppData\Local\Programs\Swift\Toolchains\0.0.0+Asserts\usr\lib\swift\pm\PluginAPI;" + $env:PATH

E.g the experimental SDK imports $sScM6sharedScMvau (MainActor.shared.unsafeMutableAddressor) while the regular SDK imports $sScM6sharedScMvgZ (static MainActor.shared.getter).

I'm pretty sure the plugin build doesn't take SDKROOT into account. It's not built with a build system. It's build just like the manifest is. Could be done though.

This doesn't seem to be the case according to BuildPlugin-state.json, which has a frontend invocation with -sdk flag that's directly correlated to the value of SDKROOT before running SPM.

BuildPlugin-state.json

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    "-lPackagePlugin",
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    "C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\SDKs\\WindowsExperimental.sdk",
    "-libc",
    "MD",
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    "-L",
    "C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\Library\\XCTest-0.0.0\\usr\\lib\\swift\\windows\\x86_64",
    "-I",
    "C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\Library\\Testing-0.0.0\\usr\\lib\\swift\\windows",
    "-L",
    "C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\Library\\Testing-0.0.0\\usr\\lib\\swift\\windows\\x86_64",
    "-use-ld=lld",
    "-g",
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    "6",
    "-package-description-version",
    "6.0.0",
    "-I",
    "C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\lib\\swift\\pm\\PluginAPI",
    "-parse-as-library",
    "-j20",
    "-Xfrontend",
    "-serialize-diagnostics-path",
    "-Xfrontend",
    "C:\\Users\\ambru\\spm-plugin-windows-static\\.build\\plugins\\cache\\BuildPlugin.dia",
    "C:\\Users\\ambru\\spm-plugin-windows-static\\Plugins\\BuildPlugin\\Plugin.swift",
    "-o",
    "C:\\Users\\ambru\\spm-plugin-windows-static\\.build\\plugins\\cache\\BuildPlugin.exe",
    "-v"
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    "ProgramData" : "C:\\ProgramData",
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-I C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\lib\\swift\\pm\\PluginAPI -no-color-diagnostics -Xcc -fno-color-diagnostics -g -debug-info-format=dwarf -dwarf-version=4 -swift-version 6 -package-description-version 6.0.0 -serialize-diagnostics-path C:\\Users\\ambru\\spm-plugin-windows-static\\.build\\plugins\\cache\\BuildPlugin.dia -empty-abi-descriptor -enable-anonymous-context-mangled-names -file-compilation-dir C:\\Users\\ambru\\spm-plugin-windows-static -no-auto-bridging-header-chaining -module-name BuildPlugin -in-process-plugin-server-path C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\bin\\SwiftInProcPluginServer.dll -plugin-path C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\bin -plugin-path C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\local\\bin -emit-module-doc-path C:\\Users\\ambru\\AppData\\Local\\Temp\\TemporaryDirectory.blvnv5\\BuildPlugin-1.swiftdoc -emit-module-source-info-path C:\\Users\\ambru\\AppData\\Local\\Temp\\TemporaryDirectory.blvnv5\\BuildPlugin-1.swiftsourceinfo -parse-as-library -o C:\\Users\\ambru\\AppData\\Local\\Temp\\TemporaryDirectory.blvnv5\\BuildPlugin-1.swiftmodule\r\nC:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\bin\\swift-frontend.exe -modulewrap C:\\Users\\ambru\\AppData\\Local\\Temp\\TemporaryDirectory.blvnv5\\BuildPlugin-1.swiftmodule -target x86_64-unknown-windows-msvc -o C:\\Users\\ambru\\AppData\\Local\\Temp\\TemporaryDirectory.blvnv5\\BuildPlugin-1.o\r\nC:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\bin\\clang.exe --rsp-quoting=windows -target x86_64-unknown-windows-msvc -fuse-ld=lld -nostartfiles -L C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\lib\\swift\\windows -L C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\lib\\swift\\windows\\x86_64 -L C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\SDKs\\WindowsExperimental.sdk\\usr\\lib\\swift\\windows -L C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\SDKs\\WindowsExperimental.sdk\\usr\\lib\\swift\\windows\\x86_64 C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\SDKs\\WindowsExperimental.sdk\\usr\\lib\\swift\\windows\\x86_64\\swiftrt.obj C:\\Users\\ambru\\AppData\\Local\\Temp\\TemporaryDirectory.blvnv5\\BuildPlugin-1.o C:\\Users\\ambru\\AppData\\Local\\Temp\\TemporaryDirectory.blvnv5\\Plugin-1.o -L C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\lib\\swift\\pm\\PluginAPI -L C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\Library\\XCTest-0.0.0\\usr\\lib\\swift\\windows\\x86_64 -L C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\Library\\Testing-0.0.0\\usr\\lib\\swift\\windows\\x86_64 -I C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Platforms\\0.0.0\\Windows.platform\\Developer\\SDKs\\WindowsExperimental.sdk -v -o C:\\Users\\ambru\\spm-plugin-windows-static\\.build\\plugins\\cache\\BuildPlugin.exe -lPackagePlugin\r\nSwift version 6.4-dev (LLVM 446ed12475b4e6a, Swift 66a1326cdb60036)\r\nTarget: x86_64-unknown-windows-msvc\r\ncompnerd.org clang version 21.1.6\r\r\nTarget: x86_64-unknown-windows-msvc\r\r\nThread model: posix\r\r\nInstalledDir: C:\\Users\\ambru\\AppData\\Local\\Programs\\Swift\\Toolchains\\0.0.0+Asserts\\usr\\bin\r\r\nBuild config: +assertions\r\r\n \"C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Toolchains\\\\0.0.0+Asserts\\\\usr\\\\bin\\\\lld-link\" \"-out:C:\\\\Users\\\\ambru\\\\spm-plugin-windows-static\\\\.build\\\\plugins\\\\cache\\\\BuildPlugin.exe\" \"-libpath:C:\\\\Program Files\\\\Microsoft Visual Studio\\\\2022\\\\Community\\\\VC\\\\Tools\\\\MSVC\\\\14.44.35207\\\\lib\\\\x64\" \"-libpath:C:\\\\Program Files\\\\Microsoft Visual Studio\\\\2022\\\\Community\\\\VC\\\\Tools\\\\MSVC\\\\14.44.35207\\\\atlmfc\\\\lib\\\\x64\" \"-libpath:C:\\\\Program Files (x86)\\\\Windows Kits\\\\10\\\\Lib\\\\10.0.22621.0\\\\ucrt\\\\x64\" \"-libpath:C:\\\\Program Files (x86)\\\\Windows Kits\\\\10\\\\Lib\\\\10.0.22621.0\\\\um\\\\x64\" \"-libpath:C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Toolchains\\\\0.0.0+Asserts\\\\usr\\\\lib\\\\swift\\\\windows\" \"-libpath:C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Toolchains\\\\0.0.0+Asserts\\\\usr\\\\lib\\\\swift\\\\windows\\\\x86_64\" \"-libpath:C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Platforms\\\\0.0.0\\\\Windows.platform\\\\Developer\\\\SDKs\\\\WindowsExperimental.sdk\\\\usr\\\\lib\\\\swift\\\\windows\" \"-libpath:C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Platforms\\\\0.0.0\\\\Windows.platform\\\\Developer\\\\SDKs\\\\WindowsExperimental.sdk\\\\usr\\\\lib\\\\swift\\\\windows\\\\x86_64\" \"-libpath:C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Toolchains\\\\0.0.0+Asserts\\\\usr\\\\lib\\\\swift\\\\pm\\\\PluginAPI\" \"-libpath:C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Platforms\\\\0.0.0\\\\Windows.platform\\\\Developer\\\\Library\\\\XCTest-0.0.0\\\\usr\\\\lib\\\\swift\\\\windows\\\\x86_64\" \"-libpath:C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Platforms\\\\0.0.0\\\\Windows.platform\\\\Developer\\\\Library\\\\Testing-0.0.0\\\\usr\\\\lib\\\\swift\\\\windows\\\\x86_64\" -nologo \"C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Programs\\\\Swift\\\\Platforms\\\\0.0.0\\\\Windows.platform\\\\Developer\\\\SDKs\\\\WindowsExperimental.sdk\\\\usr\\\\lib\\\\swift\\\\windows\\\\x86_64\\\\swiftrt.obj\" \"C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Temp\\\\TemporaryDirectory.blvnv5\\\\BuildPlugin-1.o\" \"C:\\\\Users\\\\ambru\\\\AppData\\\\Local\\\\Temp\\\\TemporaryDirectory.blvnv5\\\\Plugin-1.o\" PackagePlugin.lib\r\r\n",
  "result" : {
    "exit" : {
      "code" : 0
    }
  }
}

My SPM invocation:

$env:SDKROOT="C:\Users\ambru\AppData\Local\Programs\Swift\Platforms\0.0.0\Windows.platform\Developer\SDKs\WindowsExperimental.sdk"
swift build -c release --very-verbose --triple x86_64-unknown-windows-msvc  -Xswiftc -static-stdlib -debug-info-format none