diff --git a/webapp/app.py b/webapp/app.py
index 0bbf747..c17ab03 100644
--- a/webapp/app.py
+++ b/webapp/app.py
@@ -15,6 +15,7 @@ This file is the route surface; most logic lives in ``services/``:
 import json
 import os
 import shutil
+import tempfile
 from datetime import datetime
 from pathlib import Path
 
@@ -506,13 +507,29 @@ def imaging_set_laps(serial):
     if not isinstance(pw, str):
         return {"ok": False, "error": "password must be string"}, 400
     if pw == "":
-        # Clear by direct read-modify-write since update_session skips empty values.
-        state = imaging_status.get_session(serial) or {}
-        if "laps_password" in state:
-            state.pop("laps_password", None)
-            # Re-feed everything (minus laps_password) through update_session.
-            state["serial"] = serial
-            imaging_status.update_session(state)
+        # Clear by direct file write. update_session() merges payload INTO
+        # existing state and skips empty values, so it cannot remove a key.
+        # Pop the laps_password key directly from the session JSON and
+        # write the result atomically.
+        path = imaging_status._path_for(serial)
+        if os.path.isfile(path):
+            try:
+                with open(path, "r") as f:
+                    state = json.load(f)
+            except (json.JSONDecodeError, OSError):
+                state = {}
+            if "laps_password" in state:
+                state.pop("laps_password", None)
+                state["last_updated"] = imaging_status._now_iso()
+                fd, tmp = tempfile.mkstemp(dir=config.IMAGING_DIR, prefix=".tmp-", suffix=".json")
+                try:
+                    with os.fdopen(fd, "w") as f:
+                        json.dump(state, f, indent=2)
+                    os.replace(tmp, path)
+                except Exception:
+                    try: os.unlink(tmp)
+                    except OSError: pass
+                    raise
         return {"ok": True, "cleared": True}
     imaging_status.update_session({"serial": serial, "laps_password": pw})
     return {"ok": True}