diff --git a/python/tk_framework_desktopserver/command.py b/python/tk_framework_desktopserver/command.py
index de0d30463..a30e3cea8 100644
--- a/python/tk_framework_desktopserver/command.py
+++ b/python/tk_framework_desktopserver/command.py
@@ -226,11 +226,18 @@ def _call_cmd_win32(args, env):
             )
             process.wait()
 
-            # Read back the output from the two.
-            with open(stdout_path, "rt") as stdout_file:
+            # Read back the output from the two. Decode as utf-8 with
+            # errors="replace" rather than relying on the OS locale codec:
+            # on non-utf-8 locales (e.g. cp932 on Japanese Windows) open()
+            # would otherwise raise UnicodeDecodeError on non-ASCII output.
+            with open(
+                stdout_path, "rt", encoding="utf-8", errors="replace"
+            ) as stdout_file:
                 stdout_lines = [l for l in stdout_file]
 
-            with open(stderr_path) as stderr_file:
+            with open(
+                stderr_path, "rt", encoding="utf-8", errors="replace"
+            ) as stderr_file:
                 stderr_lines = [l for l in stderr_file]
 
             # Track the result code.
@@ -240,7 +247,7 @@ def _call_cmd_win32(args, env):
             logger.exception("Error running subprocess:")
 
             ret = 1
-            stderr_lines = [traceback.format_exc().split()]
+            stderr_lines = traceback.format_exc().split()
             stderr_lines.append("%s" % args)
 
         # Don't lose any sleep over temporary files that can't be deleted.
diff --git a/tests/test_command.py b/tests/test_command.py
new file mode 100644
index 000000000..af6b6effb
--- /dev/null
+++ b/tests/test_command.py
@@ -0,0 +1,97 @@
+# Copyright (c) 2026 Shotgun Software Inc.
+#
+# CONFIDENTIAL AND PROPRIETARY
+#
+# This work is provided "AS IS" and subject to the Shotgun Pipeline Toolkit
+# Source Code License included in this distribution package. See LICENSE.
+# By accessing, using, copying or modifying this work you indicate your
+# agreement to the Shotgun Pipeline Toolkit Source Code License. All rights
+# not expressly granted therein are reserved by Shotgun Software Inc.
+
+import os
+import sys
+import tempfile
+import unittest
+from unittest.mock import Mock, patch
+
+import sgtk
+
+# The framework package imports Qt at import time; mock it since Qt is not available
+# under test (same approach as the other suites, e.g. tests/settings/test_settings.py).
+sgtk.platform.qt.QtCore = Mock()
+sgtk.platform.qt.QtGui = Mock()
+
+repo_root = os.path.abspath(os.path.join(os.path.dirname(__file__), ".."))
+sys.path.insert(0, os.path.join(repo_root, "python"))
+
+from tk_framework_desktopserver.command import Command  # noqa: E402
+
+# The defects fixed here (SG-44961) live in _call_cmd_win32, so these tests only
+# make sense on Windows.
+windows_only = unittest.skipUnless(
+    sys.platform == "win32", "command._call_cmd_win32 is Windows-only"
+)
+
+
+@windows_only
+class TestCallCmdWin32(unittest.TestCase):
+    """
+    Regression tests for SG-44961.
+
+    Bug 1: the stdout/stderr temp files were read with the OS locale codec (cp932 on
+    Japanese Windows), so non-ASCII subprocess output raised UnicodeDecodeError.
+    Bug 2: the win32 exception handler wrapped the traceback list inside another list,
+    so call_cmd's "".join(stderr_lines) raised
+    "TypeError: sequence item 0: expected str instance, list found".
+    """
+
+    def _write_child(self, body):
+        """Write a small child script to a temp .py file and return its path."""
+        handle, path = tempfile.mkstemp(suffix=".py", prefix="sg44961_child_")
+        os.close(handle)
+        with open(path, "w", encoding="utf-8") as f:
+            f.write(body)
+        self.addCleanup(lambda: os.path.exists(path) and os.remove(path))
+        return path
+
+    def test_non_ascii_stderr_does_not_crash(self):
+        """
+        Bug 1: a child emitting bytes that are illegal under the OS locale codec must
+        not crash the reader. The pair 0x81 0xff is undecodable under cp932, cp1252 and
+        strict utf-8 alike, so on the pre-fix code open() raises UnicodeDecodeError
+        regardless of the machine locale; with the fix, errors="replace" turns the bytes
+        into U+FFFD and the call returns cleanly.
+        """
+        child = self._write_child(
+            "import sys\n"
+            "sys.stderr.buffer.write(b'engine bootstrap line\\n')\n"
+            "sys.stderr.buffer.write(bytes([0x81, 0xff]))\n"
+            "sys.stderr.buffer.write(b'\\ntrailing line\\n')\n"
+            "sys.exit(3)\n"
+        )
+
+        ret, out, err = Command.call_cmd([sys.executable, child])
+
+        self.assertEqual(ret, 3)
+        self.assertIsInstance(err, str)
+        self.assertIn("�", err)  # undecodable bytes replaced, not raised
+        self.assertIn("engine bootstrap line", err)
+
+    def test_exception_handler_returns_joinable_lines(self):
+        """
+        Bug 2: when the try block raises, the handler must leave stderr_lines as a flat
+        list of strings so call_cmd's "".join(...) succeeds instead of raising TypeError.
+        """
+        with patch(
+            "tk_framework_desktopserver.command.subprocess.Popen",
+            side_effect=RuntimeError("boom"),
+        ):
+            ret, out, err = Command.call_cmd([sys.executable, "-c", "pass"])
+
+        self.assertEqual(ret, 1)
+        self.assertIsInstance(err, str)
+        self.assertIn("Traceback", err)
+
+
+if __name__ == "__main__":
+    unittest.main()