diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7f729e9e..ddbcbb60 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,16 @@ accepts both, but the store flags the old spelling as deprecated
 
 ## Unreleased
 
+- Web API error responses no longer carry an exception's message (CodeQL
+  `py/stack-trace-exposure`). `describe_exception()` now returns a reason
+  code -- the exception type, plus the errno for an `OSError`
+  (`OSError:EIO`, `PermissionError:EACCES`) -- and logs the message
+  instead, so `details` still names the fault without quoting paths, URLs
+  or library internals. The display service status and the on-demand
+  start/stop `service` results keep `active`, `returncode` and `started`
+  but drop systemctl's `stdout`/`stderr`; WiFi, unit-refresh and
+  config-save failures say what failed and point at the log.
+
 ## 3.8.2
 
 The display hands freed memory back to the OS (#774), and sports consolidation
diff --git a/src/web_interface/error_handler.py b/src/web_interface/error_handler.py
index c3360def..5f5913b1 100644
--- a/src/web_interface/error_handler.py
+++ b/src/web_interface/error_handler.py
@@ -4,6 +4,7 @@ Centralized error handling for web interface.
 Provides helpers for consistent error responses across API endpoints.
 """
 
+import errno
 from typing import Any, Optional
 from flask import jsonify
 
@@ -20,10 +21,9 @@ logger = get_logger(__name__)
 _MAX_DETAIL_LENGTH = 400
 
 
-def describe_exception(exc: BaseException,
-                       max_length: int = _MAX_DETAIL_LENGTH) -> str:
+def describe_exception(exc: BaseException) -> str:
     """
-    One-line, safe-to-return description of an exception.
+    Machine-readable reason code for an exception, safe to return over HTTP.
 
     The generic "an error occurred; see logs for details" tells a user nothing
     and, when the failure is bad enough, the logs are unreachable too: a device
@@ -31,20 +31,24 @@ def describe_exception(exc: BaseException,
     *including* the log viewer, because journalctl could not be executed. The
     underlying `[Errno 5] Input/output error` named the fault immediately.
 
-    Returns "TypeName: message", credentials redacted and length capped. The
-    type alone is worth carrying -- a bare PermissionError says more than any
-    generic sentence.
+    So the type and errno still go back -- "OSError:EIO", "PermissionError:
+    EACCES", "TimeoutExpired" -- but never the exception's message, which can
+    quote paths, URLs, credentials or a library's internals (CodeQL
+    py/stack-trace-exposure). The message is logged here instead, so every
+    reason code a client sees has its full text in the log.
 
     Args:
         exc: The exception to describe
-        max_length: Truncate beyond this many characters
 
     Returns:
-        A single-line description, never empty
+        "TypeName" or "TypeName:ERRNO", never empty
     """
-    message = str(exc).strip()
-    text = f"{type(exc).__name__}: {message}" if message else type(exc).__name__
-    return redact_text(text, max_length)
+    code = type(exc).__name__
+    exc_errno = getattr(exc, 'errno', None)
+    if isinstance(exc_errno, int) and exc_errno in errno.errorcode:
+        code = f"{code}:{errno.errorcode[exc_errno]}"
+    logger.warning("Error reported to the client as %s: %s", code, redact_text(str(exc)))
+    return code
 
 
 def redact_text(text: str, max_length: int = _MAX_DETAIL_LENGTH) -> str:
diff --git a/src/wifi_manager.py b/src/wifi_manager.py
index 12d98797..0221a2d1 100644
--- a/src/wifi_manager.py
+++ b/src/wifi_manager.py
@@ -1369,7 +1369,7 @@ class WiFiManager:
                         self.enable_ap_mode(force=True)
                     except Exception as ap_error:  # nosec B110 - last-resort; do not re-raise, but log for debugging
                         logger.error("Last-resort AP mode enable failed in recovery path: %s", ap_error, exc_info=True)
-            return False, str(e)
+            return False, f"Connection failed ({type(e).__name__}); see logs for details"
     
     def _failsafe_ap(self, enabled_msg: str, failed_msg: str) -> Tuple[bool, str]:
         """Force the setup AP up after a connect that left no working network,
@@ -1585,7 +1585,7 @@ class WiFiManager:
         except Exception as e:
             logger.error(f"Error connecting with nmcli: {e}")
             self._show_led_message("Connection error", duration=5)
-            return False, str(e)
+            return False, f"Connection failed ({type(e).__name__}); see logs for details"
 
