diff --git a/anygen/agent-harness/cli_anything/anygen/core/session.py b/anygen/agent-harness/cli_anything/anygen/core/session.py
index 1fcb843..8622b64 100644
--- a/anygen/agent-harness/cli_anything/anygen/core/session.py
+++ b/anygen/agent-harness/cli_anything/anygen/core/session.py
@@ -1,11 +1,38 @@
 """Session management — undo/redo and command history for AnyGen CLI."""
 
 import json
+import os
 from dataclasses import dataclass, field
 from datetime import datetime, timezone
 from pathlib import Path
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 @dataclass
 class HistoryEntry:
     command: str
@@ -101,9 +128,7 @@ def save(self, path: str):
             "history": [e.to_dict() for e in self._history],
             "redo_stack": [e.to_dict() for e in self._redo_stack],
         }
-        Path(path).parent.mkdir(parents=True, exist_ok=True)
-        with open(path, "w") as f:
-            json.dump(data, f, indent=2, sort_keys=True, default=str)
+        _locked_save_json(path, data, indent=2, sort_keys=True, default=str)
 
     def _load(self, path: str):
         p = Path(path)
diff --git a/audacity/agent-harness/cli_anything/audacity/core/session.py b/audacity/agent-harness/cli_anything/audacity/core/session.py
index af108ce..b13f2f3 100644
--- a/audacity/agent-harness/cli_anything/audacity/core/session.py
+++ b/audacity/agent-harness/cli_anything/audacity/core/session.py
@@ -8,6 +8,32 @@
 import os
 import copy
 from typing import Dict, Any, Optional, List
+
+
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
 from datetime import datetime
 
 
@@ -119,9 +145,7 @@ def save_session(self, path: Optional[str] = None) -> str:
 
         # Save project
         self.project["metadata"]["modified"] = datetime.now().isoformat()
-        os.makedirs(os.path.dirname(os.path.abspath(save_path)), exist_ok=True)
-        with open(save_path, "w") as f:
-            json.dump(self.project, f, indent=2, sort_keys=True, default=str)
+        _locked_save_json(save_path, self.project, indent=2, sort_keys=True, default=str)
 
         self.project_path = save_path
         self._modified = False
diff --git a/blender/agent-harness/cli_anything/blender/core/session.py b/blender/agent-harness/cli_anything/blender/core/session.py
index 0359cde..67ccea2 100644
--- a/blender/agent-harness/cli_anything/blender/core/session.py
+++ b/blender/agent-harness/cli_anything/blender/core/session.py
@@ -7,6 +7,32 @@
 from datetime import datetime
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 class Session:
     """Manages project state with undo/redo history."""
 
@@ -111,8 +137,7 @@ def save_session(self, path: Optional[str] = None) -> str:
 
         # Save project
         self.project["metadata"]["modified"] = datetime.now().isoformat()
-        with open(save_path, "w") as f:
-            json.dump(self.project, f, indent=2, sort_keys=True, default=str)
+        _locked_save_json(save_path, self.project, indent=2, sort_keys=True, default=str)
 
