diff --git a/.github/actions/windows-reg-import/action.yml b/.github/actions/windows-reg-import/action.yml
new file mode 100755
index 0000000000..728a735beb
--- /dev/null
+++ b/.github/actions/windows-reg-import/action.yml
@@ -0,0 +1,39 @@
+name: windows-reg-import
+description: Import a .reg file and verify those registry values (best-effort)
+inputs:
+  reg-file:
+    description: "Path to the .reg file"
+    required: true
+runs:
+  using: "composite"
+  steps:
+    - name: Attempt to import custom registry (best-effort)
+      shell: pwsh
+      run: |
+        try {
+          $regFile = Join-Path $env:GITHUB_WORKSPACE "${{ inputs.reg-file }}"
+          if (-not (Test-Path $regFile)) {
+            Write-Warning "Registry file not found (skipping): $regFile"
+          } else {
+            Write-Host "Importing registry from $regFile (attempting, non-fatal)"
+            reg import $regFile 2>&1 | ForEach-Object { Write-Host $_ }
+          }
+        } catch {
+          Write-Warning "Registry import failed (ignored): $_"
+        }
+
+    - name: Attempt to verify registry values (best-effort)
+      shell: pwsh
+      run: |
+        try {
+          $acp = (Get-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\Nls\CodePage' -Name ACP -ErrorAction Stop).ACP
+          Write-Host "ACP = $acp"
+        } catch {
+          Write-Warning "Failed to read ACP (ignored): $_"
+        }
+        try {
+          $eb = (Get-ItemProperty -Path 'HKLM:\SYSTEM\CurrentControlSet\Control\Nls\MUILanguagePreferences' -Name EnableBetaUnicode -ErrorAction Stop).EnableBetaUnicode
+          Write-Host "EnableBetaUnicode = $eb"
+        } catch {
+          Write-Warning "Failed to read EnableBetaUnicode (ignored): $_"
+        }
diff --git a/.github/ci/windows/custom.reg b/.github/ci/windows/custom.reg
new file mode 100755
index 0000000000..cbc0213c5c
Binary files /dev/null and b/.github/ci/windows/custom.reg differ
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c1fd97e03b..99dad0bb91 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -746,6 +746,11 @@ jobs:
       with:
         fetch-depth: 0
         submodules: true
+    - name: Import custom registry and verify
+      uses: ./.github/actions/windows-reg-import
+      with:
+        reg-file: .github/ci/windows/custom.reg
+      continue-on-error: true
     - uses: VirtusLab/scala-cli-setup@v1
       with:
         jvm: "temurin:17"
@@ -778,6 +783,11 @@ jobs:
       with:
         fetch-depth: 0
         submodules: true
+    - name: Import custom registry and verify
+      uses: ./.github/actions/windows-reg-import
+      with:
+        reg-file: .github/ci/windows/custom.reg
+      continue-on-error: true
     - name: Set up Python
       uses: actions/setup-python@v6
       with:
@@ -819,6 +829,11 @@ jobs:
       with:
         fetch-depth: 0
         submodules: true
+    - name: Import custom registry and verify
+      uses: ./.github/actions/windows-reg-import
+      with:
+        reg-file: .github/ci/windows/custom.reg
+      continue-on-error: true
     - name: Set up Python
       uses: actions/setup-python@v6
       with:
@@ -860,6 +875,11 @@ jobs:
       with:
         fetch-depth: 0
         submodules: true
+    - name: Import custom registry and verify
+      uses: ./.github/actions/windows-reg-import
+      with:
+        reg-file: .github/ci/windows/custom.reg
+      continue-on-error: true
     - name: Set up Python
       uses: actions/setup-python@v6
       with:
@@ -901,6 +921,11 @@ jobs:
         with:
           fetch-depth: 0
           submodules: true
+      - name: Import custom registry and verify
+        uses: ./.github/actions/windows-reg-import
+        with:
+          reg-file: .github/ci/windows/custom.reg
+        continue-on-error: true
       - name: Set up Python
         uses: actions/setup-python@v6
         with:
@@ -942,6 +967,11 @@ jobs:
         with:
           fetch-depth: 0
           submodules: true
+      - name: Import custom registry and verify
+        uses: ./.github/actions/windows-reg-import
+        with:
+          reg-file: .github/ci/windows/custom.reg
+        continue-on-error: true
       - name: Set up Python
         uses: actions/setup-python@v6
         with:
@@ -1518,6 +1548,11 @@ jobs:
       with:
         fetch-depth: 0
         submodules: true
+    - name: Import custom registry and verify
+      uses: ./.github/actions/windows-reg-import
+      with:
+        reg-file: .github/ci/windows/custom.reg
+      continue-on-error: true
     - uses: VirtusLab/scala-cli-setup@v1
       with:
         jvm: "temurin:17"
@@ -1824,6 +1859,11 @@ jobs:
       with:
         fetch-depth: 0
         submodules: true
+    - name: Import custom registry and verify
+      uses: ./.github/actions/windows-reg-import
+      with:
+        reg-file: .github/ci/windows/custom.reg
+      continue-on-error: true
     - uses: VirtusLab/scala-cli-setup@v1
       with:
         jvm: "temurin:17"
diff --git a/build.mill.scala b/build.mill.scala
index 0f6a0513de..5c2c888bbf 100644
--- a/build.mill.scala
+++ b/build.mill.scala
@@ -126,6 +126,10 @@ object integration extends CliIntegration {
       Deps.jgit,
       Deps.jsoup
     )
+    override def forkEnv: T[Map[String, String]] = super.forkEnv() ++ Seq(
+      "JAVA_TOOL_OPTIONS" -> "-Dfile.encoding=UTF-8",
+      "BLOOP_JAVA_OPTS"   -> "-Dfile.encoding=UTF-8 -Xmx512m"
+    )
   }
   object docker extends CliIntegrationDocker {
     object test extends ScalaCliTests {
diff --git a/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala b/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala
old mode 100644
new mode 100755
index 39b841120f..fee62cf62d
--- a/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala
+++ b/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala
@@ -4,9 +4,9 @@ import com.eed3si9n.expecty.Expecty.expect
 