     # 802.11 caps an SSID at 32 octets. Control characters cannot appear in a
     # real one, and a leading "-" would be read by nmcli as an option rather
@@ -1725,7 +1725,7 @@ class WiFiManager:
                 return False, "nmcli is required to disconnect from WiFi"
         except Exception as e:
             logger.error(f"Error disconnecting from WiFi: {e}")
-            return False, str(e)
+            return False, f"Disconnect failed ({type(e).__name__}); see logs for details"
     
     def _ensure_wifi_radio_enabled(self, max_retries: int = 3) -> bool:
         """
@@ -2004,7 +2004,7 @@ class WiFiManager:
             return False, "No WiFi tools available (nmcli, hostapd, or dnsmasq required)"
         except Exception as e:
             logger.error(f"Error in enable_ap_mode: {e}")
-            return False, str(e)
+            return False, f"Could not enable AP mode ({type(e).__name__}); see logs for details"
     
     def _mark_forced(self) -> None:
         """Record that AP mode was forced on, so the periodic check leaves it
@@ -2099,10 +2099,10 @@ class WiFiManager:
                 return True, "AP mode enabled"
             except Exception as e:
                 logger.error(f"Error starting AP services: {e}")
-                return False, str(e)
+                return False, f"Could not enable AP mode ({type(e).__name__}); see logs for details"
         except Exception as e:
             logger.error(f"Error enabling AP mode: {e}")
-            return False, str(e)
+            return False, f"Could not enable AP mode ({type(e).__name__}); see logs for details"
     
     def _enable_ap_mode_nmcli_hotspot(self) -> Tuple[bool, str]:
         """
@@ -2227,7 +2227,7 @@ class WiFiManager:
             logger.error(f"Error starting AP mode with nmcli: {e}")
             self._remove_nm_dnsmasq_captive_conf()
             self._show_led_message("Setup mode error", duration=5)
-            return False, str(e)
+            return False, f"Could not enable AP mode ({type(e).__name__}); see logs for details"
     
     def _get_ap_status_nmcli(self) -> Dict:
         """
@@ -2409,10 +2409,10 @@ class WiFiManager:
                 return True, "AP mode disabled"
             except Exception as e:
                 logger.error(f"Error stopping AP services: {e}")
-                return False, str(e)
+                return False, f"Could not disable AP mode ({type(e).__name__}); see logs for details"
         except Exception as e:
             logger.error(f"Error disabling AP mode: {e}")
-            return False, str(e)
+            return False, f"Could not disable AP mode ({type(e).__name__}); see logs for details"
     
     def _create_hostapd_config(self):
         """Create hostapd configuration file"""
diff --git a/test/test_api_v3_display_modes.py b/test/test_api_v3_display_modes.py
index a59913f0..323894d3 100644
--- a/test/test_api_v3_display_modes.py
+++ b/test/test_api_v3_display_modes.py
@@ -147,7 +147,8 @@ class TestOneBadConfigSectionDoesNotBlankTheList:
             side_effect=RuntimeError("disk is gone"))
         resp = api_v3_client.get('/api/v3/display/modes')
         assert resp.status_code == 500
-        assert 'disk is gone' in resp.get_json()['details']
+        assert resp.get_json()['details'] == 'RuntimeError'
+        assert 'disk is gone' not in json.dumps(resp.get_json())
 
