diff --git a/scripts/dev_server.py b/scripts/dev_server.py
index 7a7dd945..4372f14b 100644
--- a/scripts/dev_server.py
+++ b/scripts/dev_server.py
@@ -142,17 +142,18 @@ def find_plugin_dir(plugin_id: str) -> Optional[Path]:
 
 
 def load_config_defaults(plugin_dir: 'str | Path') -> Dict[str, Any]:
-    """Extract default values from config_schema.json."""
+    """Extract default values from config_schema.json.
+
+    The same extraction a device and the plugin harness use
+    (src/plugin_system/testing/loading.py), nested defaults included.
+    """
+    from src.plugin_system.testing.loading import (
+        load_config_defaults as _load_config_defaults,
+    )
     schema_path = resolve_under(plugin_dir, 'config_schema.json')
     if schema_path is None or not schema_path.exists():
         return {}
-    with open(schema_path, 'r') as f:
-        schema = json.load(f)
-    defaults: Dict[str, Any] = {}
-    for key, prop in schema.get('properties', {}).items():
-        if 'default' in prop:
-            defaults[key] = prop['default']
-    return defaults
+    return _load_config_defaults(schema_path.parent)
 
 
 # --------------------------------------------------------------------------
@@ -303,10 +304,13 @@ def _parse_render_request(data):
     with open(manifest_path, 'r') as f:
         manifest = json.load(f)
 
-    # Build config: schema defaults + user overrides
-    config = {'enabled': True}
-    config.update(load_config_defaults(trusted_dir))
-    config.update(data.get('config', {}))
+    # Build config the way a device would: schema defaults under a forced
+    # enabled, with the user's overrides deep-merged on top
+    from src.plugin_system.testing.loading import build_config
+    overrides = data.get('config') or {}
+    if not isinstance(overrides, dict):
+        raise ValueError('config must be a JSON object')
+    config = build_config(trusted_dir, overrides)
 
     return trusted_dir, manifest, config, data.get('mock_data', {}), data.get('skip_update', False)
 
diff --git a/src/display_controller.py b/src/display_controller.py
index 0434528e..7f5d7a66 100644
--- a/src/display_controller.py
+++ b/src/display_controller.py
@@ -2976,6 +2976,14 @@ class DisplayController:
                                        _pid: str = plugin_id, _plugin: Any = plugin_instance) -> None:
                 """Callback for plugin config changes."""
                 try:
+                    # ConfigService hands over the raw config.json section.
+                    # Prepare it as loading did (legacy booleans read as
+                    # objects, schema defaults filled in), so the plugin gets
+                    # the same shape it was constructed with.
+                    prepare = getattr(self.plugin_manager, 'prepare_plugin_config', None)
+                    prepared = prepare(_pid, new_config) if callable(prepare) else None
+                    if isinstance(prepared, dict):
+                        new_config = prepared
                     _plugin.on_config_change(new_config)
                     logger.debug("Plugin %s notified of config change", _pid)
                 except Exception as e:
diff --git a/src/plugin_system/plugin_manager.py b/src/plugin_system/plugin_manager.py
index c587acc8..43879a82 100644
--- a/src/plugin_system/plugin_manager.py
+++ b/src/plugin_system/plugin_manager.py
@@ -22,7 +22,9 @@ from src.logging_config import get_logger
 from src.plugin_system.plugin_loader import PluginLoader
 from src.plugin_system.plugin_executor import PluginExecutor
 from src.plugin_system.plugin_state import PluginStateManager, PluginState
-from src.plugin_system.schema_manager import SchemaManager, normalize_legacy_booleans
+from src.plugin_system.schema_manager import (
+    CORE_VEGAS_TUNING_KEYS, SchemaManager, normalize_legacy_booleans,
+)
 from src.common.permission_utils import (
     ensure_directory_permissions,
     get_plugin_dir_mode
@@ -369,32 +371,11 @@ class PluginManager:
                         f"The schema may be invalid. Please verify the schema file at: {schema_path}"
                     )
 
-            # A plugin that turned an on/off boolean into an {enabled, ...}
-            # object still finds the boolean in config.json until the user saves
-            # its settings form, which carries it over (plugin_config.html).
-            # Read it the same way here, before the defaults fill in the rest
-            # of the object and before schema validation, so the plugin doesn't
-            # start with a schema warning and a degraded flag. In memory only:
-            # config.json is written by saves, never by loading a plugin.
-            if schema:
-                upgraded: List[str] = []
-                config = normalize_legacy_booleans(config, schema, upgraded)
-                if upgraded:
-                    self.logger.info(
-                        "Plugin %s: reading legacy boolean setting %s as "
-                        "{\"enabled\": ...}; saving the plugin's settings "
-                        "stores the new shape",
-                        plugin_id, ", ".join(upgraded),
-                    )
-
-            # Merge config with schema defaults to ensure all defaults are applied
-            try:
-                defaults = self.schema_manager.generate_default_config(plugin_id, use_cache=True)
-                config = self.schema_manager.merge_with_defaults(config, defaults)
-                self.logger.debug(f"Merged config with schema defaults for {plugin_id}")
-            except Exception as e:
-                self.logger.warning(f"Could not apply schema defaults for {plugin_id}: {e}")
-                # Continue with original config if defaults can't be applied
+            # Legacy booleans read as objects, then schema defaults: the same
+            # preparation saves, GET /plugins/config and hot reload apply
+            # (prepare_plugin_config). In memory only: config.json is written
+            # by saves, never by loading a plugin.
+            config = self.prepare_plugin_config(plugin_id, config, schema=schema)
             
             # Use PluginLoader to load plugin
             plugin_instance, module = self.plugin_loader.load_plugin(
@@ -489,11 +470,55 @@ class PluginManager:
     #:
     #: Read by: ``vegas_mode/plugin_adapter.py`` (``vegas_width_pct``,
     #: ``vegas_overflow``) and ``base_plugin.py`` (``vegas_max_width_screens``).
-    CORE_OWNED_CONFIG_KEYS = frozenset({
-        'vegas_width_pct',
-        'vegas_overflow',
-        'vegas_max_width_screens',
-    })
+    #:
+    #: The list itself lives with the other core-owned per-plugin properties in
+    #: ``schema_manager.CORE_PLUGIN_PROPERTIES``, which the web save path also
+    #: uses to keep these keys.
+    CORE_OWNED_CONFIG_KEYS = CORE_VEGAS_TUNING_KEYS
+
+    def prepare_plugin_config(self, plugin_id: str, config: Any,
+                              schema: Optional[Dict[str, Any]] = None) -> Dict[str, Any]:
+        """The config a plugin runs with, built from its raw config.json section.
+
+        A plugin that turned an on/off boolean into an ``{enabled, ...}``
+        object still finds the boolean in config.json until its settings are
+        next saved; it is read as the object, and schema defaults fill in the
+        rest (``SchemaManager.prepare_plugin_config``). Used when loading a
+        plugin and on hot reload (DisplayController), so ``on_config_change``
+        receives the same shape the plugin was constructed with.
+
+        Never raises: on failure the legacy-boolean pass alone is applied, or
+        failing that the section is returned as it was.
+        """
+        if schema is None:
+            try:
+                schema = self.schema_manager.load_schema(plugin_id)
+            except Exception as e:
+                self.logger.debug("Could not load schema for %s: %s", plugin_id, e)
+                schema = None
+        upgraded: List[str] = []
+        try:
+            prepared = self.schema_manager.prepare_plugin_config(
+                plugin_id, config, schema=schema, changed_paths=upgraded)
+            self.logger.debug("Merged config with schema defaults for %s", plugin_id)
+        except Exception as e:
+            self.logger.warning("Could not apply schema defaults for %s: %s", plugin_id, e)
+            # Continue without defaults if they can't be applied
+            upgraded = []
+            prepared = config if isinstance(config, dict) else {}
+            if schema:
+                try:
+                    prepared = normalize_legacy_booleans(prepared, schema, upgraded)
+                except Exception:
+                    pass
+        if upgraded:
+            self.logger.info(
+                "Plugin %s: reading legacy boolean setting %s as "
+                "{\"enabled\": ...}; saving the plugin's settings "
+                "stores the new shape",
+                plugin_id, ", ".join(upgraded),
+            )
+        return prepared
 
     def _strip_core_owned_keys(self, config: Dict[str, Any]) -> Dict[str, Any]:
         """A shallow copy of ``config`` without the core's own tuning keys.
diff --git a/src/plugin_system/schema_manager.py b/src/plugin_system/schema_manager.py
index 2be222ef..d4eb70b8 100644
--- a/src/plugin_system/schema_manager.py
+++ b/src/plugin_system/schema_manager.py
@@ -95,6 +95,222 @@ def normalize_legacy_booleans(config: Any, schema: Any,
     return result
 
