diff --git a/buckaroo/server/websocket_handler.py b/buckaroo/server/websocket_handler.py
index da5917932..0a00202eb 100644
--- a/buckaroo/server/websocket_handler.py
+++ b/buckaroo/server/websocket_handler.py
@@ -51,7 +51,21 @@ def on_message(self, message):
     def _handle_buckaroo_state_change(self, new_state):
         sessions = self.application.settings["sessions"]
         session = sessions.get(self.session_id)
-        if not session or session.mode != "buckaroo":
+        if not session:
+            return
+        if session.mode != "buckaroo":
+            # Lazy mode (and any future read-only mode) doesn't currently
+            # plumb state_change → filter into its dataflow. Return a
+            # structured error rather than the pre-#793 silent drop so the
+            # client can render "filtering not supported in this mode"
+            # instead of hanging on the WS read.
+            self.write_message(json.dumps({
+                "type": "error",
+                "error_code": "state_change_unsupported_mode",
+                "message": (
+                    f"buckaroo_state_change is not supported in "
+                    f"mode={session.mode!r}; this reader is read-only "
+                    "(no filter / cleaning / post-processing).")}))
             return
         dataflow = session.xorq_dataflow if session.backend == "xorq" else session.dataflow
         if dataflow is None:
diff --git a/tests/unit/server/test_load_expr.py b/tests/unit/server/test_load_expr.py
index e4d6cc9db..a53ede645 100644
--- a/tests/unit/server/test_load_expr.py
+++ b/tests/unit/server/test_load_expr.py
@@ -141,6 +141,48 @@ async def test_ws_search_pushdown(self):
         finally:
             shutil.rmtree(builds_root, ignore_errors=True)
 
+    @tornado.testing.gen_test
+    async def test_lazy_state_change_returns_explicit_error(self):
+        """Regression for #793: ``_handle_buckaroo_state_change``
+        returned silently for ``mode != "buckaroo"`` — lazy-polars
+        sessions had their state_change messages dropped on the floor
+        with no response, hanging the client until its read timed out.
+
+        Must return a structured error instead so callers can render
+        a sensible "not supported" message.
+        """
+        import asyncio
+        csv_fd, csv_path = tempfile.mkstemp(suffix=".csv")
+        os.close(csv_fd)
+        try:
+            pd.DataFrame({"a": [1, 2, 3], "b": ["x", "y", "z"]}).to_csv(csv_path, index=False)
+            await _post(self.get_http_port(), "/load",
+                {"session": "lazy-sc", "path": csv_path, "mode": "lazy"})
+
+            ws = await tornado.websocket.websocket_connect(
+                f"ws://localhost:{self.get_http_port()}/ws/lazy-sc")
+            await ws.read_message()  # discard initial_state
+
+            ws.write_message(json.dumps({
+                "type": "buckaroo_state_change",
+                "new_state": {
+                    "post_processing": "", "cleaning_method": "",
+                    "quick_command_args": {"search": ["x"]},
+                    "df_display": "main", "show_commands": False,
+                    "sampled": False, "search_string": "x",
+                }}))
+
+            # Today: nothing comes back; client times out.
+            # After fix: a structured error frame within a couple seconds.
+            frame = await asyncio.wait_for(ws.read_message(), timeout=3.0)
+            self.assertIsNotNone(frame)
+            d = json.loads(frame)
+            self.assertEqual(d.get("type"), "error")
+            self.assertIn("error_code", d)
+            ws.close()
+        finally:
+            os.unlink(csv_path)
+
     @tornado.testing.gen_test
     async def test_session_reuse_xorq_then_pandas(self):
         """A client that POSTs /load_expr and then POSTs /load with the