     def test_credentials_in_the_exception_are_redacted(self, api_v3_module, api_v3_client):
         """describe_exception is what makes returning detail safe."""
diff --git a/test/test_api_v3_no_exception_text.py b/test/test_api_v3_no_exception_text.py
new file mode 100644
index 00000000..5f930576
--- /dev/null
+++ b/test/test_api_v3_no_exception_text.py
@@ -0,0 +1,147 @@
+"""No API response carries an exception's message (CodeQL py/stack-trace-exposure).
+
+One representative route per file that had open alerts. Each forces a failure
+whose message holds a marker and asserts the marker is nowhere in the body:
+the message goes to the log, the client gets a fixed message plus a reason
+code (describe_exception: the type, and the errno for an OSError).
+"""
+
+import json
+import sys
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+from test._api_v3_test_helpers import api_v3_client, api_v3_module  # noqa: F401,E402
+
+LEAK = "LEAKED-/home/pi/secret token=abc123"
+API = "web_interface.blueprints.api_v3"
+
+
+def _assert_no_leak(response):
+    body = response.get_data(as_text=True)
+    assert "LEAKED" not in body, body
+    assert "abc123" not in body, body
+    return json.loads(body)
+
+
+def test_display_service_status_drops_systemctl_output(api_v3_module, api_v3_client,
+                                                      monkeypatch):
+    """display.py: the on-demand routes return the service status verbatim."""
+    api_v3_module.api_v3.cache_manager.get.return_value = None
+    monkeypatch.setattr(f"{API}.display.display_state.read_state", lambda: None)
+    with patch(f"{API}.subprocess.run", side_effect=OSError(13, LEAK)):
+        body = _assert_no_leak(api_v3_client.get("/api/v3/display/on-demand/status"))
+    assert body["data"]["service"] == {"active": False, "returncode": -1}
+
+
+@pytest.mark.parametrize("helper", ["_ensure_display_service_running",
+                                    "_stop_display_service"])
+def test_service_results_keep_returncode_but_not_output(api_v3_module, helper):
+    """display.py start/stop: returncode/active/started stay, stdout/stderr go."""
+    failed = MagicMock(returncode=1, stdout=LEAK, stderr=LEAK)
+    with patch(f"{API}.subprocess.run", return_value=failed):
+        result = getattr(api_v3_module, helper)()
+    assert "LEAKED" not in json.dumps(result)
+    assert result["returncode"] == 1 and result["active"] is False
+    assert "stdout" not in result and "stderr" not in result
+
+
+def test_wifi_connect_failure(api_v3_client):
+    """wifi.py: a raising connect, and the attempt /wifi/status reports after."""
+    with patch("src.wifi_manager.WiFiManager") as cls:
+        cls.return_value._is_ap_mode_active.return_value = False
+        cls.return_value.connect_to_network.side_effect = RuntimeError(LEAK)
+        body = _assert_no_leak(api_v3_client.post(
+            "/api/v3/wifi/connect", json={"ssid": "HomeNet", "password": "pw"}))
+        assert body["details"] == "RuntimeError"
+        cls.return_value.get_wifi_status.return_value = MagicMock(
+            connected=False, ssid=None, ip_address=None, signal=0, ap_mode_active=False)
+        cls.return_value.config = {}
+        status = _assert_no_leak(api_v3_client.get("/api/v3/wifi/status"))
+    assert status["data"]["last_connect_attempt"]["message"] == (
+        "Failed to connect to network (RuntimeError)")
+
+
+def test_wifi_manager_messages_carry_no_exception_text():
+    """src/wifi_manager.py: its (success, message) is what the wifi routes return."""
+    from src.wifi_manager import WiFiManager
+    manager = WiFiManager.__new__(WiFiManager)  # no __init__: no host access
+    manager.get_wifi_status = MagicMock(side_effect=OSError(5, LEAK))
+    success, message = manager.disconnect_from_network()
+    assert success is False
+    assert "LEAKED" not in message and "OSError" in message
+
+
+def test_system_action_exception(api_v3_client):
+    """system.py: execute_system_action's catch-all."""