 
+#: Per-plugin settings the **core** owns: it reads them out of each plugin's
+#: config section, so they are allowed in every plugin's config whether or not
+#: the plugin's schema declares them. The one list for validation, for the web
+#: save filter and for the load-time checks -- a private copy is how JSON saves
+#: came to drop ``skin`` and the ``vegas_*`` keys while the validator accepted
+#: them.
+#:
+#: Values are the schema used when the plugin does not declare the property.
+CORE_PLUGIN_PROPERTIES: Dict[str, Dict[str, Any]] = {
+    # Defaults match BasePlugin behavior: enabled=True, display_duration=15,
+    # live_priority=False.
+    "enabled": {
+        "type": "boolean",
+        "default": True,
+        "description": "Enable or disable this plugin"
+    },
+    "display_duration": {
+        "type": "number",
+        "default": 15,
+        "minimum": 1,
+        "maximum": 300,
+        "description": "How long to display this plugin in seconds"
+    },
+    "live_priority": {
+        "type": "boolean",
+        "default": False,
+        "description": "Enable live priority takeover when plugin has live content"
+    },
+    # Skin selection (docs/SKIN_SYSTEM.md). Deliberately NOT an enum here:
+    # validation must keep passing when a configured skin gets uninstalled
+    # (rendering falls back to built-in). The install-dependent enum is
+    # injected only at serve time (inject_skin_selector) for the web UI
+    # dropdown.
+    "skin": {
+        "type": ["string", "object", "null"],
+        "description": "Visual skin id, or a per-mode mapping like {\"live\": \"my-skin\"}"
+    },
+    "skin_options": {
+        "type": "object",
+        "description": "Options passed through to the selected skin"
+    },
+    # Vegas tuning read by vegas_mode/plugin_adapter.py and base_plugin.py.
+    # Left untyped: the adapter validates them itself and ignores a bad
+    # value with a log line, so a stored one must never block a save.
+    "vegas_width_pct": {
+        "description": "Vegas mode: width of this plugin's card, as a percentage of the panel"
+    },
+    "vegas_overflow": {
+        "description": "Vegas mode: 'rotate' or 'truncate' when this plugin's content overflows"
+    },
+    "vegas_max_width_screens": {
+        "description": "Vegas mode: widest this plugin's card may be, in screens"
+    },
+}
+
+#: The keys of CORE_PLUGIN_PROPERTIES that are Vegas tuning rather than plugin
+#: state. PluginManager strips these before its soft validation (see
+#: PluginManager.CORE_OWNED_CONFIG_KEYS).
+CORE_VEGAS_TUNING_KEYS = frozenset({
+    'vegas_width_pct', 'vegas_overflow', 'vegas_max_width_screens',
+})
+
+
+def with_core_plugin_properties(schema: Dict[str, Any]) -> Dict[str, Any]:
+    """A deep copy of a plugin schema with CORE_PLUGIN_PROPERTIES allowed.
+
+    Properties the plugin declares itself are left as declared. Core
+    properties are removed from ``required``: they are system-managed.
+    """
+    enhanced = copy.deepcopy(schema) if isinstance(schema, dict) else {}
+    properties = enhanced.setdefault("properties", {})
+    for name, definition in CORE_PLUGIN_PROPERTIES.items():
+        if name not in properties:
+            properties[name] = copy.deepcopy(definition)
+    if "required" in enhanced:
+        enhanced["required"] = [field for field in enhanced["required"]
+                                if field not in CORE_PLUGIN_PROPERTIES]
+    return enhanced
+
+
+def extract_schema_defaults(schema: Dict[str, Any]) -> Dict[str, Any]:
+    """Default values of a JSON Schema's properties, recursively.
+
+    A property's own ``default`` wins; otherwise a nested object contributes
+    its children's defaults, and an array contributes ``[]`` (or a one-item
+    list of its ``items`` default). This is what a device runs with, so the
+    dev tools use it too (src/plugin_system/testing/loading.py).
+    """
+    defaults: Dict[str, Any] = {}
+    properties = schema.get('properties', {}) if isinstance(schema, dict) else {}
+    if not isinstance(properties, dict):
+        return defaults
+
+    for key, prop_schema in properties.items():
+        if not isinstance(prop_schema, dict):
+            continue
+        # If property has a default, use it
+        if 'default' in prop_schema:
+            defaults[key] = prop_schema['default']
+            continue
+
+        # Handle nested objects
+        if prop_schema.get('type') == 'object' and 'properties' in prop_schema:
+            nested_defaults = extract_schema_defaults(prop_schema)
+            if nested_defaults:
+                defaults[key] = nested_defaults
+
+        # Handle arrays with object items
+        elif prop_schema.get('type') == 'array' and 'items' in prop_schema:
+            items_schema = prop_schema['items']
+            if items_schema.get('type') == 'object' and 'properties' in items_schema:
+                # For arrays of objects, use empty array as default
+                # Individual objects will use their defaults when created
+                defaults[key] = []
+            elif 'default' in items_schema:
+                # Array with default item value
+                defaults[key] = [items_schema['default']]
+            else:
+                # Empty array as default
+                defaults[key] = []
+
+        # For other types without defaults, don't add to defaults dict
+        # This allows plugins to handle missing values as needed
+
+    return defaults
+
+
+def plugin_config_defaults(schema: Optional[Dict[str, Any]]) -> Dict[str, Any]:
+    """Every default a plugin's config gets: the schema's plus the core ones.
+
+    A plugin with no schema gets the minimal ``enabled: False,
+    display_duration: 15``. Device location is not applied here; that needs a
+    config manager (SchemaManager.generate_default_config).
+    """
+    if not schema:
+        return {
+            'enabled': False,
+            'display_duration': 15
+        }
+
+    defaults = extract_schema_defaults(schema)
+
+    # Ensure core properties have defaults (they may not be in the schema)
+    # These match BasePlugin behavior
+    for name in ('enabled', 'display_duration', 'live_priority'):
+        if name not in defaults:
+            defaults[name] = CORE_PLUGIN_PROPERTIES[name]['default']
+    return defaults
+
+
+def merge_config_defaults(config: Dict[str, Any], defaults: Dict[str, Any]) -> Dict[str, Any]:
+    """Merge configuration with defaults, preserving user values.
+
+    Also replaces None values with defaults so a config never starts with
+    None where a default exists. Neither argument is mutated.
+    """
+    merged = copy.deepcopy(defaults)
+
+    def deep_merge(target: Dict[str, Any], source: Dict[str, Any], default_dict: Dict[str, Any]) -> None:
+        """Recursively merge source into target, replacing None with defaults."""
+        for key, value in source.items():
+            default_value = default_dict.get(key)
+
+            if key in target and isinstance(target[key], dict) and isinstance(value, dict):
+                # Both are dicts, recursively merge
+                if isinstance(default_value, dict):
+                    deep_merge(target[key], value, default_value)
+                else:
+                    deep_merge(target[key], value, {})
+            elif value is None and default_value is not None:
+                # Value is None and we have a default, use the default
+                target[key] = copy.deepcopy(default_value) if isinstance(default_value, (dict, list)) else default_value
+            else:
+                # Normal merge: user value takes precedence (copy if dict/list)
+                if isinstance(value, (dict, list)):
+                    target[key] = copy.deepcopy(value)
+                else:
+                    target[key] = value
+
+    deep_merge(merged, config, defaults)
+
+    # Final pass: replace any remaining None values at any level with defaults
+    def replace_none_with_defaults(target: Dict[str, Any], default_dict: Dict[str, Any]) -> None:
+        """Recursively replace None values with defaults."""
+        for key in list(target.keys()):
+            value = target[key]
+            default_value = default_dict.get(key)
+
+            if value is None and default_value is not None:
+                # Replace None with default
+                target[key] = copy.deepcopy(default_value) if isinstance(default_value, (dict, list)) else default_value
+            elif isinstance(value, dict) and isinstance(default_value, dict):
+                # Recursively process nested dicts
+                replace_none_with_defaults(value, default_value)
+
+    replace_none_with_defaults(merged, defaults)
+    return merged
+
+
+def prepare_plugin_config(config: Any, schema: Optional[Dict[str, Any]],
+                          defaults: Dict[str, Any],
+                          changed_paths: Optional[List[str]] = None) -> Dict[str, Any]:
+    """The config a plugin runs with, from its stored (or submitted) section.
+
+    Legacy booleans are read as ``{"enabled": ...}`` objects
+    (normalize_legacy_booleans), then schema defaults fill in whatever is
+    missing. Loading a plugin, both config saves, GET /plugins/config, hot
+    reload and the dev tools all go through this, so a plugin sees the same
+    shape however its config reached it.
+    """
+    config = config if isinstance(config, dict) else {}
+    if schema:
+        config = normalize_legacy_booleans(config, schema, changed_paths)
+    return merge_config_defaults(config, defaults)
+
+
 class SchemaManager:
     """
     Manages plugin configuration schemas with caching and validation.
@@ -264,57 +480,12 @@ class SchemaManager:
     def extract_defaults_from_schema(self, schema: Dict[str, Any], prefix: str = '') -> Dict[str, Any]:
         """
         Recursively extract default values from a JSON Schema.
-        
-        Handles nested objects, arrays, and all schema types.
-        
-        Args:
-            schema: JSON Schema dictionary
-            prefix: Optional prefix for logging/debugging
-            
-        Returns:
-            Dictionary of default values
+
+        See :func:`extract_schema_defaults`; ``prefix`` is accepted for
+        compatibility and unused.
         """
-        defaults = {}
-        
-        # Handle schema with properties
-        properties = schema.get('properties', {})
-        if not properties:
-            return defaults
-        
-        for key, prop_schema in properties.items():
-            field_path = f"{prefix}.{key}" if prefix else key
-            
-            # If property has a default, use it
-            if 'default' in prop_schema:
-                defaults[key] = prop_schema['default']
-                self.logger.debug(f"Found default for {field_path}: {prop_schema['default']}")
-                continue
-            
-            # Handle nested objects
-            if prop_schema.get('type') == 'object' and 'properties' in prop_schema:
-                nested_defaults = self.extract_defaults_from_schema(prop_schema, field_path)
-                if nested_defaults:
-                    defaults[key] = nested_defaults
-            
-            # Handle arrays with object items
-            elif prop_schema.get('type') == 'array' and 'items' in prop_schema:
-                items_schema = prop_schema['items']
-                if items_schema.get('type') == 'object' and 'properties' in items_schema:
-                    # For arrays of objects, use empty array as default
-                    # Individual objects will use their defaults when created
-                    defaults[key] = []
-                elif 'default' in items_schema:
-                    # Array with default item value
-                    defaults[key] = [items_schema['default']]
-                else:
-                    # Empty array as default
-                    defaults[key] = []
-            
-            # For other types without defaults, don't add to defaults dict
-            # This allows plugins to handle missing values as needed
-        
-        return defaults
-    
+        return extract_schema_defaults(schema)
+
     def get_device_location(self) -> Optional[Dict[str, Any]]:
         """
         Return the device-wide ``location`` block from config.json, or None.
@@ -393,31 +564,39 @@ class SchemaManager:
         schema = self.load_schema(plugin_id, use_cache=use_cache)
         if not schema:
             # Return minimal defaults if no schema
-            return {
-                'enabled': False,
-                'display_duration': 15
-            }
-        
-        # Extract defaults from schema
-        defaults = self.extract_defaults_from_schema(schema)
-        
-        # Ensure core properties have defaults (they may not be in the schema)
-        # These match BasePlugin behavior
-        if 'enabled' not in defaults:
-            defaults['enabled'] = schema.get('properties', {}).get('enabled', {}).get('default', True)
-        
-        if 'display_duration' not in defaults:
-            defaults['display_duration'] = schema.get('properties', {}).get('display_duration', {}).get('default', 15)
-        
-        if 'live_priority' not in defaults:
-            defaults['live_priority'] = schema.get('properties', {}).get('live_priority', {}).get('default', False)
-        
+            return plugin_config_defaults(None)
+
+        # Schema defaults plus the core properties' (they may not be in the
+        # schema)
+        defaults = plugin_config_defaults(schema)
+
         # Cache the defaults *before* the device location is layered on, so a
         # later change to the device location is picked up by the next call.
         self._defaults_cache[plugin_id] = defaults.copy()
         
