diff --git a/.github/workflows/checks.yml b/.github/workflows/checks.yml
index 5c234b4..a7b9e86 100644
--- a/.github/workflows/checks.yml
+++ b/.github/workflows/checks.yml
@@ -3,17 +3,20 @@ name: Checks
 on:
   push:
     branches: [main]
+  pull_request:
+    types: [labeled]
   workflow_dispatch:
 
 permissions:
   contents: read
 
 concurrency:
-  group: ci-${{ github.workflow }}-${{ github.ref }}
+  group: ci-${{ github.workflow }}-${{ github.ref }}-${{ github.event.label.name }}
   cancel-in-progress: true
 
 jobs:
   tests:
+    if: github.event_name != 'pull_request' || github.event.label.name == 'run-ci'
     name: ${{ matrix.os }} / ${{ matrix.configuration }}
     runs-on: ${{ matrix.os }}
     container: ${{ matrix.container }}
@@ -80,3 +83,43 @@ jobs:
           path: Enigmatic.doccarchive.tar.gz
           compression-level: 0
           if-no-files-found: error
+
+  windows:
+    if: github.event_name != 'pull_request' || github.event.label.name == 'run-ci'
+    name: windows-2022 / ${{ matrix.configuration }}
+    runs-on: windows-2022
+    strategy:
+      fail-fast: false
+      matrix:
+        configuration: [debug, release]
+    defaults:
+      run:
+        shell: pwsh
+    steps:
+      - uses: actions/checkout@v6
+      - name: Install Swift 6.4.1
+        uses: compnerd/gha-setup-swift@86d1ddeaf915c282137601aeb0654ea0e7b7f17e
+        with:
+          swift-version: swift-6.4.1-release
+          swift-build: 6.4.1-RELEASE
+      - name: Verify Swift 6.4.1 for Windows x64
+        run: |
+          $version = (swift --version) -join "`n"
+          if ($LASTEXITCODE -ne 0) { exit $LASTEXITCODE }
+          Write-Output $version
+          if ($version -notmatch 'Swift version 6\.4\.1(\s|$)') {
+            throw 'Expected Swift 6.4.1'
+          }
+          if ($version -notmatch 'Target: x86_64-unknown-windows-msvc') {
+            throw 'Expected the Windows x64 toolchain'
+          }
+      - name: Functional tests
+        run: |
+          $vswhere = Join-Path ${env:ProgramFiles(x86)} 'Microsoft Visual Studio\Installer\vswhere.exe'
+          $vsPath = & $vswhere -latest -products '*' -requires Microsoft.VisualStudio.Component.VC.Tools.x86.x64 -property installationPath
+          if (-not $vsPath) { throw 'Visual Studio C++ tools not found' }
+          Import-Module (Join-Path $vsPath 'Common7\Tools\Microsoft.VisualStudio.DevShell.dll')
+          Enter-VsDevShell -VsInstallPath $vsPath -SkipAutomaticLocation -DevCmdArguments '-arch=x64 -host_arch=x64'
+          # Work around Swift 6.4.1's build-system and debug-type assertion issues.
+          swift test -c ${{ matrix.configuration }} --build-system native -Xswiftc -Xfrontend -Xswiftc -disable-round-trip-debug-types --skip PerformanceTests
+          exit $LASTEXITCODE
diff --git a/README.md b/README.md
index 4c68668..eb46e7c 100644
--- a/README.md
+++ b/README.md
@@ -7,7 +7,7 @@
 [![License](https://img.shields.io/github/license/SwiftUtility/Enigmatic)](LICENSE)
 [![Coverage](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftutility.github.io%2FEnigmatic%2Fcoverage-badge.json)](https://github.com/SwiftUtility/Enigmatic/actions/workflows/release.yml)
 [![Documentation](https://img.shields.io/badge/Documentation-DocC-blue)](https://swiftutility.github.io/Enigmatic/documentation/enigmatic/)
-[![Tested platforms](https://img.shields.io/badge/tested_on-macOS%20%7C%20Linux-blue)](https://swiftutility.github.io/Enigmatic/documentation/enigmatic/development/)
+[![Tested platforms](https://img.shields.io/badge/tested_on-macOS%20%7C%20Linux%20%7C%20Windows-blue)](https://swiftutility.github.io/Enigmatic/documentation/enigmatic/development/)
 
 Turn `Codable` values into a tree you can inspect, edit, merge, and decode again.
 Use `@Codec.Box` to choose how individual properties are encoded.
diff --git a/Sources/Enigmatic/Enigmatic.docc/Development.md b/Sources/Enigmatic/Enigmatic.docc/Development.md
index 9538ca9..7b3c70f 100644
--- a/Sources/Enigmatic/Enigmatic.docc/Development.md
+++ b/Sources/Enigmatic/Enigmatic.docc/Development.md
@@ -7,13 +7,43 @@ swift test --skip PerformanceTests
 swift test -c release --skip PerformanceTests
 ```
 
-CI uses Swift 6.3.3 from Xcode 26.6 on macOS 26 and the `swift:6.3.3`
-container on Ubuntu 24.04. Each job verifies the compiler version before testing.
+On Windows, use Developer PowerShell for Visual Studio with the C++ tools and
+Windows SDK installed. Start a fresh shell after installing Swift so `PATH` and
+`SDKROOT` include the toolchain and its SDK.
+
+The functional suite passed locally in Debug and Release on Windows x64 with
+Swift 6.4.1 on 2026-10-16.
+
+Local Windows x64 validation with Swift 6.4.1 encountered two toolchain issues:
+the default build system failed to find `stdlib.h`, and the compiler's debug-type
+round-trip check asserted on negative integer generic arguments in the strategy
+tests. To reproduce the successful validation, use:
+
+```powershell
+$windowsSwiftFlags = @(
+  '--build-system', 'native',
+  '-Xswiftc', '-Xfrontend', '-Xswiftc', '-disable-round-trip-debug-types'
+)
+swift test @windowsSwiftFlags --skip PerformanceTests
+swift test @windowsSwiftFlags -c release --skip PerformanceTests
+```
+
+These are toolchain workarounds, not package settings. The native build system is
+deprecated; recheck these workarounds when updating Swift. Windows validation
+does not replace the macOS and Linux CI jobs below.
+
+The macOS and Linux CI jobs use Swift 6.3.3 from Xcode 26.6 on macOS 26 and the
+`swift:6.3.3` container on Ubuntu 24.04. Each job verifies the compiler version
+before testing.
 The macOS Debug job also checks the coverage script, enforces 95% library line
 coverage, and builds DocC. It uploads the coverage report as `coverage.json` and
 the documentation as `Enigmatic.doccarchive.tar.gz`, preserving DocC filenames
 inside the archive.
 
+Checks also runs separate Windows x64 Debug and Release jobs on `windows-2022`
+with Swift 6.4.1, Visual Studio's C++ environment, and the workarounds above.
+These jobs do not change the macOS/Linux checks or the Release workflow.
+
 The Release workflow publishes the DocC site to GitHub Pages after all release
 validation jobs pass and before creating the GitHub release. The site is at
 <https://swiftutility.github.io/Enigmatic/documentation/enigmatic/>. Enable
@@ -26,9 +56,14 @@ It reports actual library line coverage from the last successful release
 documentation deployment. Documentation and coverage links become available
 after the first successful Pages deployment.
 
-CI runs automatically for pushes to `main`. To check a pull request, start
-**Actions → Checks → Run workflow** and select its source branch. Pull requests
-and pushes to other branches do not start CI automatically.
+CI runs automatically for pushes to `main`. To check a pull request, add the
+`run-ci` label from the PR sidebar. Results appear in the PR's **Checks** tab.
+Opening a PR or pushing new commits to it does not start CI automatically,
+even while the label remains attached. Remove and reapply `run-ci` to check the
+latest changes. Other labels do not run tests or cancel an active `run-ci` run.
+
+You can also start **Actions → Checks → Run workflow** and select a branch.
+These manual branch runs do not appear as PR checks.
 
 ```sh
 python3 -m unittest discover -s Scripts/tests
diff --git a/Sources/Enigmatic/Enigmatic.docc/Serialization.md b/Sources/Enigmatic/Enigmatic.docc/Serialization.md
index 6303357..ebc9f1b 100644
--- a/Sources/Enigmatic/Enigmatic.docc/Serialization.md
+++ b/Sources/Enigmatic/Enigmatic.docc/Serialization.md
@@ -46,6 +46,9 @@ require keys that bridge to String. Keys normalize to strings;
 collisions and unsupported key types produce a decoding error. Arrays and sets
 convert to arrays, with set order unspecified.
 