+    with patch("subprocess.run", side_effect=OSError(5, LEAK)):
+        body = _assert_no_leak(api_v3_client.post(
+            "/api/v3/system/action", json={"action": "stop_display"}))
+    assert body["details"] == "OSError:EIO"
+
+
+def test_calendar_registration_failure(api_v3_client, tmp_path, monkeypatch):
+    """plugin_calendar.py: the auth script could not be run."""
+    plugin_dir = tmp_path / "calendar"
+    plugin_dir.mkdir()
+    (plugin_dir / "credentials.json").write_text("{}", encoding="utf-8")
+    (plugin_dir / "calendar_registration.py").write_text("", encoding="utf-8")
+    monkeypatch.setattr(f"{API}._calendar_plugin_dir", lambda: plugin_dir)
+    with patch(f"{API}.subprocess.run", side_effect=OSError(13, LEAK)):
+        body = _assert_no_leak(api_v3_client.post(
+            "/api/v3/plugins/calendar/authenticate", json={"code": "x"}))
+    assert "EACCES" in body["message"]
+
+
+def test_health_failure(api_v3_client, monkeypatch):
+    """misc.py: get_health's catch-all."""
+    def boom():
+        raise RuntimeError(LEAK)
+    monkeypatch.setattr(f"{API}.misc._get_display_service_status", boom)
+    body = _assert_no_leak(api_v3_client.get("/api/v3/health"))
+    assert body["details"] == "RuntimeError"
+
+
+def test_config_route_failure(api_v3_module, api_v3_client):
+    """error_handler.py: create_error_response, as config.py's routes use it."""
+    api_v3_module.api_v3.config_manager.load_config.side_effect = RuntimeError(LEAK)
+    body = _assert_no_leak(api_v3_client.get("/api/v3/config/schedule"))
+    assert body["details"] == "RuntimeError"
+
+
+def test_plugin_route_failure(api_v3_module, api_v3_client):
+    """plugins.py: an unhandled error in a plugin route."""
+    api_v3_module.api_v3.plugin_catalog.get_all_plugin_info.side_effect = RuntimeError(LEAK)
+    body = _assert_no_leak(api_v3_client.get("/api/v3/plugins/installed"))
+    assert body["details"] == "RuntimeError"
+
+
+def test_starlark_route_failure(api_v3_client):
+    """starlark.py: one of its catch-alls."""
+    with patch(f"{API}._get_starlark_plugin", side_effect=RuntimeError(LEAK)):
+        body = _assert_no_leak(api_v3_client.get("/api/v3/starlark/status"))
+    assert body["details"] == "RuntimeError"
+
+
+def test_unit_refresh_failure(monkeypatch):
+    """system.py git_pull: perform_core_update appends unit_refresh's message."""
+    from web_interface import unit_refresh
+
+    def boom(*_a, **_k):
+        raise RuntimeError(LEAK)
+    monkeypatch.setattr(unit_refresh, "stale_units", boom)
+    result = unit_refresh.refresh_after_update()
+    assert result["status"] == unit_refresh.FAILED
+    assert "LEAKED" not in result["message"]
+
+
+def test_install_base_requirements_failure(api_v3_client):
+    """system.py: a pip install that could not start, in the action's output."""
+    with patch(f"{API}.system._pip_install_requirements", side_effect=OSError(5, LEAK)):
+        body = _assert_no_leak(api_v3_client.post(
+            "/api/v3/system/action", json={"action": "install_base_requirements"}))
+    assert "Failed: OSError:EIO" in body["output"]
diff --git a/test/test_api_v3_registry_endpoints.py b/test/test_api_v3_registry_endpoints.py
index b4e6dd1f..ba0e5f96 100644
--- a/test/test_api_v3_registry_endpoints.py
+++ b/test/test_api_v3_registry_endpoints.py
@@ -95,9 +95,10 @@ class TestRefreshPluginStore:
             RuntimeError("failed at /home/user/LEDMatrix/src/secret.py line 42"))
         body = api_v3_client.post(self.URL, json={}).get_json()
         assert "Traceback" not in str(body)
-        # `details` is describe_exception output: one line, type-named,
-        # credential-redacted. It may quote the message, but never a stack.
-        assert body["details"].startswith("RuntimeError:")
+        # `details` is describe_exception output: the type, never the
+        # message or a stack.
+        assert body["details"] == "RuntimeError"
+        assert "secret.py" not in str(body)
         assert "\n" not in body["details"]
 