         return self.apply_device_location(defaults)
     
+    def prepare_plugin_config(self, plugin_id: str, config: Any,
+                              schema: Optional[Dict[str, Any]] = None,
+                              changed_paths: Optional[List[str]] = None) -> Dict[str, Any]:
+        """
+        The config a plugin runs with: see :func:`prepare_plugin_config`.
+
+        Args:
+            plugin_id: Plugin identifier
+            config: The plugin's stored or submitted config section
+            schema: The plugin's schema, when the caller already has it
+            changed_paths: Receives the dotted path of each legacy boolean
+                read as an object
+
+        Returns:
+            A new dict; ``config`` is not mutated
+        """
+        if schema is None:
+            schema = self.load_schema(plugin_id, use_cache=True)
+        defaults = self.generate_default_config(plugin_id, use_cache=True)
+        return prepare_plugin_config(config, schema, defaults, changed_paths)
+
     def validate_config_against_schema(self, config: Dict[str, Any], schema: Dict[str, Any], 
                                       plugin_id: Optional[str] = None) -> Tuple[bool, List[str]]:
         """
@@ -438,80 +617,18 @@ class SchemaManager:
         errors = []
         
         try:
-            # Core plugin properties that should always be allowed
-            # These are handled by the base plugin system and should not cause validation failures
-            # Defaults match BasePlugin behavior: enabled=True, display_duration=15, live_priority=False
-            core_properties = {
-                "enabled": {
-                    "type": "boolean",
-                    "default": True,
-                    "description": "Enable or disable this plugin"
-                },
-                "display_duration": {
-                    "type": "number",
-                    "default": 15,
-                    "minimum": 1,
-                    "maximum": 300,
-                    "description": "How long to display this plugin in seconds"
-                },
-                "live_priority": {
-                    "type": "boolean",
-                    "default": False,
-                    "description": "Enable live priority takeover when plugin has live content"
-                },
-                # Skin selection (docs/SKIN_SYSTEM.md). Deliberately NOT an
-                # enum here: validation must keep passing when a configured
-                # skin gets uninstalled (rendering falls back to built-in).
-                # The install-dependent enum is injected only at serve time
-                # (inject_skin_selector) for the web UI dropdown.
-                "skin": {
-                    "type": ["string", "object", "null"],
-                    "description": "Visual skin id, or a per-mode mapping like {\"live\": \"my-skin\"}"
-                },
-                "skin_options": {
-                    "type": "object",
-                    "description": "Options passed through to the selected skin"
-                }
-            }
-            
-            # Create a deep copy of the schema to modify (to avoid mutating the original)
-            enhanced_schema = copy.deepcopy(schema)
-            if "properties" not in enhanced_schema:
-                enhanced_schema["properties"] = {}
-            
-            # Inject core properties if they're not already defined in the schema
-            # This ensures core properties are always allowed even if not in the plugin's schema
-            properties_added = []
-            for prop_name, prop_def in core_properties.items():
-                if prop_name not in enhanced_schema["properties"]:
-                    enhanced_schema["properties"][prop_name] = copy.deepcopy(prop_def)
-                    properties_added.append(prop_name)
-            
-            # Log if we added any core properties (for debugging)
-            if properties_added and plugin_id:
+            # Core plugin properties (CORE_PLUGIN_PROPERTIES) are handled by
+            # the base plugin system and should not cause validation failures:
+            # they are allowed even when the plugin's schema doesn't declare
+            # them, and never required.
+            enhanced_schema = with_core_plugin_properties(schema)
+            if plugin_id:
+                declared = schema.get("properties", {}) if isinstance(schema, dict) else {}
                 self.logger.debug(
-                    f"Injected core properties into schema for {plugin_id}: {properties_added}"
+                    "Injected core properties into schema for %s: %s", plugin_id,
+                    [name for name in CORE_PLUGIN_PROPERTIES if name not in declared]
                 )
-            
-            # Remove core properties from required array (they're system-managed)
-            # Core properties should be allowed but not required for validation
-            if "required" in enhanced_schema:
-                core_prop_names = list(core_properties.keys())
-                removed_from_required = [
-                    field for field in enhanced_schema["required"]
-                    if field in core_prop_names
-                ]
-                enhanced_schema["required"] = [
-                    field for field in enhanced_schema["required"] 
-                    if field not in core_prop_names
-                ]
-                
-                # Log if we removed any core properties from required (for debugging)
-                if removed_from_required and plugin_id:
-                    self.logger.debug(
-                        f"Removed core properties from required array for {plugin_id}: {removed_from_required}"
-                    )
-            
+
             # Create validator with enhanced schema
             validator = Draft7Validator(enhanced_schema)
             
@@ -628,55 +745,15 @@ class SchemaManager:
         """
         Merge configuration with defaults, preserving user values.
         Also replaces None values with defaults to ensure config never has None from the start.
-        
+
         Args:
             config: User configuration
             defaults: Default values from schema
-            
+
         Returns:
             Merged configuration with defaults applied where missing or None
         """
-        merged = copy.deepcopy(defaults)
-        
-        def deep_merge(target: Dict[str, Any], source: Dict[str, Any], default_dict: Dict[str, Any]) -> None:
-            """Recursively merge source into target, replacing None with defaults."""
-            for key, value in source.items():
-                default_value = default_dict.get(key)
-                
-                if key in target and isinstance(target[key], dict) and isinstance(value, dict):
-                    # Both are dicts, recursively merge
-                    if isinstance(default_value, dict):
-                        deep_merge(target[key], value, default_value)
-                    else:
-                        deep_merge(target[key], value, {})
-                elif value is None and default_value is not None:
-                    # Value is None and we have a default, use the default
-                    target[key] = copy.deepcopy(default_value) if isinstance(default_value, (dict, list)) else default_value
-                else:
-                    # Normal merge: user value takes precedence (copy if dict/list)
-                    if isinstance(value, (dict, list)):
-                        target[key] = copy.deepcopy(value)
-                    else:
-                        target[key] = value
-        
-        deep_merge(merged, config, defaults)
-        
-        # Final pass: replace any remaining None values at any level with defaults
-        def replace_none_with_defaults(target: Dict[str, Any], default_dict: Dict[str, Any]) -> None:
-            """Recursively replace None values with defaults."""
-            for key in list(target.keys()):
-                value = target[key]
-                default_value = default_dict.get(key)
-                
-                if value is None and default_value is not None:
-                    # Replace None with default
-                    target[key] = copy.deepcopy(default_value) if isinstance(default_value, (dict, list)) else default_value
-                elif isinstance(value, dict) and isinstance(default_value, dict):
-                    # Recursively process nested dicts
-                    replace_none_with_defaults(value, default_value)
-        
-        replace_none_with_defaults(merged, defaults)
-        return merged
+        return merge_config_defaults(config, defaults)
 
     def detect_config_key_collisions(
         self,
diff --git a/src/plugin_system/testing/harness.py b/src/plugin_system/testing/harness.py
index 267774c2..90351542 100644
--- a/src/plugin_system/testing/harness.py
+++ b/src/plugin_system/testing/harness.py
@@ -27,7 +27,7 @@ from PIL import Image, ImageChops
 
 from src.logging_config import get_logger
 from .bounds_display_manager import BoundsCheckingDisplayManager
-from .loading import load_config_defaults, load_manifest, merge_config
+from .loading import build_config, load_manifest
 from .sizes import DEFAULT_TEST_SIZES, safe_mode_filename, size_label
 
 logger = get_logger("[Plugin Harness]")
@@ -305,9 +305,7 @@ def render_plugin_matrix(
     manifest = load_manifest(plugin_dir)
     # Start from config_schema.json defaults so the plugin behaves like a real
     # install; explicit caller config still wins over a schema default.
-    config = merge_config(
-        merge_config({"enabled": True}, load_config_defaults(plugin_dir)),
-        config or {})
+    config = build_config(plugin_dir, config)
     sizes = sizes or DEFAULT_TEST_SIZES
     results: List[RenderResult] = []
 
diff --git a/src/plugin_system/testing/loading.py b/src/plugin_system/testing/loading.py
index a7992231..2a0dd8e0 100644
--- a/src/plugin_system/testing/loading.py
+++ b/src/plugin_system/testing/loading.py
@@ -42,29 +42,6 @@ def load_manifest(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
         return json.load(f)
 
 
-def _defaults_from_properties(properties: Dict[str, Any]) -> Dict[str, Any]:
-    """Defaults for one `properties` block, recursing into nested objects.
-
-    An object property carries its defaults on its children, not on itself, so
-    reading only the top level dropped everything nested. That is most of the
-    fleet: config organised by league, or under customization/display_options,
-    lost 2,386 defaults across 37 of 44 plugins -- soccer-scoreboard alone lost
-    539 of 565 -- and the harness rendered them with a config no install would
-    ever have.
-    """
-    defaults: Dict[str, Any] = {}
-    for key, prop in (properties or {}).items():
-        if not isinstance(prop, dict):
-            continue
-        if prop.get('type') == 'object' and isinstance(prop.get('properties'), dict):
-            nested = _defaults_from_properties(prop['properties'])
-            if nested:
-                defaults[key] = nested
-        elif 'default' in prop:
-            defaults[key] = prop['default']
-    return defaults
-
-
 def merge_config(base: Dict[str, Any], override: Dict[str, Any]) -> Dict[str, Any]:
     """Deep-merge override onto base, without dropping sibling defaults.
 
