diff --git a/install.ps1 b/install.ps1
index a4f26313..4d971fd1 100644
--- a/install.ps1
+++ b/install.ps1
@@ -8,7 +8,7 @@ param(
 
 $ErrorActionPreference = "Stop"
 $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-$defaultInstallSha256 = "1cb76eb63b57e1390bfb4b6c2b24a5991819541414150aa88edc76d1d1188cd6"
+$defaultInstallSha256 = "7dbb5c48a834d31a470b1952a9b8659b4c1f49f8d37c709a4032a56abe495f27"
 $maximumInstallerBytes = 2MB
 $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }
 $temporary = $null
diff --git a/install.py b/install.py
index 873e1d27..2a2b5f13 100644
--- a/install.py
+++ b/install.py
@@ -106,7 +106,15 @@ DEFAULT_DESTS = {
 
 
 def detect_os() -> str:
-    """Return normalized OS identifier."""
+    """Return normalized OS identifier.
+
+    RETROBIOS_OS names the platform outright (linux, wsl, windows, darwin),
+    for a run that has to behave like another host: the PowerShell wrapper
+    tests drive a Windows layout on a Linux runner.
+    """
+    forced = os.environ.get("RETROBIOS_OS", "").strip().lower()
+    if forced in ("linux", "wsl", "windows", "darwin"):
+        return forced
     system = platform.system().lower()
     if system == "linux":
         proc_version = Path("/proc/version")
diff --git a/install.sh b/install.sh
index 96ff02bf..6d9750a6 100755
--- a/install.sh
+++ b/install.sh
@@ -18,7 +18,7 @@ esac
 TEMP_INSTALLER=""
 TEMP_DIRECTORY=""
 DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-DEFAULT_INSTALL_SHA256="1cb76eb63b57e1390bfb4b6c2b24a5991819541414150aa88edc76d1d1188cd6"
+DEFAULT_INSTALL_SHA256="7dbb5c48a834d31a470b1952a9b8659b4c1f49f8d37c709a4032a56abe495f27"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {
diff --git a/tests/test_install.py b/tests/test_install.py
index ac3ee4db..379d7d90 100644
--- a/tests/test_install.py
+++ b/tests/test_install.py
@@ -852,6 +852,8 @@ class TestLaunchboxDetectionPowershell(unittest.TestCase):
                 USERPROFILE=str(userprofile),
                 APPDATA=str(tempfile.mkdtemp()),
                 RETROBIOS_BASE_URL=f"http://127.0.0.1:{httpd.server_address[1]}",
+                # A LaunchBox layout is a Windows layout: the runner is Linux.
+                RETROBIOS_OS="wsl",
             )
             proc = subprocess.run(
                 [
@@ -953,6 +955,18 @@ class TestLaunchboxDetectionPowershell(unittest.TestCase):
         self.assertTrue(landed.exists(), proc.stdout)
 
 
+class TestForcedOs(unittest.TestCase):
+    """RETROBIOS_OS overrides detection with a known name only."""
+
+    def test_known_name_wins(self):
+        with unittest.mock.patch.dict(os.environ, {"RETROBIOS_OS": "Windows"}):
+            self.assertEqual(install.detect_os(), "windows")
+
+    def test_unknown_name_is_ignored(self):
+        with unittest.mock.patch.dict(os.environ, {"RETROBIOS_OS": "amiga"}):
+            self.assertIn(install.detect_os(), ("linux", "wsl", "windows", "darwin"))
+
+
 class TestDetectFrontends(unittest.TestCase):
     """Frontends have no BIOS directory of their own but hint at the setup."""