 
diff --git a/test/test_web_error_detail.py b/test/test_web_error_detail.py
index 2fb3a2f7..f0e376fd 100644
--- a/test/test_web_error_detail.py
+++ b/test/test_web_error_detail.py
@@ -11,26 +11,32 @@ than even logging it.
 
 import pytest
 
-from src.web_interface.error_handler import describe_exception
+from src.web_interface.error_handler import describe_exception, redact_text
 
 
 class TestDescribeException:
-    def test_names_the_type_and_message(self):
-        detail = describe_exception(OSError(5, "Input/output error", "systemctl"))
-        assert detail == "OSError: [Errno 5] Input/output error: 'systemctl'"
+    """describe_exception is a reason code: type and errno, never the message.
 
-    def test_the_reported_failure_is_legible(self):
-        # The whole point: this string is the diagnosis.
-        assert "Input/output error" in describe_exception(
-            OSError(5, "Input/output error", "systemctl"))
+    The message can quote paths, URLs or credentials (CodeQL
+    py/stack-trace-exposure), so it goes to the log; the code still names the
+    fault, as "[Errno 5]" did.
+    """
+
+    def test_an_oserror_names_its_errno(self):
+        assert describe_exception(
+            OSError(5, "Input/output error", "systemctl")) == "OSError:EIO"
 
     def test_a_bare_exception_still_names_its_type(self):
-        # A PermissionError with no message still says more than "unknown".
         assert describe_exception(PermissionError()) == "PermissionError"
         assert describe_exception(Exception()) == "Exception"
 
-    def test_message_is_kept_when_present(self):
-        assert describe_exception(ValueError("bad port")) == "ValueError: bad port"
+    def test_the_message_never_reaches_the_code(self):
+        assert describe_exception(ValueError("bad port /etc/secret")) == "ValueError"
+
+    def test_the_message_is_logged_instead(self, caplog):
+        describe_exception(RuntimeError("disk on fire token=abc123"))
+        assert "disk on fire" in caplog.text
+        assert "abc123" not in caplog.text
 
 
 class TestCredentialRedaction:
@@ -56,47 +62,44 @@ class TestCredentialRedaction:
         ("authorization: barecredential", "barecredential"),
     ])
     def test_credentials_never_reach_the_response(self, secret_text, leaked):
-        detail = describe_exception(RuntimeError(secret_text))
+        detail = redact_text(secret_text)
         assert leaked not in detail
         assert "<redacted>" in detail
 
     def test_the_parameter_name_survives_redaction(self):
         # Knowing *which* credential was involved is part of the diagnosis.
-        detail = describe_exception(RuntimeError("https://x/y?api_key=SEC123"))
+        detail = redact_text("https://x/y?api_key=SEC123")
         assert "api_key" in detail
 
     def test_unknown_schemes_keep_their_name(self):
         for scheme in ("ApiKey", "Negotiate", "NTLM", "AWS4-HMAC-SHA256"):
-            detail = describe_exception(
-                RuntimeError("Authorization: %s SECRETVALUE" % scheme))
+            detail = redact_text("Authorization: %s SECRETVALUE" % scheme)
             assert scheme in detail, detail
             assert "SECRETVALUE" not in detail, detail
 
     def test_auth_scheme_and_username_survive(self):
         # Which kind of credential, and whose, without the credential itself.
-        assert "Bearer" in describe_exception(
-            RuntimeError("Authorization: Bearer eyJ.SECRET.sig"))
-        assert "user" in describe_exception(
-            RuntimeError("https://user:hunter2@example.com"))
+        assert "Bearer" in redact_text("Authorization: Bearer eyJ.SECRET.sig")
+        assert "user" in redact_text("https://user:hunter2@example.com")
 
     def test_non_secret_context_is_preserved(self):
-        detail = describe_exception(RuntimeError("https://api.x.com/v1?city=Tampa"))
+        detail = redact_text("https://api.x.com/v1?city=Tampa")
         assert "city=Tampa" in detail
         assert "<redacted>" not in detail
 
 
 class TestBounds:
     def test_long_messages_are_truncated(self):
-        detail = describe_exception(ValueError("x" * 5000))
+        detail = redact_text("x" * 5000)
         assert len(detail) <= 400
 
     def test_newlines_are_collapsed_to_one_line(self):
-        detail = describe_exception(ValueError("line one\nline two\tthree"))
+        detail = redact_text("line one\nline two\tthree")
         assert "\n" not in detail and "\t" not in detail
-        assert detail == "ValueError: line one line two three"
+        assert detail == "line one line two three"
 
     def test_custom_length_is_honoured(self):
-        assert len(describe_exception(ValueError("y" * 500), max_length=50)) <= 50
+        assert len(redact_text("y" * 500, max_length=50)) <= 50
 
 
 class TestHandlersCarryDetail:
@@ -319,10 +322,10 @@ class TestHandlersCarryDetail:
         assert resp.status_code == 405, "a wrong method must stay a 405"
         assert resp.get_json()["error_code"] == "METHOD_NOT_ALLOWED"
 
