diff --git a/repeater/web/api_endpoints.py b/repeater/web/api_endpoints.py
index fa6b480..59641a7 100644
--- a/repeater/web/api_endpoints.py
+++ b/repeater/web/api_endpoints.py
@@ -1983,8 +1983,18 @@ class APIEndpoints:
             if "repeater" not in self.config:
                 self.config["repeater"] = {}
             self.config["repeater"]["mode"] = new_mode
+
+            # The engine reads the mode live from the shared config dict;
+            # update_and_save writes it to disk so the choice survives a
+            # restart.
+            result = self.config_manager.update_and_save(
+                updates={}, live_update=True, live_update_sections=["repeater"]
+            )
+            if not result.get("saved", False):
+                return self._error(result.get("error", "Failed to save configuration to file"))
+
             logger.info(f"Mode changed to: {new_mode}")
-            return {"success": True, "mode": new_mode}
+            return {"success": True, "mode": new_mode, "persisted": True}
         except cherrypy.HTTPError:
             # Re-raise HTTP errors (like 405 Method Not Allowed) without logging
             raise
diff --git a/tests/test_api_endpoints_core_coverage.py b/tests/test_api_endpoints_core_coverage.py
index 9aed860..f84a267 100644
--- a/tests/test_api_endpoints_core_coverage.py
+++ b/tests/test_api_endpoints_core_coverage.py
@@ -1726,10 +1726,14 @@ def test_set_mode_and_set_duty_cycle_paths(cherrypy_ctx):
     invalid = api.set_mode()
     assert invalid["success"] is False
 
+    api.config_manager.update_and_save.return_value = {"saved": True}
     request.json = {"mode": "monitor"}
     ok = api.set_mode()
-    assert ok == {"success": True, "mode": "monitor"}
+    assert ok == {"success": True, "mode": "monitor", "persisted": True}
     assert api.config["repeater"]["mode"] == "monitor"
+    api.config_manager.update_and_save.assert_called_once_with(
+        updates={}, live_update=True, live_update_sections=["repeater"]
+    )
 
     request.json = {"enabled": False}
     duty = api.set_duty_cycle()
@@ -1737,6 +1741,21 @@ def test_set_mode_and_set_duty_cycle_paths(cherrypy_ctx):
     assert api.config["duty_cycle"]["enforcement_enabled"] is False
 
 
+def test_set_mode_save_failure_reports_error(cherrypy_ctx):
+    request, _ = cherrypy_ctx
+    api = _make_api({"repeater": {}})
+    api.config_manager.update_and_save.return_value = {"saved": False, "error": "disk full"}
+
+    request.method = "POST"
+    request.json = {"mode": "no_tx"}
+    resp = api.set_mode()
+    assert resp["success"] is False
+    assert "disk full" in resp["error"]
+    # The live switch already happened on the shared dict; only the
+    # persistence failed, and the error says so instead of a silent loss.
+    assert api.config["repeater"]["mode"] == "no_tx"
+
+
 def test_update_duty_cycle_config_branches(cherrypy_ctx):
     request, _ = cherrypy_ctx
     api = _make_api({"duty_cycle": {}})