         self.project_path = save_path
         self._modified = False
diff --git a/cli-anything-plugin/HARNESS.md b/cli-anything-plugin/HARNESS.md
index 9b16737..155f91e 100644
--- a/cli-anything-plugin/HARNESS.md
+++ b/cli-anything-plugin/HARNESS.md
@@ -67,6 +67,36 @@ designed for humans, without needing a display or mouse.
 5. **Add rendering/export** — The export pipeline calls the backend module.
    Generate valid intermediate files, then invoke the real software for conversion.
 6. **Add session management** — State persistence, undo/redo
+
+   **Session file locking** — When saving session JSON, use exclusive file locking
+   to prevent concurrent writes from corrupting data. Never use bare
+   `open("w") + json.dump()` — `open("w")` truncates the file before any lock
+   can be acquired. Instead, open with `"r+"`, lock, then truncate inside the lock:
+   ```python
+   def _locked_save_json(path, data, **dump_kwargs) -> None:
+       """Atomically write JSON with exclusive file locking."""
+       try:
+           f = open(path, "r+")            # no truncation on open
+       except FileNotFoundError:
+           os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+           f = open(path, "w")             # first save — file doesn't exist yet
+       with f:
+           _locked = False
+           try:
+               import fcntl
+               fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+               _locked = True
+           except (ImportError, OSError):
+               pass                        # Windows / unsupported FS — proceed unlocked
+           try:
+               f.seek(0)
+               f.truncate()                # truncate INSIDE the lock
+               json.dump(data, f, **dump_kwargs)
+               f.flush()
+           finally:
+               if _locked:
+                   fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+   ```
 7. **Add the REPL with unified skin** — Interactive mode wrapping the subcommands.
    - Copy `repl_skin.py` from the plugin (`cli-anything-plugin/repl_skin.py`) into
      `utils/repl_skin.py` in your CLI package
diff --git a/drawio/agent-harness/cli_anything/drawio/core/session.py b/drawio/agent-harness/cli_anything/drawio/core/session.py
index 9a9d056..722beae 100644
--- a/drawio/agent-harness/cli_anything/drawio/core/session.py
+++ b/drawio/agent-harness/cli_anything/drawio/core/session.py
@@ -14,6 +14,33 @@
 from ..utils import drawio_xml
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    path = str(path)
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 SESSION_DIR = Path.home() / ".drawio-cli" / "sessions"
 MAX_UNDO_DEPTH = 50
 
@@ -126,8 +153,7 @@ def save_session_state(self) -> str:
             "timestamp": time.time(),
         }
         path = SESSION_DIR / f"{self.session_id}.json"
-        with open(path, "w") as f:
-            json.dump(state, f, indent=2, sort_keys=True)
+        _locked_save_json(path, state, indent=2, sort_keys=True)
         return str(path)
 
     @classmethod
diff --git a/gimp/agent-harness/cli_anything/gimp/core/session.py b/gimp/agent-harness/cli_anything/gimp/core/session.py
index 233834d..dc97c13 100644
--- a/gimp/agent-harness/cli_anything/gimp/core/session.py
+++ b/gimp/agent-harness/cli_anything/gimp/core/session.py
@@ -7,6 +7,32 @@
 from datetime import datetime
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 class Session:
     """Manages project state with undo/redo history."""
 
@@ -111,8 +137,7 @@ def save_session(self, path: Optional[str] = None) -> str:
 
         # Save project
         self.project["metadata"]["modified"] = datetime.now().isoformat()
-        with open(save_path, "w") as f:
-            json.dump(self.project, f, indent=2, sort_keys=True, default=str)
+        _locked_save_json(save_path, self.project, indent=2, sort_keys=True, default=str)
 
         self.project_path = save_path
         self._modified = False
diff --git a/gimp/agent-harness/cli_anything/gimp/tests/test_core.py b/gimp/agent-harness/cli_anything/gimp/tests/test_core.py
index 4d9f56c..86dd064 100644
--- a/gimp/agent-harness/cli_anything/gimp/tests/test_core.py
+++ b/gimp/agent-harness/cli_anything/gimp/tests/test_core.py
@@ -476,3 +476,76 @@ def test_max_undo(self):
         for i in range(10):
             sess.snapshot(f"action {i}")