-        # A genuine server fault still reports as one, with its detail.
+        # A genuine server fault still reports as one, with its reason code.
         resp = client.get("/boom")
         assert resp.status_code == 500
-        assert "Input/output error" in resp.get_json()["details"]
+        assert resp.get_json()["details"] == "OSError:EIO"
 
     def test_global_handler_reports_the_underlying_error(self):
         from flask import Flask, jsonify
@@ -345,4 +348,5 @@ class TestHandlersCarryDetail:
         client = app.test_client()
         body = client.get("/boom").get_json()
         assert body["error_code"] == "UNKNOWN_ERROR"
-        assert "Input/output error" in body["details"]
+        assert body["details"] == "OSError:EIO"
+        assert "Input/output error" not in str(body)
diff --git a/test/web_interface/test_api_v3_unhandled_errors.py b/test/web_interface/test_api_v3_unhandled_errors.py
index 49ab0dbc..d8c9652a 100644
--- a/test/web_interface/test_api_v3_unhandled_errors.py
+++ b/test/web_interface/test_api_v3_unhandled_errors.py
@@ -114,12 +114,11 @@ def test_the_answer_is_what_the_catch_all_returned(client, caplog, method, url,
     assert records[-1].exc_info[1] is FORCED
 
 
-def test_credentials_are_redacted_from_the_detail(client):
+def test_the_exception_message_never_reaches_the_detail(client):
     body = client.get("/api/v3/plugins/installed").get_json()
-    for secret in ("SECRET123", "pw1", "K1"):
-        assert secret not in body["details"]
-    assert "<redacted>" in body["details"]
-    assert body["details"].startswith("RuntimeError: forced failure")
+    for secret in ("SECRET123", "pw1", "K1", "forced failure"):
+        assert secret not in str(body)
+    assert body["details"] == "RuntimeError"
 
 
 def _raise_415():
@@ -218,7 +217,7 @@ class TestPluginActionStep1:
             encoding="utf-8")
         return d
 
-    def test_the_script_error_reaches_the_response(self, plugin_dir, monkeypatch):
+    def test_the_script_error_is_reported_by_type(self, plugin_dir, monkeypatch):
         from unittest.mock import MagicMock
         manager = MagicMock()
         manager.get_plugin_directory.return_value = str(plugin_dir)
@@ -232,5 +231,6 @@ class TestPluginActionStep1:
 
         assert resp.status_code == 500
         body = resp.get_json()
-        assert body["details"] == "RuntimeError: the auth script failed"
+        assert body["details"] == "RuntimeError"
+        assert "the auth script failed" not in str(body)
         assert body["message"] == 'An error occurred; see logs for details'
diff --git a/test/web_interface/test_starlark_pixlet_routes.py b/test/web_interface/test_starlark_pixlet_routes.py
index 92102c48..55c55928 100644
--- a/test/web_interface/test_starlark_pixlet_routes.py
+++ b/test/web_interface/test_starlark_pixlet_routes.py
@@ -946,21 +946,27 @@ class TestTheStoreReportsWhyItIsEmpty:
 
 
 class TestACrashCarriesItsDetail:
-    """Seventeen Starlark handlers answered 5xx with no detail at all."""
+    """Seventeen Starlark handlers answered 5xx with no detail at all.
 
-    def test_browse_returns_the_exception_detail(self, client):
+    The detail is a reason code (the exception type), not the exception's
+    message, which stays in the log (CodeQL py/stack-trace-exposure).
+    """
+
+    def test_browse_returns_the_reason_code(self, client):
         with patch('web_interface.blueprints.api_v3._get_tronbyte_repository_class',
                    side_effect=ImportError("No module named 'yaml'")):
             body = client.get('/api/v3/starlark/repository/browse').get_json()
 
-        assert 'yaml' in body.get('details', ''), body
+        assert body.get('details') == 'ImportError', body
+        assert 'yaml' not in str(body), body
 
-    def test_status_returns_the_exception_detail(self, client):
+    def test_status_returns_the_reason_code(self, client):
         with patch('web_interface.blueprints.api_v3._get_starlark_plugin',
                    side_effect=RuntimeError("plugin manager is not attached")):
             body = client.get('/api/v3/starlark/status').get_json()
 
-        assert 'plugin manager is not attached' in body.get('details', ''), body
+        assert body.get('details') == 'RuntimeError', body
+        assert 'plugin manager is not attached' not in str(body), body
 