@@ -81,14 +58,45 @@ def merge_config(base: Dict[str, Any], override: Dict[str, Any]) -> Dict[str, An
     return merged
 
 
-def load_config_defaults(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
-    """Extract default values from a plugin's config_schema.json (empty if none)."""
+def load_schema(plugin_dir: Union[str, Path]) -> Optional[Dict[str, Any]]:
+    """A plugin's config_schema.json, or None when it has none."""
     schema_path = Path(plugin_dir) / 'config_schema.json'
     if not schema_path.exists():
-        return {}
+        return None
     with open(schema_path, 'r', encoding='utf-8') as f:
-        schema = json.load(f)
-    return _defaults_from_properties(schema.get('properties', {}))
+        return json.load(f)
+
+
+def load_config_defaults(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
+    """Default values from a plugin's config_schema.json (empty if none).
+
+    The device's own extraction (schema_manager.extract_schema_defaults), so a
+    harness run starts from the config an install would have: nested objects
+    contribute their children's defaults (organised-by-league configs lost
+    thousands of them when only the top level was read), and arrays without a
+    default start as [].
+    """
+    from src.plugin_system.schema_manager import extract_schema_defaults
+    schema = load_schema(plugin_dir)
+    return extract_schema_defaults(schema) if schema else {}
+
+
+def build_config(plugin_dir: Union[str, Path],
+                 overrides: Optional[Dict[str, Any]] = None) -> Dict[str, Any]:
+    """The config a device would give this plugin, with ``overrides`` applied.
+
+    Starts from a forced ``enabled: True``, deep-merges ``overrides`` onto it
+    (merge_config), then prepares the result exactly as the device does when it
+    loads a plugin: legacy booleans read as objects, and schema plus core
+    defaults filled in (schema_manager.prepare_plugin_config). Used by the
+    harness, check_plugin, render_plugin and the dev preview server.
+    """
+    from src.plugin_system.schema_manager import (
+        plugin_config_defaults, prepare_plugin_config,
+    )
+    schema = load_schema(plugin_dir)
+    requested = merge_config({"enabled": True}, overrides or {})
+    return prepare_plugin_config(requested, schema, plugin_config_defaults(schema))
 
 
 def load_harness_spec(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
@@ -143,16 +151,14 @@ def build_full_config(
 