         assert len(sess._undo_stack) == 5
+
+
+# ── Concurrent Save Tests ───────────────────────────────────────
+
+class TestLockedSaveJson:
+    """Tests for _locked_save_json atomic file writes."""
+
+    def test_basic_save(self, tmp_path):
+        from cli_anything.gimp.core.session import _locked_save_json
+        path = str(tmp_path / "test.json")
+        _locked_save_json(path, {"key": "value"}, indent=2)
+        with open(path) as f:
+            data = json.load(f)
+        assert data == {"key": "value"}
+
+    def test_overwrite_existing(self, tmp_path):
+        from cli_anything.gimp.core.session import _locked_save_json
+        path = str(tmp_path / "test.json")
+        _locked_save_json(path, {"version": 1}, indent=2)
+        _locked_save_json(path, {"version": 2}, indent=2)
+        with open(path) as f:
+            data = json.load(f)
+        assert data == {"version": 2}
+
+    def test_overwrite_shorter_data(self, tmp_path):
+        """Ensure truncation works — shorter data doesn't leave old bytes."""
+        from cli_anything.gimp.core.session import _locked_save_json
+        path = str(tmp_path / "test.json")
+        _locked_save_json(path, {"key": "a" * 1000}, indent=2)
+        _locked_save_json(path, {"k": 1}, indent=2)
+        with open(path) as f:
+            data = json.load(f)
+        assert data == {"k": 1}
+
+    def test_creates_parent_dirs(self, tmp_path):
+        from cli_anything.gimp.core.session import _locked_save_json
+        path = str(tmp_path / "nested" / "dir" / "test.json")
+        _locked_save_json(path, {"nested": True})
+        with open(path) as f:
+            data = json.load(f)
+        assert data == {"nested": True}
+
+    def test_concurrent_writes_produce_valid_json(self, tmp_path):
+        """Multiple threads writing to the same file should not corrupt it."""
+        from cli_anything.gimp.core.session import _locked_save_json
+        import threading
+
+        path = str(tmp_path / "concurrent.json")
+        errors = []
+
+        def writer(thread_id):
+            try:
+                for i in range(50):
+                    _locked_save_json(
+                        path,
+                        {"thread": thread_id, "iteration": i},
+                        indent=2, sort_keys=True,
+                    )
+            except Exception as e:
+                errors.append(e)
+
+        threads = [threading.Thread(target=writer, args=(t,)) for t in range(4)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        assert not errors, f"Errors during concurrent writes: {errors}"
+        # Final file must be valid JSON
+        with open(path) as f:
+            data = json.load(f)
+        assert "thread" in data
+        assert "iteration" in data
diff --git a/inkscape/agent-harness/cli_anything/inkscape/core/session.py b/inkscape/agent-harness/cli_anything/inkscape/core/session.py
index 3df553e..2db41a8 100644
--- a/inkscape/agent-harness/cli_anything/inkscape/core/session.py
+++ b/inkscape/agent-harness/cli_anything/inkscape/core/session.py
@@ -7,6 +7,32 @@
 from datetime import datetime
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 class Session:
     """Manages project state with undo/redo history."""
 
@@ -110,9 +136,7 @@ def save_session(self, path: Optional[str] = None) -> str:
             raise ValueError("No save path specified.")
 
         self.project["metadata"]["modified"] = datetime.now().isoformat()
-        os.makedirs(os.path.dirname(os.path.abspath(save_path)), exist_ok=True)
-        with open(save_path, "w") as f:
-            json.dump(self.project, f, indent=2, sort_keys=True, default=str)
+        _locked_save_json(save_path, self.project, indent=2, sort_keys=True, default=str)
 
         self.project_path = save_path
         self._modified = False
diff --git a/kdenlive/agent-harness/cli_anything/kdenlive/core/session.py b/kdenlive/agent-harness/cli_anything/kdenlive/core/session.py
index 13b1a66..cf532bc 100644
--- a/kdenlive/agent-harness/cli_anything/kdenlive/core/session.py
+++ b/kdenlive/agent-harness/cli_anything/kdenlive/core/session.py
@@ -7,6 +7,32 @@
 from datetime import datetime
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 class Session:
     """Manages project state with undo/redo history."""
 