 
 class TestTheListingIsNotCappedAtOneThousand:
diff --git a/web_interface/blueprints/api_v3/__init__.py b/web_interface/blueprints/api_v3/__init__.py
index b40105be..f20d223d 100644
--- a/web_interface/blueprints/api_v3/__init__.py
+++ b/web_interface/blueprints/api_v3/__init__.py
@@ -222,7 +222,7 @@ def _save_config_atomic(config_manager, config_data, create_backup=True):
             config_manager.save_config(config_data)
             return True, None
         except Exception as e:
-            return False, str(e)
+            return False, f"Failed to save configuration ({describe_exception(e)})"
 def _coerce_to_bool(value):
     """
     Coerce a form value to a proper Python boolean.
@@ -246,7 +246,11 @@ def _coerce_to_bool(value):
         return value.lower() in ('true', 'on', '1', 'yes')
     return False
 def _get_display_service_status():
-    """Return status information about the ledmatrix service."""
+    """Return status information about the ledmatrix service.
+
+    active/returncode only: this goes back in API responses, and systemctl's
+    output (or an exception's text) is logged rather than returned.
+    """
     try:
         result = subprocess.run(
             ['systemctl', 'is-active', 'ledmatrix'],
@@ -254,26 +258,18 @@ def _get_display_service_status():
             text=True,
             timeout=3
         )
+        if result.stderr.strip():
+            logger.debug('systemctl is-active ledmatrix: %s', result.stderr.strip())
         return {
             'active': result.stdout.strip() == 'active',
             'returncode': result.returncode,
-            'stdout': result.stdout.strip(),
-            'stderr': result.stderr.strip()
         }
     except subprocess.TimeoutExpired:
-        return {
-            'active': False,
-            'returncode': -1,
-            'stdout': '',
-            'stderr': 'timeout'
-        }
-    except Exception as err:
-        return {
-            'active': False,
-            'returncode': -1,
-            'stdout': '',
-            'stderr': str(err)
-        }
+        logger.warning('systemctl is-active ledmatrix timed out')
+        return {'active': False, 'returncode': -1}
+    except Exception:
+        logger.warning('Could not query ledmatrix.service status', exc_info=True)
+        return {'active': False, 'returncode': -1}
 def _run_systemctl_command(args):
     """Run a systemctl command safely."""
     try:
@@ -295,18 +291,26 @@ def _run_systemctl_command(args):
             'stderr': 'timeout'
         }
     except Exception as err:
+        logger.warning('%s failed', ' '.join(args), exc_info=True)
         return {
             'returncode': -1,
             'stdout': '',
-            'stderr': str(err)
+            'stderr': describe_exception(err)
         }
+def _public_service_result(result):
+    """A _run_systemctl_command result fit for a response: no stdout/stderr."""
+    if result.get('returncode') != 0:
+        logger.error('systemctl exited %s: %s', result.get('returncode'),
+                     (result.get('stderr') or '').strip())
+    return {k: v for k, v in result.items() if k not in ('stdout', 'stderr')}
 def _ensure_display_service_running():
     """Ensure the ledmatrix display service is running."""
     status = _get_display_service_status()
     if status.get('active'):
         status['started'] = False
         return status
-    result = _run_systemctl_command(['sudo', 'systemctl', 'start', 'ledmatrix.service'])
+    result = _public_service_result(
+        _run_systemctl_command(['sudo', 'systemctl', 'start', 'ledmatrix.service']))
     service_status = _get_display_service_status()
     result['started'] = result.get('returncode') == 0
     result['active'] = service_status.get('active')
@@ -314,7 +318,8 @@ def _ensure_display_service_running():
     return result
 def _stop_display_service():
     """Stop the ledmatrix display service."""
-    result = _run_systemctl_command(['sudo', 'systemctl', 'stop', 'ledmatrix.service'])
+    result = _public_service_result(
+        _run_systemctl_command(['sudo', 'systemctl', 'stop', 'ledmatrix.service']))
     status = _get_display_service_status()
     result['active'] = status.get('active')
     result['status'] = status
@@ -712,7 +717,7 @@ def _do_transactional_uninstall(plugin_id, preserve_config):
         success = api_v3.plugin_store_manager.uninstall_plugin(plugin_id)
     except Exception as remove_err:
         _rollback()
-        return False, f"Failed to remove plugin {plugin_id}: {remove_err}"
+        return False, f"Failed to remove plugin {plugin_id} ({describe_exception(remove_err)})"
 