 import java.io.{ByteArrayOutputStream, File}
 import java.nio.charset.Charset
+import java.nio.charset.StandardCharsets.UTF_8
 
 import scala.cli.integration.util.DockerServer
-import scala.io.Codec
 import scala.jdk.CollectionConverters.*
 import scala.util.Properties
 
@@ -1057,31 +1057,130 @@ abstract class RunTestDefinitions
   }
 
   test("UTF-8") {
-    val message  = "Hello from TestÅÄÖåäö"
-    val fileName = "TestÅÄÖåäö.scala"
-    val inputs   = TestInputs(
+    def utf8tag   = "ÅÄÖåäöΔЖ"
+    val classname = s"Q$utf8tag"
+    val basename  = classname // if (Properties.isWin) s"Test_ascii" else classname
+
+    val fileName     = s"$basename.scala"
+    val message      = s"Hello $classname"
+    val utfPropnames = Seq("file.encoding", "sun.jnu.encoding", "native.encoding")
+    val utfProps     = utfPropnames.map(s => s"-D$s=UTF-8")
+    val utfOptions   = utfProps ++ Seq("-Dtest.scala-cli.debug-charset-issue=true")
+
+    def cliOptions = utfOptions.flatMap(opt => Seq("--java-opt", opt))
+
+    def code =
+      """
+object MAINCLASS {
+  def props(s: String): String = Option(sys.props(s)).getOrElse("")
+  val fileName = "FILENAME"
+  val utfPropnames = Seq("file.encoding", "sun.jnu.encoding", "native.encoding", "java.runtime.version")
+  utfPropnames.foreach { (str: String) => System.err.println(s"$str = ${props(str)}") }
+  if (sys.props("os.name").toLowerCase.contains("windows")) {
+    import scala.sys.process._
+    System.err.println(s"chcp.com code-page: ${"chcp.com".!!.trim}")
+    try {
+      val codepage = "reg query 'HKLM\\SYSTEM\\CurrentControlSet\\Control\\Nls\\CodePage' /v ACP".!!.trim
+      System.err.println(s"registry code-page: ${codepage.replaceAll("[^0-9]", "")}")
+    } catch {
+      case _: Throwable =>
+    }
+  }
+  import java.nio.charset.Charset
+  System.err.println(s"Charset.defaultCharset: ${Charset.defaultCharset}")
+  System.err.println(s"fileName Chars: ${fileName.map(c => f"U+$c%04x").mkString(" ")}")
+  def main(args: Array[String]): Unit = {
+    print("""" + message + """") // no newline needed here
+  }
+}
+""".trim
+    val scriptContents =
+      code.replaceAll("MAINCLASS", classname).replaceAll("FILENAME", fileName)
+    // System.err.printf("%s\n", scriptContents)
+
+    val inputs = TestInputs(
       os.rel / fileName ->
-        s"""object TestÅÄÖåäö {
-           |  def main(args: Array[String]): Unit = {
-           |    println("$message")
-           |  }
-           |}
-           |""".stripMargin
+        scriptContents
     )
+
+    val jvmDir = os.pwd.toString.replace('\\', '/').replaceAll("/out/.*", "") + "/jvm"
+
+    // the build jvm respects system code page, temurin:17 does not
+    val testCli = if Properties.isWin && TestUtil.cli.contains("-jar") then
+      val i                  = TestUtil.cli.indexOf("-jar")
+      val (left, right)      = TestUtil.cli.splitAt(i)
+      val arr: Array[String] = (left ++ utfOptions ++ right).toArray
+      arr(0) = s"$jvmDir/bin/java.exe"
+      arr.toSeq
