diff --git a/agent/core/session_uploader.py b/agent/core/session_uploader.py
index 404fd224..989e3f0b 100644
--- a/agent/core/session_uploader.py
+++ b/agent/core/session_uploader.py
@@ -20,6 +20,7 @@
 import json
 import os
 import sys
+from contextlib import contextmanager
 from datetime import datetime
 from pathlib import Path
 from typing import Any
@@ -300,16 +301,37 @@ def _url_field(format: str) -> str:
     return "personal_upload_url" if format == "claude_code" else "upload_url"
 
 
-def _read_session_file(session_file: str) -> dict:
-    """Read a local session file while respecting uploader file locks."""
+@contextmanager
+def _file_lock(file_obj, *, exclusive: bool):
+    """Lock a session file across uploader processes on Unix and Windows."""
+    if os.name == "nt":
+        import msvcrt
+
+        file_obj.seek(0)
+        mode = msvcrt.LK_LOCK
+        try:
+            msvcrt.locking(file_obj.fileno(), mode, 1)
+            yield
+        finally:
+            file_obj.seek(0)
+            msvcrt.locking(file_obj.fileno(), msvcrt.LK_UNLCK, 1)
+        return
+
     import fcntl
 
+    lock = fcntl.LOCK_EX if exclusive else fcntl.LOCK_SH
+    fcntl.flock(file_obj, lock)
+    try:
+        yield
+    finally:
+        fcntl.flock(file_obj, fcntl.LOCK_UN)
+
+
+def _read_session_file(session_file: str) -> dict:
+    """Read a local session file while respecting uploader file locks."""
     with open(session_file, "r") as f:
-        fcntl.flock(f, fcntl.LOCK_SH)
-        try:
+        with _file_lock(f, exclusive=False):
             return json.load(f)
-        finally:
-            fcntl.flock(f, fcntl.LOCK_UN)
 
 
 def _update_upload_status(
@@ -325,11 +347,8 @@ def _update_upload_status(
     local session JSON file. Re-read under an exclusive lock so one uploader
     cannot clobber fields written by the other.
     """
-    import fcntl
-
     with open(session_file, "r+") as f:
-        fcntl.flock(f, fcntl.LOCK_EX)
-        try:
+        with _file_lock(f, exclusive=True):
             data = json.load(f)
             data[status_key] = status
             if dataset_url is not None:
@@ -340,8 +359,6 @@ def _update_upload_status(
             f.truncate()
             f.flush()
             os.fsync(f.fileno())
-        finally:
-            fcntl.flock(f, fcntl.LOCK_UN)
 
 
 def dataset_card_readme(repo_id: str) -> str:
diff --git a/tests/unit/test_prioritize_backlog.py b/tests/unit/test_prioritize_backlog.py
index 9a8fd316..fd6dce2f 100644
--- a/tests/unit/test_prioritize_backlog.py
+++ b/tests/unit/test_prioritize_backlog.py
@@ -718,4 +718,4 @@ def test_cli_defaults_without_live_network_or_llm():
     assert args.github_report_label == mod.DEFAULT_GITHUB_REPORT_LABEL
     assert args.output_dir is None
     assert out.name == "20260504T123000Z"
-    assert "scratch/backlog-prioritization" in str(out)
+    assert out.parts[-3:-1] == ("scratch", "backlog-prioritization")