     if not success:
         _rollback()
diff --git a/web_interface/blueprints/api_v3/starlark.py b/web_interface/blueprints/api_v3/starlark.py
index c6d679bf..1ea32d6a 100644
--- a/web_interface/blueprints/api_v3/starlark.py
+++ b/web_interface/blueprints/api_v3/starlark.py
@@ -983,7 +983,6 @@ def stop_pixlet_editor():
                     'status': 'error',
                     'message': 'Editor force-stopped, but the display could not be '
                                'restarted automatically - start it manually.',
-                    'details': (result.get('stderr') or '').strip(),
                     'data': {'running': False}}), 500
             return jsonify({'status': 'success',
                             'message': 'Editor force-stopped; the display has been '
diff --git a/web_interface/blueprints/api_v3/system.py b/web_interface/blueprints/api_v3/system.py
index 0b22e20d..35375332 100644
--- a/web_interface/blueprints/api_v3/system.py
+++ b/web_interface/blueprints/api_v3/system.py
@@ -689,7 +689,7 @@ def execute_system_action():
                     logger.warning("install_base_requirements timed out for %s", label)
                 except OSError as install_err:
                     all_ok = False
-                    outputs.append(f"== {label} ==\nFailed: {install_err}")
+                    outputs.append(f"== {label} ==\nFailed: {describe_exception(install_err)}")
                     logger.warning("install_base_requirements errored for %s: %s", label, install_err)
             return jsonify({
                 'status': 'success' if all_ok else 'error',
@@ -784,11 +784,10 @@ def execute_system_action():
         return jsonify({'status': 'error', 'message': 'Command timed out', 'returncode': -1, 'stderr': 'timeout'})
     except Exception as e:
         logger.error("execute_system_action failed: %s", e, exc_info=True)
-        detail = describe_exception(e)
         resp = {
             'status': 'error',
-            'message': _sudo_hint_for(detail) or 'Action failed; see logs for details',
-            'details': detail,
+            'message': _sudo_hint_for(str(e)) or 'Action failed; see logs for details',
+            'details': describe_exception(e),
         }
         return jsonify(resp), 500
 @api_v3.route('/system/git-info', methods=['GET'])
diff --git a/web_interface/blueprints/api_v3/wifi.py b/web_interface/blueprints/api_v3/wifi.py
index c38f0462..8d26f989 100644
--- a/web_interface/blueprints/api_v3/wifi.py
+++ b/web_interface/blueprints/api_v3/wifi.py
@@ -67,7 +67,7 @@ def _run_background_connect(ssid, password):
         payload = _connect_result_payload(ssid, success, message)
     except Exception as e:
         logger.error("Background WiFi connect failed", exc_info=True)
-        payload = {'status': 'error', 'message': describe_exception(e)}
+        payload = {'status': 'error', 'message': f'Failed to connect to network ({describe_exception(e)})'}
     _record_connect_result(ssid, payload)
 
 
@@ -276,7 +276,7 @@ def connect_wifi():
     try:
         success, message = wifi_manager.connect_to_network(ssid, password)
     except Exception as e:
-        _record_connect_result(ssid, {'status': 'error', 'message': describe_exception(e)})
+        _record_connect_result(ssid, {'status': 'error', 'message': f'Failed to connect to network ({describe_exception(e)})'})
         raise
     payload = _connect_result_payload(ssid, success, message)
     _record_connect_result(ssid, payload)
diff --git a/web_interface/unit_refresh.py b/web_interface/unit_refresh.py
index 611a69a9..52580404 100644
--- a/web_interface/unit_refresh.py
+++ b/web_interface/unit_refresh.py
@@ -86,7 +86,7 @@ def refresh_after_update(run=None, systemd_dir=None, helper_source=None, helper_
     except Exception as e:  # a broken template must not fail the update itself
         if type(e).__name__ != 'UnitsUnreadable':
             logger.warning("Could not compare the installed systemd units with the new templates: %s", e)
-            return _result(FAILED, f'The service settings could not be checked: {e}.')
+            return _result(FAILED, 'The service settings could not be checked; see logs for details.')
         # Units installed mode 0600 (install_service.sh run on its own, before
         # it set 0644): only root can compare them, so let the helper decide.
         stale = []
@@ -110,7 +110,7 @@ def refresh_after_update(run=None, systemd_dir=None, helper_source=None, helper_
                      timeout=TIMEOUT_SECONDS)
     except (subprocess.SubprocessError, OSError) as e:
         logger.warning("Refreshing the systemd units failed: %s", e)
-        return _result(FAILED, f'Updating the service settings ({names}) failed: {e}.', stale)
+        return _result(FAILED, f'Updating the service settings ({names}) failed; see logs for details.', stale)
     if result.returncode == 0:
         # The helper says what it did: "units refreshed: a b" or "units: up to date".
         done = next((line.split(':', 1)[1].split() for line in (result.stdout or '').splitlines()