@@ -106,11 +132,7 @@ def save_session(self, path: Optional[str] = None) -> str:
             raise ValueError("No save path specified.")
 
         self.project["metadata"]["modified"] = datetime.now().isoformat()
-        parent = os.path.dirname(os.path.abspath(save_path))
-        if parent:
-            os.makedirs(parent, exist_ok=True)
-        with open(save_path, "w") as f:
-            json.dump(self.project, f, indent=2, sort_keys=True, default=str)
+        _locked_save_json(save_path, self.project, indent=2, sort_keys=True, default=str)
 
         self.project_path = save_path
         self._modified = False
diff --git a/libreoffice/agent-harness/cli_anything/libreoffice/core/session.py b/libreoffice/agent-harness/cli_anything/libreoffice/core/session.py
index 91a29db..2f2f5ce 100644
--- a/libreoffice/agent-harness/cli_anything/libreoffice/core/session.py
+++ b/libreoffice/agent-harness/cli_anything/libreoffice/core/session.py
@@ -7,6 +7,32 @@
 from datetime import datetime
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 class Session:
     """Manages document project state with undo/redo history."""
 
@@ -117,9 +143,7 @@ def save_session(self, path: Optional[str] = None) -> str:
             raise ValueError("No save path specified.")
 
         self.project["metadata"]["modified"] = datetime.now().isoformat()
-        os.makedirs(os.path.dirname(os.path.abspath(save_path)), exist_ok=True)
-        with open(save_path, "w") as f:
-            json.dump(self.project, f, indent=2, sort_keys=True, default=str)
+        _locked_save_json(save_path, self.project, indent=2, sort_keys=True, default=str)
 
         self.project_path = save_path
         self._modified = False
diff --git a/obs-studio/agent-harness/cli_anything/obs_studio/core/session.py b/obs-studio/agent-harness/cli_anything/obs_studio/core/session.py
index 089f33f..e5275b5 100644
--- a/obs-studio/agent-harness/cli_anything/obs_studio/core/session.py
+++ b/obs-studio/agent-harness/cli_anything/obs_studio/core/session.py
@@ -7,6 +7,32 @@
 from datetime import datetime
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 class Session:
     """Manages project state with undo/redo history."""
 
@@ -106,9 +132,7 @@ def save_session(self, path: Optional[str] = None) -> str:
             raise ValueError("No save path specified.")
 
         self.project["metadata"]["modified"] = datetime.now().isoformat()
-        os.makedirs(os.path.dirname(os.path.abspath(save_path)), exist_ok=True)
-        with open(save_path, "w") as f:
-            json.dump(self.project, f, indent=2, sort_keys=True, default=str)
+        _locked_save_json(save_path, self.project, indent=2, sort_keys=True, default=str)
 
         self.project_path = save_path
         self._modified = False
diff --git a/shotcut/agent-harness/cli_anything/shotcut/core/session.py b/shotcut/agent-harness/cli_anything/shotcut/core/session.py
index 7341c81..21fa977 100644
--- a/shotcut/agent-harness/cli_anything/shotcut/core/session.py
+++ b/shotcut/agent-harness/cli_anything/shotcut/core/session.py
@@ -15,6 +15,33 @@
 from ..utils import mlt_xml
 
 
+def _locked_save_json(path, data, **dump_kwargs) -> None:
+    """Atomically write JSON with exclusive file locking."""
+    path = str(path)
+    try:
+        f = open(path, "r+")
+    except FileNotFoundError:
+        os.makedirs(os.path.dirname(os.path.abspath(path)), exist_ok=True)
+        f = open(path, "w")
+    with f:
+        _locked = False
+        try:
+            import fcntl
+            fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+            _locked = True
+        except (ImportError, OSError):
+            pass
+        try:
+            f.seek(0)
+            f.truncate()
+            json.dump(data, f, **dump_kwargs)
+            f.flush()
+        finally:
+            if _locked:
+                import fcntl
+                fcntl.flock(f.fileno(), fcntl.LOCK_UN)
+
+
 SESSION_DIR = Path.home() / ".shotcut-cli" / "sessions"
 MAX_UNDO_DEPTH = 50
 
@@ -154,8 +181,7 @@ def save_session_state(self) -> str:
             "timestamp": time.time(),
         }
         path = SESSION_DIR / f"{self.session_id}.json"
-        with open(path, "w") as f:
-            json.dump(state, f, indent=2, sort_keys=True)
+        _locked_save_json(path, state, indent=2, sort_keys=True)
         return str(path)
 
     @classmethod