+    else
+      TestUtil.cli ++ cliOptions
+
+    if (false && Properties.isWin) { // TODO: not sure how enable this in debug or verbose mode
+      def props(s: String): String = Option(sys.props(s)).getOrElse("")
+      utfPropnames.foreach(s => System.err.println(s"$s = ${props(s)}"))
+      System.err.println(s"Charset.defaultCharset: ${Charset.defaultCharset}")
+      if (sys.props("os.name").toLowerCase.contains("windows")) {
+        import scala.sys.process.*
+        System.err.println(s"chcp.com code-page: ${"chcp.com".!!.trim}")
+        try {
+          val codepage =
+            "reg query 'HKLM\\SYSTEM\\CurrentControlSet\\Control\\Nls\\CodePage' /v ACP".!!.trim
+          System.err.println(s"registry code-page: ${codepage.replaceAll("[^0-9]", "")}")
+        }
+        catch {
+          case _: Throwable =>
+        }
+      }
+      System.err.println(s"[DEBUG] fileName string: [$fileName]")
+      System.err.println(s"[DEBUG] fileName.length: ${fileName.length}")
+      System.err.println(s"[DEBUG] Chars: ${fileName.map(c => f"U+$c%04x").mkString(" ")}")
+    }
+    System.err.println(s"""
+  os.proc(
+  ${testCli.mkString(" ")},
+  ${extraOptions.mkString(" ")},
+  ${fileName.replace('\\', '/')}
+)""".trim)
+    val runenv = if (Properties.isWin)
+      Map(
+        "JAVA_TOOL_OPTIONS" -> "-Dfile.encoding=UTF-8",
+        "JAVA_HOME"         -> jvmDir
+      )
+    else
+      Map(
+        "JAVA_TOOL_OPTIONS" -> "-Dfile.encoding=UTF-8"
+      )
+
     inputs.fromRoot { root =>
+      System.err.println("================== pre os.proc.call")
       val res = os.proc(
-        TestUtil.cli,
-        "-Dtest.scala-cli.debug-charset-issue=true",
-        "run",
+        testCli,
         extraOptions,
         fileName
       )
-        .call(cwd = root)
-      if (res.out.text(Codec.default).trim != message) {
-        pprint.err.log(res.out.text(Codec.default).trim)
-        pprint.err.log(message)
+        .call(
+          cwd = root,
+          check = false,
+          env = runenv
+        )
+      System.err.println(s"===== post os.proc.call stdout:\n[${res.out}]")
+
+      val stdoutbytes = res.out.bytes
+      val utf8str     = new String(stdoutbytes, UTF_8).trim
+
+      pprint.err.log(utf8str)
+      pprint.err.log(message)
+
+      if (utf8str != message) {
+        val expectbytes = message.getBytes(UTF_8)
+        val expectMsg   = expectbytes.map("%02x".format(_)).mkString(" ")
+        val stdoutMsg   = stdoutbytes.map("%02x".format(_)).mkString(" ")
+        pprint.err.log("stdout bytes:" + stdoutMsg)
+        pprint.err.log("expect bytes:" + expectMsg)
       }
-      expect(res.out.text(Codec.default).trim == message)
+      expect(utf8str == message)
+      // bogus failure on Windows occurs only in mill test environment
+      expect(Properties.isWin || utf8str == message)
     }
   }
 
@@ -2457,4 +2556,5 @@ abstract class RunTestDefinitions
         processes.foreach { case (p, _) => expect(p.exitCode() == 0) }
       }
     }
+
 }