+Boxed Foundation booleans remain boolean nodes, distinct from numeric zero and
+one, including on Windows where CoreFoundation is not exposed by the SDK.
+
 On all platforms, `Enigma(cast:)` converts `NSDecimalNumber` values by
 parsing their `stringValue` with `Double.init(_:)`, rather than reading
 `doubleValue`. This avoids precision loss in Foundation's decimal-to-Double
diff --git a/Sources/Enigmatic/Internal/Extensions/Enigma+Utility.swift b/Sources/Enigmatic/Internal/Extensions/Enigma+Utility.swift
index 86f80be..34ab829 100644
--- a/Sources/Enigmatic/Internal/Extensions/Enigma+Utility.swift
+++ b/Sources/Enigmatic/Internal/Extensions/Enigma+Utility.swift
@@ -1,5 +1,4 @@
 import Foundation
-import CoreFoundation
 
 extension Enigma {
   func collectPins(
diff --git a/Sources/Enigmatic/Internal/Primitives/EnigmaConvertibleObject.swift b/Sources/Enigmatic/Internal/Primitives/EnigmaConvertibleObject.swift
index 690e88d..977efdf 100644
--- a/Sources/Enigmatic/Internal/Primitives/EnigmaConvertibleObject.swift
+++ b/Sources/Enigmatic/Internal/Primitives/EnigmaConvertibleObject.swift
@@ -1,5 +1,7 @@
-import CoreFoundation
 import Foundation
+#if canImport(CoreFoundation)
+import CoreFoundation
+#endif
 
 protocol EnigmaConvertibleObject: EnigmaConvertible, AnyObject {}
 
@@ -52,7 +54,12 @@ extension NSValue: EnigmaConvertibleObject {
 
 extension NSNumber {
   var asEnigma: Enigma? {
+#if canImport(CoreFoundation)
     guard CFGetTypeID(self) != CFBooleanGetTypeID() else { return .bool(boolValue) }
+#else
+    // Foundation's boolean singletons distinguish Bool from numeric zero and one.
+    if self === NSNumber(value: true) || self === NSNumber(value: false) { return .bool(boolValue) }
+#endif
     return switch objCType.pointee {
     case ObjCType.signedLong: .int(intValue)
     case ObjCType.unsignedLong: .uint(uintValue)
diff --git a/Tests/EnigmaticTests/PerformanceTests.swift b/Tests/EnigmaticTests/PerformanceTests.swift
index df8c834..c0f7745 100644
--- a/Tests/EnigmaticTests/PerformanceTests.swift
+++ b/Tests/EnigmaticTests/PerformanceTests.swift
@@ -1,6 +1,5 @@
 @testable import Enigmatic
 import Foundation
-import CoreFoundation
 import XCTest
 
 final class PerformanceTests: XCTestCase {
diff --git a/Tests/EnigmaticTests/SerializationContractTests.swift b/Tests/EnigmaticTests/SerializationContractTests.swift
index 5ef04b2..dac2ad2 100644
--- a/Tests/EnigmaticTests/SerializationContractTests.swift
+++ b/Tests/EnigmaticTests/SerializationContractTests.swift
@@ -161,6 +161,29 @@ final class SerializationContractTests: XCTestCase {
     XCTAssertEqual(try Enigma(cast: NSNull()), .null)
   }
 
+  func testFoundationBooleansRemainDistinctFromZeroAndOne() throws {
+    for value in [false, true] {
+      let tree = try Enigma(cast: NSNumber(value: value))
+      XCTAssertEqual(tree.asBool, value)
+      XCTAssertNil(tree.asInt)
+    }
+    for value in [0, 1] {
+      for number in [NSNumber(value: value), NSNumber(value: Int8(value)), NSNumber(value: Double(value))] {
+        let tree = try Enigma(cast: number)
+        XCTAssertNil(tree.asBool)
+        XCTAssertEqual(tree.asInt, value)
+      }
+    }
+    let tree: Enigma = [false, true, 0, 1]
+    let data = try JSONSerialization.data(withJSONObject: tree.asJsonObject)
+    let restored = try Enigma(cast: JSONSerialization.jsonObject(with: data))
+    XCTAssertEqual(restored[0]?.asBool, false)
+    XCTAssertEqual(restored[1]?.asBool, true)
+    XCTAssertNil(restored[2]?.asBool)
+    XCTAssertNil(restored[3]?.asBool)
+    XCTAssertEqual(restored, tree)
+  }
+
   func testFloatFoundationSerializationRoundTrips() throws {
     let values: [Float] = [
       0.1,