diff --git a/test/test_api_v3_on_demand_socket.py b/test/test_api_v3_on_demand_socket.py
index d6769fb1..8498b8bd 100644
--- a/test/test_api_v3_on_demand_socket.py
+++ b/test/test_api_v3_on_demand_socket.py
@@ -127,6 +127,20 @@ class TestMailboxFallback:
         assert resp.get_json()["data"]["socket_error"] == "internal"
         assert len(_mailbox_writes(service["cache"])) == 1
 
+    def test_an_unknown_reason_is_reported_as_other(self, api_v3_client, service):
+        # Only known codes are echoed back; anything else stays server-side.
+        with patch(f"{CLIENT}.on_demand_start",
+                   side_effect=control_client.ControlError("/run/secret/path", "x")):
+            data = api_v3_client.post(START_URL, json={"plugin_id": "weather"}).get_json()["data"]
+        assert data["transport"] == "mailbox"
+        assert data["socket_error"] == "other"
+        assert len(_mailbox_writes(service["cache"])) == 1
+
+    def test_every_display_error_code_is_reportable(self):
+        from web_interface.blueprints.api_v3 import display
+        codes = {v for k, v in vars(c.ErrorCode).items() if not k.startswith("_")}
+        assert codes <= set(display._REPORTABLE_SOCKET_REASONS)
+
     def test_stop_falls_back(self, api_v3_client, service):
         with patch(f"{CLIENT}.on_demand_stop",
                    side_effect=control_client.ControlError("timeout")):
diff --git a/web_interface/blueprints/api_v3/display.py b/web_interface/blueprints/api_v3/display.py
index 3b38c955..a2fc74ba 100644
--- a/web_interface/blueprints/api_v3/display.py
+++ b/web_interface/blueprints/api_v3/display.py
@@ -34,6 +34,21 @@ def _cache_manager():
 #: older than the socket, Windows, or switched off. Not worth a log line.
 _QUIET_SOCKET_REASONS = frozenset({'no_socket', 'disabled', 'unsupported'})
 
+#: Every reason code a response may echo as ``socket_error``: the client's
+#: transport reasons plus the display's ErrorCode values. Anything else is
+#: reported as ``other``, so no text taken from an exception reaches a reply.
+_REPORTABLE_SOCKET_REASONS = (
+    'disabled', 'unsupported', 'no_socket', 'refused', 'timeout', 'closed',
+    'bad_response', 'invalid_request',
+    'bad_json', 'bad_request', 'message_too_large', 'unsupported_version',
+    'unknown_command', 'invalid_args', 'busy', 'forbidden', 'internal',
+)
+
+
+def _socket_reason_code(reason):
+    """``reason`` as one of _REPORTABLE_SOCKET_REASONS, else ``'other'``."""
+    return next((code for code in _REPORTABLE_SOCKET_REASONS if code == reason), 'other')
+
 
 def _deliver_on_demand(payload):
     """Hand an on-demand request to the display: control socket, else mailbox.
@@ -58,7 +73,7 @@ def _deliver_on_demand(payload):
             control_client.on_demand_stop(payload['request_id'])
         return 'socket', None
     except control_client.ControlError as e:
-        reason = e.reason
+        reason = _socket_reason_code(e.reason)
         if reason in _QUIET_SOCKET_REASONS:
             logger.debug("On-demand %s via the mailbox: %s", payload['action'], e)
         else: