diff --git a/tests/tui_gateway/test_compute_host_phase1.py b/tests/tui_gateway/test_compute_host_phase1.py
index bb7368d..edf78cf 100644
--- a/tests/tui_gateway/test_compute_host_phase1.py
+++ b/tests/tui_gateway/test_compute_host_phase1.py
@@ -1,6 +1,7 @@
 import io
 import json
 import os
+import subprocess
 import sys
 import threading
 import time
@@ -118,6 +119,35 @@ def test_append_log_record_single_write_lines(tmp_path):
     assert all(line.endswith("x" * 2000) for line in lines)
 
 
+def test_append_log_record_across_processes_preserves_unicode(tmp_path):
+    path = tmp_path / "nested" / "shared.log"
+    child_code = """
+import sys, time
+from tui_gateway.host_supervisor import append_log_record
+for i in range(100):
+    append_log_record(sys.argv[1], f'{sys.argv[2]}:{i}:' + '\u0130\u015f\u011f\U0001f680' * 100)
+    time.sleep(0.001)
+"""
+    children = [subprocess.Popen(
+        [sys.executable, "-c", child_code, str(path), str(index)],
+        stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+        creationflags=getattr(subprocess, "CREATE_NO_WINDOW", 0),
+    ) for index in range(4)]
+    try:
+        for child in children:
+            _, errors = child.communicate(timeout=30)
+            assert child.returncode == 0, errors.decode("utf-8", errors="replace")
+    finally:
+        for child in children:
+            if child.poll() is None:
+                child.kill()
+                child.communicate(timeout=10)
+    lines = path.read_text(encoding="utf-8").splitlines()
+    expected = {f"{process}:{index}:" + "İşğ🚀" * 100 for process in range(4) for index in range(100)}
+    assert len(lines) == len(expected)
+    assert set(lines) == expected
+
+
 def test_supervisor_startup_reconcile_pid_reuse_guard(tmp_path, monkeypatch):
     registry = tmp_path / "dashboard-compute-host.json"
     registry.write_text(json.dumps({"host_pid": os.getpid(), "boot_id": "stale"}), encoding="utf-8")
diff --git a/tui_gateway/host_supervisor.py b/tui_gateway/host_supervisor.py
index 9f8a7bd..ca50e3a 100644
--- a/tui_gateway/host_supervisor.py
+++ b/tui_gateway/host_supervisor.py
@@ -55,14 +55,35 @@ _LATE_CONTROL_MAX = 64
 
 
 def append_log_record(path: str | Path, record: str) -> None:
-    """Append one log record using O_APPEND and exactly one os.write call."""
+    """Append a record with one native write and kernel-owned EOF placement."""
     p = Path(path)
     p.parent.mkdir(parents=True, exist_ok=True)
     text = record if record.endswith("\n") else f"{record}\n"
     data = text.encode("utf-8", errors="replace")
+    if sys.platform == "win32":
+        import ntsecuritycon
+        import win32con
+        import win32file
+
+        # The Windows CRT's O_APPEND seeks before writing, so independent file
+        # descriptors can choose the same offset. Append-only access delegates
+        # EOF placement to the kernel, without granting overwrite permission.
+        handle = win32file.CreateFile(
+            str(p), ntsecuritycon.FILE_APPEND_DATA,
+            win32con.FILE_SHARE_READ | win32con.FILE_SHARE_WRITE | win32con.FILE_SHARE_DELETE,
+            None, win32con.OPEN_ALWAYS, win32con.FILE_ATTRIBUTE_NORMAL, None,
+        )
+        try:
+            _, written = win32file.WriteFile(handle, data)
+            if written != len(data):
+                raise OSError("Incomplete log record write")
+        finally:
+            handle.Close()
+        return
     fd = os.open(str(p), os.O_WRONLY | os.O_CREAT | os.O_APPEND, 0o600)
     try:
-        os.write(fd, data)
+        if os.write(fd, data) != len(data):
+            raise OSError("Incomplete log record write")
     finally:
         os.close(fd)