     Merge order: config_schema.json defaults, then a forced ``enabled: True``,
     then harness.json's config overlay, then the caller's explicit config --
-    most specific wins. `enabled` is re-asserted *after* the schema defaults
-    so a plugin that reasonably ships `enabled: false` (e.g. a seasonal or
-    opt-in plugin) can't silently make every harness run test "disabled, do
-    nothing" by accident -- callers that genuinely want to test the disabled
-    path can still do so via `cli_config={"enabled": False}`.
+    most specific wins, and each layer deep-merges (a nested override such as
+    ``{"nhl": {"enabled": true}}`` keeps the other nhl defaults). `enabled` is
+    asserted over the schema defaults so a plugin that reasonably ships
+    `enabled: false` (e.g. a seasonal or opt-in plugin) can't silently make
+    every harness run test "disabled, do nothing" by accident -- callers that
+    genuinely want to test the disabled path can still do so via
+    `cli_config={"enabled": False}`. See build_config.
     """
     spec = spec or {}
-    config: Dict[str, Any] = {}
-    config.update(load_config_defaults(plugin_dir))
-    config["enabled"] = True
-    config.update(spec.get("config", {}))
-    config.update(cli_config or {})
-    return config
+    overrides = merge_config(spec.get("config", {}) or {}, cli_config or {})
+    return build_config(plugin_dir, overrides)
diff --git a/test/test_plugin_config_preparation.py b/test/test_plugin_config_preparation.py
new file mode 100644
index 00000000..e1ffb6bb
--- /dev/null
+++ b/test/test_plugin_config_preparation.py
@@ -0,0 +1,205 @@
+"""One preparation for a plugin's config, wherever the config comes from.
+
+A plugin's stored section becomes the config it runs with through
+schema_manager.prepare_plugin_config: legacy booleans (#588) read as
+``{"enabled": ...}`` objects, then schema and core defaults filled in. Loading
+did that; hot reload handed plugins the raw section instead (a legacy
+``dynamic_duration: true`` came back as a boolean, turning dynamic duration
+off), and the dev tools each built configs their own way:
+
+- dev_server read only top-level defaults and let a schema ``enabled: false``
+  override its forced ``enabled: True``;
+- check_plugin/render_plugin (build_full_config) merged overrides with
+  dict.update, so ``{"nhl": {"enabled": true}}`` dropped every other nhl
+  default;
+- the harness and the device disagreed on object and array defaults.
+
+The web saves and GET are covered in
+test/web_interface/test_plugin_config_json_saves.py.
+"""
+
+import importlib.util
+import json
+from pathlib import Path
+from unittest.mock import MagicMock
+
+import pytest
+
+from src.plugin_system.plugin_manager import PluginManager
+from src.plugin_system.schema_manager import (
+    CORE_PLUGIN_PROPERTIES, SchemaManager, extract_schema_defaults,
+)
+from src.plugin_system.testing import loading
+
+REPO = Path(__file__).resolve().parent.parent
+
+SCHEMA = {
+    "type": "object",
+    "properties": {
+        "enabled": {"type": "boolean", "default": False},
+        "global": {
+            "type": "object",
+            "properties": {
+                "dynamic_duration": {
+                    "type": "object",
+                    "properties": {
+                        "enabled": {"type": "boolean", "default": True},
+                        "max_duration_seconds": {"type": "integer", "default": 300},
+                    },
+                },
+            },
+        },
+        "nhl": {
+            "type": "object",
+            "properties": {
+                "enabled": {"type": "boolean", "default": False},
+                "favorite_teams": {"type": "array", "default": ["TB"]},
+                "show_records": {"type": "boolean", "default": True},
+            },
+        },
+        "colors": {
+            "type": "object",
+            "default": {"text": [255, 255, 255]},
+            "properties": {"text": {"type": "array", "default": [1, 2, 3]}},
+        },
+        "feeds": {"type": "array", "items": {"type": "string"}},
+    },
+}
+
+
+@pytest.fixture
+def plugin_dir(tmp_path):
+    pdir = tmp_path / "plugins" / "demo"
+    pdir.mkdir(parents=True)
+    (pdir / "config_schema.json").write_text(json.dumps(SCHEMA))
+    (pdir / "manifest.json").write_text(json.dumps({"id": "demo"}))
+    return pdir
+
+
+@pytest.fixture
+def plugin_manager(plugin_dir, tmp_path):
+    manager = PluginManager.__new__(PluginManager)  # skip the heavy constructor
+    manager.logger = MagicMock()
+    manager.schema_manager = SchemaManager(plugins_dir=plugin_dir.parent, project_root=tmp_path)
+    return manager
+
+
+class TestPluginManagerPreparation:
+    def test_legacy_boolean_and_defaults(self, plugin_manager):
+        prepared = plugin_manager.prepare_plugin_config(
+            "demo", {"enabled": True, "global": {"dynamic_duration": True}})
+        assert prepared["global"]["dynamic_duration"] == {
+            "enabled": True, "max_duration_seconds": 300}
+        assert prepared["nhl"]["favorite_teams"] == ["TB"]
+        assert prepared["display_duration"] == 15
+
+    def test_does_not_mutate_the_raw_section(self, plugin_manager):
+        raw = {"global": {"dynamic_duration": True}}
+        plugin_manager.prepare_plugin_config("demo", raw)
+        assert raw == {"global": {"dynamic_duration": True}}
+
+    def test_never_raises(self, plugin_manager):
+        plugin_manager.schema_manager = MagicMock()
+        plugin_manager.schema_manager.load_schema.return_value = SCHEMA
+        plugin_manager.schema_manager.prepare_plugin_config.side_effect = RuntimeError("boom")
+        prepared = plugin_manager.prepare_plugin_config("demo", {"global": {"dynamic_duration": False}})
+        assert prepared["global"]["dynamic_duration"] == {"enabled": False}
+
+
+class TestHotReload:
+    def test_on_config_change_gets_the_prepared_config(self, test_display_controller, plugin_manager):
+        controller = test_display_controller
+        plugin = MagicMock()
+        plugin.modes = ["demo"]
+        pm = controller.plugin_manager
+        pm.discover_plugins.return_value = ["demo"]
+        pm.load_plugin.return_value = True
+        pm.plugin_manifests = {}
+        pm.get_plugin.side_effect = lambda pid: plugin if pid == "demo" else None
+        pm.prepare_plugin_config.side_effect = plugin_manager.prepare_plugin_config
+        controller.config_service.get_config = lambda: {"demo": {"enabled": True}}
+        controller._reconcile_enabled_plugins()
+
+        callback = controller._plugin_config_callbacks["demo"]
+        callback({"enabled": True}, {"enabled": True, "global": {"dynamic_duration": True}})
+
+        new_config = plugin.on_config_change.call_args[0][0]
+        assert new_config["global"]["dynamic_duration"] == {
+            "enabled": True, "max_duration_seconds": 300}
+        assert new_config["nhl"]["show_records"] is True
+
+    def test_raw_section_still_delivered_without_a_preparer(self, test_display_controller):
+        controller = test_display_controller
+        plugin = MagicMock()
+        plugin.modes = ["demo"]
+        pm = controller.plugin_manager
+        pm.discover_plugins.return_value = ["demo"]
+        pm.load_plugin.return_value = True
+        pm.plugin_manifests = {}
+        pm.get_plugin.side_effect = lambda pid: plugin if pid == "demo" else None
+        pm.prepare_plugin_config.return_value = None
+        controller.config_service.get_config = lambda: {"demo": {"enabled": True}}
+        controller._reconcile_enabled_plugins()
+
+        raw = {"enabled": False}
+        controller._plugin_config_callbacks["demo"]({}, raw)
+        plugin.on_config_change.assert_called_once_with(raw)
+
+
+class TestDevToolsMatchTheDevice:
+    def test_harness_defaults_are_the_device_defaults(self, plugin_dir):
+        assert loading.load_config_defaults(plugin_dir) == extract_schema_defaults(SCHEMA)
+        defaults = loading.load_config_defaults(plugin_dir)
+        # An object's own default wins, as on a device; arrays start empty
+        assert defaults["colors"] == {"text": [255, 255, 255]}
+        assert defaults["feeds"] == []
+
+    def test_nested_override_keeps_sibling_defaults(self, plugin_dir):
+        config = loading.build_full_config(plugin_dir, cli_config={"nhl": {"enabled": True}})
+        assert config["nhl"] == {"enabled": True, "favorite_teams": ["TB"], "show_records": True}
+
+    def test_spec_and_cli_overrides_both_deep_merge(self, plugin_dir):
+        config = loading.build_full_config(
+            plugin_dir, spec={"config": {"nhl": {"show_records": False}}},
+            cli_config={"nhl": {"enabled": True}})
+        assert config["nhl"] == {"enabled": True, "favorite_teams": ["TB"], "show_records": False}
+
+    def test_build_config_matches_the_device_load(self, plugin_dir, plugin_manager):
+        overrides = {"enabled": True, "global": {"dynamic_duration": False}}
+        assert loading.build_config(plugin_dir, overrides) == \
+            plugin_manager.prepare_plugin_config("demo", overrides)
+
+    def test_core_defaults_are_present(self, plugin_dir):
+        config = loading.build_full_config(plugin_dir)
+        assert config["enabled"] is True
+        assert config["display_duration"] == CORE_PLUGIN_PROPERTIES["display_duration"]["default"]
+        assert config["live_priority"] is False
+
+    def test_render_plugin_matrix_config(self, plugin_dir, monkeypatch):
+        from src.plugin_system.testing import harness
+        seen = {}
+
+        def fake_render_size(plugin_id, manifest, pdir, config, *args, **kwargs):
+            seen["config"] = config
+            return []
+
+        monkeypatch.setattr(harness, "_render_size", fake_render_size)
+        harness.render_plugin_matrix("demo", plugin_dir, config={"nhl": {"enabled": True}},
+                                     sizes=[(64, 32)], run_update=False)
+        assert seen["config"]["enabled"] is True, "a schema enabled:false must not win"
+        assert seen["config"]["nhl"]["favorite_teams"] == ["TB"]
+
+    def test_dev_server_render_config(self, plugin_dir, monkeypatch):
+        spec = importlib.util.spec_from_file_location("dev_server_under_test",
+                                                      REPO / "scripts" / "dev_server.py")
+        dev_server = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(dev_server)
+        monkeypatch.setattr(dev_server, "find_plugin_dir", lambda pid: plugin_dir)
+        monkeypatch.setattr(dev_server, "_trusted_plugin_dir", lambda d: plugin_dir)
+
+        _, _, config, _, _ = dev_server._parse_render_request(
+            {"plugin_id": "demo", "config": {"nhl": {"enabled": True}}})
+        assert config["enabled"] is True, "a schema enabled:false must not win"
+        assert config["nhl"] == {"enabled": True, "favorite_teams": ["TB"], "show_records": True}
+        assert config["global"]["dynamic_duration"]["max_duration_seconds"] == 300
+        assert dev_server.load_config_defaults(plugin_dir) == extract_schema_defaults(SCHEMA)
diff --git a/test/web_interface/test_plugin_config_json_saves.py b/test/web_interface/test_plugin_config_json_saves.py
new file mode 100644
index 00000000..5d0c50f6
--- /dev/null
+++ b/test/web_interface/test_plugin_config_json_saves.py
@@ -0,0 +1,258 @@
+"""JSON plugin-config saves store what the device would load, and only what was sent.
+
+Runs a real ConfigManager and SchemaManager over tmp_path, like
+test_api_v3_secret_roundtrip.py, so assertions are on config.json itself.
+
+- POST /plugins/config with a partial ``config`` reset every unsent setting to
+  its schema default: the JSON path built on defaults, not the stored section.
+- Legacy booleans (#588) were normalized only at load, so posting back what
+  GET /plugins/config returned failed validation.
+- The JSON save's filter kept only enabled/display_duration/live_priority, so
+  a submitted skin, skin_options or vegas_* tuning key was silently dropped.
+- Plugin sections posted to /config/main skipped all of that and were stored
+  verbatim, including values /plugins/config rejects.
+"""
+
+import json
+from unittest.mock import MagicMock
+
+import pytest
+from flask import Flask
+
+from src.config_manager import ConfigManager
+from src.plugin_system.schema_manager import SchemaManager
+from web_interface.blueprints.api_v3 import api_v3
+
+PLUGIN_ID = "demo"
+
+SCHEMA = {
+    "type": "object",
+    "additionalProperties": False,
+    "properties": {
+        "enabled": {"type": "boolean", "default": True},
+        "city": {"type": "string", "default": "Dallas"},
+        "update_interval": {"type": "integer", "default": 300, "minimum": 30},
+        "api_key": {"type": "string", "x-secret": True, "default": ""},
+        "display": {
+            "type": "object",
+            "additionalProperties": False,
+            "properties": {
+                "brightness": {"type": "integer", "default": 50},
+                "show_icons": {"type": "boolean", "default": True},
+            },
+        },
+        "global": {
+            "type": "object",
+            "additionalProperties": False,
+            "properties": {
+                "dynamic_duration": {
+                    "type": "object",
+                    "additionalProperties": False,
+                    "properties": {
+                        "enabled": {"type": "boolean", "default": True},
+                        "min_duration_seconds": {"type": "integer", "default": 30},
+                    },
+                },
+            },
+        },
+    },
+}
+
+STORED = {
+    "enabled": True,
+    "city": "Paris",
+    "update_interval": 600,
+    "display": {"brightness": 80, "show_icons": False},
+    "global": {"dynamic_duration": {"enabled": False, "min_duration_seconds": 45}},
+    "vegas_width_pct": 60,
+    "skin": "neon",
+}
+
+_ATTRS = ('config_manager', 'plugin_manager', 'plugin_store_manager',
+          'plugin_state_manager', 'saved_repositories_manager', 'schema_manager',
+          'operation_queue', 'operation_history', 'cache_manager')
+
+
+@pytest.fixture
+def env(tmp_path):
+    config_file = tmp_path / "config.json"
+    secrets_file = tmp_path / "config_secrets.json"
+    plugins_dir = tmp_path / "plugins"
+    plugin_dir = plugins_dir / PLUGIN_ID
+    plugin_dir.mkdir(parents=True)
+    (plugin_dir / "config_schema.json").write_text(json.dumps(SCHEMA))
+    (plugin_dir / "manifest.json").write_text(json.dumps({"id": PLUGIN_ID}))
+
+    sentinel = object()
+    originals = {name: getattr(api_v3, name, sentinel) for name in _ATTRS}
+
+    config_manager = ConfigManager(config_path=str(config_file),
+                                   secrets_path=str(secrets_file))
+    config_manager.template_path = str(tmp_path / "no-template.json")
+    plugin_manager = MagicMock()
+    plugin_manager.plugin_manifests = {PLUGIN_ID: {"id": PLUGIN_ID}}
+    plugin_manager.plugins_dir = plugins_dir
+    plugin_manager.get_plugin.return_value = None
+
+    for name in _ATTRS:
+        setattr(api_v3, name, MagicMock())
+    api_v3.config_manager = config_manager
+    api_v3.schema_manager = SchemaManager(plugins_dir=plugins_dir, project_root=tmp_path)
+    api_v3.plugin_manager = plugin_manager
+    api_v3.operation_queue = None
+
+    app = Flask(__name__)
+    app.config["TESTING"] = True
+    app.register_blueprint(api_v3, url_prefix="/api/v3")
+
+    class Env:
+        client = app.test_client()
+
+        @staticmethod
+        def store(section):
+            config_file.write_text(json.dumps({"timezone": "UTC", PLUGIN_ID: section}))
+
+        @staticmethod
+        def main():
+            return json.loads(config_file.read_text())
+
+        @staticmethod
+        def stored():
+            return json.loads(config_file.read_text())[PLUGIN_ID]
+
+        @staticmethod
+        def secrets():
+            return json.loads(secrets_file.read_text()) if secrets_file.exists() else {}
+
+        @staticmethod
+        def save(config):
+            return app.test_client().post("/api/v3/plugins/config",
+                                          json={"plugin_id": PLUGIN_ID, "config": config})
+
+    Env.store(json.loads(json.dumps(STORED)))
+    yield Env
+
+    for name, original in originals.items():
+        if original is sentinel:
+            if hasattr(api_v3, name):
+                delattr(api_v3, name)
+        else:
+            setattr(api_v3, name, original)
+
+
+class TestPartialJsonSave:
+    def test_unsent_settings_keep_their_values(self, env):
+        resp = env.save({"enabled": True})
+        assert resp.status_code == 200, resp.get_json()
+        stored = env.stored()
+        assert stored["city"] == "Paris"
+        assert stored["update_interval"] == 600
+        assert stored["display"] == {"brightness": 80, "show_icons": False}
+        assert stored["global"]["dynamic_duration"] == {"enabled": False, "min_duration_seconds": 45}
+
+    def test_a_nested_partial_keeps_its_siblings(self, env):
+        resp = env.save({"display": {"brightness": 20}})
+        assert resp.status_code == 200, resp.get_json()
+        assert env.stored()["display"] == {"brightness": 20, "show_icons": False}
+
+    def test_a_sent_setting_still_changes(self, env):
+        assert env.save({"city": "Lyon", "enabled": False}).status_code == 200
+        stored = env.stored()
+        assert stored["city"] == "Lyon" and stored["enabled"] is False
+
+    def test_an_invalid_sent_value_is_still_rejected(self, env):
+        resp = env.save({"update_interval": 5})
+        assert resp.status_code == 400
+        assert env.stored()["update_interval"] == 600
+
+    def test_config_must_be_an_object(self, env):
+        assert env.save(["city"]).status_code == 400
+
+
+class TestCoreOwnedKeysSurviveTheFilter:
+    def test_submitted_core_keys_are_stored(self, env):
+        env.store({"enabled": True, "city": "Paris"})
+        resp = env.save({
+            "vegas_width_pct": 50, "vegas_overflow": "truncate",
+            "vegas_max_width_screens": 2, "skin": "retro",
+            "skin_options": {"accent": "#ff0000"}, "live_priority": True,
+        })
+        assert resp.status_code == 200, resp.get_json()
+        stored = env.stored()
+        assert stored["vegas_width_pct"] == 50
+        assert stored["vegas_overflow"] == "truncate"
+        assert stored["vegas_max_width_screens"] == 2
+        assert stored["skin"] == "retro"
+        assert stored["skin_options"] == {"accent": "#ff0000"}
+        assert stored["live_priority"] is True
+
+    def test_stored_core_keys_survive_an_unrelated_save(self, env):
+        assert env.save({"city": "Nice"}).status_code == 200
+        stored = env.stored()
+        assert stored["vegas_width_pct"] == 60 and stored["skin"] == "neon"
+
+    def test_a_non_core_unknown_key_is_still_filtered(self, env):
+        assert env.save({"not_in_schema": 1}).status_code == 200
+        assert "not_in_schema" not in env.stored()
+
+
+class TestLegacyBooleans:
+    LEGACY = {"enabled": True, "city": "Paris", "global": {"dynamic_duration": True}}
+
+    def test_get_returns_the_object_shape(self, env):
+        env.store(dict(self.LEGACY))
+        data = env.client.get(f"/api/v3/plugins/config?plugin_id={PLUGIN_ID}").get_json()["data"]
+        assert data["global"]["dynamic_duration"] == {"enabled": True, "min_duration_seconds": 30}
+
+    def test_get_output_posts_back(self, env):
+        env.store(dict(self.LEGACY))
+        data = env.client.get(f"/api/v3/plugins/config?plugin_id={PLUGIN_ID}").get_json()["data"]
+        resp = env.save(data)
+        assert resp.status_code == 200, resp.get_json()
+        assert env.stored()["global"]["dynamic_duration"] == {"enabled": True, "min_duration_seconds": 30}
+
+    def test_an_unrelated_json_save_upgrades_the_stored_boolean(self, env):
+        env.store(dict(self.LEGACY))
+        resp = env.save({"city": "Lyon"})
+        assert resp.status_code == 200, resp.get_json()
+        assert env.stored()["global"]["dynamic_duration"] == {"enabled": True, "min_duration_seconds": 30}
+
+    def test_a_posted_legacy_boolean_is_read_as_the_object(self, env):
+        resp = env.save({"global": {"dynamic_duration": False}})
+        assert resp.status_code == 200, resp.get_json()
+        assert env.stored()["global"]["dynamic_duration"]["enabled"] is False
+
+
+class TestPluginSectionsInConfigMain:
+    def _post(self, env, body):
+        return env.client.post("/api/v3/config/main", json=body)
+
+    def test_a_value_plugins_config_rejects_is_rejected_here_too(self, env):
+        resp = self._post(env, {PLUGIN_ID: {"update_interval": "abc"}})
+        assert resp.status_code == 400
+        assert env.stored()["update_interval"] == 600
+        assert env.save({"update_interval": "abc"}).status_code == 400
+
+    def test_nothing_is_saved_when_a_plugin_section_fails(self, env):
+        resp = self._post(env, {"timezone": "Europe/Paris", PLUGIN_ID: {"update_interval": 1}})
+        assert resp.status_code == 400
+        assert env.main()["timezone"] == "UTC"
+        assert env.stored()["update_interval"] == 600
+
+    def test_partial_section_merges_and_keeps_core_keys(self, env):
+        resp = self._post(env, {PLUGIN_ID: {"city": "Lyon", "vegas_overflow": "rotate"}})
+        assert resp.status_code == 200, resp.get_json()
+        stored = env.stored()
+        assert stored["city"] == "Lyon"
+        assert stored["display"] == {"brightness": 80, "show_icons": False}
+        assert stored["vegas_overflow"] == "rotate"
+        assert stored["vegas_width_pct"] == 60 and stored["skin"] == "neon"
+
+    def test_secrets_still_go_to_the_secrets_file(self, env):
+        resp = self._post(env, {PLUGIN_ID: {"api_key": "s3cret"}})
+        assert resp.status_code == 200, resp.get_json()
+        assert "api_key" not in env.stored() or env.stored()["api_key"] in ("", None)
+        assert env.secrets()[PLUGIN_ID]["api_key"] == "s3cret"
+
+    def test_a_non_object_section_is_rejected(self, env):
+        assert self._post(env, {PLUGIN_ID: "on"}).status_code == 400
diff --git a/web_interface/blueprints/api_v3/__init__.py b/web_interface/blueprints/api_v3/__init__.py
index 93ff7703..4b0a3374 100644
--- a/web_interface/blueprints/api_v3/__init__.py
+++ b/web_interface/blueprints/api_v3/__init__.py
@@ -1254,8 +1254,13 @@ def _set_missing_booleans_to_false(plugin_config, schema_props, form_keys, prefi
                     )
 def _enhance_schema_with_core_properties(schema):
     """
-    Enhance schema with core plugin properties (enabled, display_duration, live_priority).
-    These properties are system-managed and should always be allowed even if not in the plugin's schema.
+    Enhance schema with the core-owned per-plugin properties.
+
+    ``enabled``, ``display_duration``, ``live_priority``, ``skin``,
+    ``skin_options`` and the ``vegas_*`` tuning keys are system-managed and
+    always allowed, even when the plugin's schema doesn't declare them. The
+    list is ``schema_manager.CORE_PLUGIN_PROPERTIES``, the one validation uses,
+    so the save filter keeps exactly what validation accepts.
 
     Args:
         schema: The original JSON schema dict
@@ -1263,44 +1268,31 @@ def _enhance_schema_with_core_properties(schema):
     Returns:
         Enhanced schema dict with core properties injected
     """
-    import copy
+    from src.plugin_system.schema_manager import with_core_plugin_properties
 
     if not schema:
         return schema
+    return with_core_plugin_properties(schema)
 
-    # Core plugin properties that should always be allowed
-    # These match the definitions in SchemaManager.validate_config_against_schema()
-    core_properties = {
-        "enabled": {
-            "type": "boolean",
-            "default": True,
-            "description": "Enable or disable this plugin"
-        },
-        "display_duration": {
-            "type": "number",
-            "default": 15,
-            "minimum": 1,
-            "maximum": 300,
-            "description": "How long to display this plugin in seconds"
-        },
-        "live_priority": {
-            "type": "boolean",
-            "default": False,
-            "description": "Enable live priority takeover when plugin has live content"
-        }
-    }
 
-    # Create a deep copy of the schema to modify (to avoid mutating the original)
-    enhanced_schema = copy.deepcopy(schema)
-    if "properties" not in enhanced_schema:
-        enhanced_schema["properties"] = {}
+def _prepared_plugin_config(plugin_id, raw_config):
+    """A plugin's config section as the plugin runs with it, for on_config_change.
+
+    Loading a plugin reads legacy booleans as objects and fills in schema
+    defaults (PluginManager.prepare_plugin_config); a save's notification must
+    hand over the same shape. Falls back to the raw section.
+    """
+    prepare = getattr(api_v3.plugin_manager, 'prepare_plugin_config', None)
+    if callable(prepare):
+        try:
+            prepared = prepare(plugin_id, raw_config)
+            if isinstance(prepared, dict):
+                return prepared
+        except Exception:
+            logger.debug("Could not prepare config for %s", plugin_id, exc_info=True)
+    return raw_config
 
-    # Inject core properties if they're not already defined in the schema
-    for prop_name, prop_def in core_properties.items():
-        if prop_name not in enhanced_schema["properties"]:
-            enhanced_schema["properties"][prop_name] = copy.deepcopy(prop_def)
 
-    return enhanced_schema
 def _filter_config_by_schema(config, schema, prefix=''):
     """
     Filter config to only include fields defined in the schema.
diff --git a/web_interface/blueprints/api_v3/config.py b/web_interface/blueprints/api_v3/config.py
index d7513284..896c6bd9 100644
--- a/web_interface/blueprints/api_v3/config.py
+++ b/web_interface/blueprints/api_v3/config.py
@@ -999,33 +999,34 @@ def save_main_config():
                 current_config['display']['display_durations'][mode_key] = int_value
 
         # Handle plugin configurations dynamically
-        # Any key that matches a plugin ID should be saved as plugin config
-        # This includes proper secret field handling from schema
+        # Any key that matches a plugin ID is that plugin's settings. They go
+        # through the same preparation as POST /plugins/config -- merged onto
+        # the stored section, legacy booleans and schema defaults applied,
+        # filtered, validated, secrets split out -- so this route can't store
+        # a config that one rejects (stored verbatim, it left the plugin
+        # flagged degraded at its next load).
         plugin_keys_to_remove = []
+        plugin_secrets_updates = {}
         for key in data:
             # Check if this key is a plugin ID
             if api_v3.plugin_manager and key in api_v3.plugin_manager.plugin_manifests:
                 plugin_id = key
-                plugin_config = data[key]
+                submitted_config = data[key]
+                if not isinstance(submitted_config, dict):
+                    return error_response(
+                        ErrorCode.VALIDATION_ERROR,
+                        f"Settings for plugin '{plugin_id}' must be a JSON object",
+                        status_code=400
+                    )
 
-                # Load plugin schema to identify secret fields (same logic as save_plugin_config)
-                secret_fields = set()
-                if api_v3.plugin_manager:
-                    plugins_dir = api_v3.plugin_manager.plugins_dir
-                else:
-                    plugin_system_config = current_config.get('plugin_system', {})
-                    plugins_dir_name = plugin_system_config.get('plugins_directory', 'plugin-repos')
-                    if os.path.isabs(plugins_dir_name):
-                        plugins_dir = Path(plugins_dir_name)
-                    else:
-                        plugins_dir = PROJECT_ROOT / plugins_dir_name
                 # plugin_id is already known to be a loaded plugin (the
                 # membership test above), so this cannot currently traverse --
                 # but the path is built from a request key, and the guard and
-                # the join are far enough apart that a later edit could
-                # separate them. Build it through the shared helper instead.
-                schema_path = resolve_under(plugins_dir, plugin_id, 'config_schema.json')
-
+                # the schema load are far enough apart that a later edit could
+                # separate them. Refuse rather than save without knowing which
+                # fields are secrets.
+                schema_path = resolve_under(api_v3.plugin_manager.plugins_dir,
+                                            plugin_id, 'config_schema.json')
                 if schema_path is None:
                     return error_response(
                         ErrorCode.VALIDATION_ERROR,
@@ -1033,77 +1034,48 @@ def save_main_config():
                         status_code=400
                     )
 
-                if schema_path.exists():
-                    try:
-                        with open(schema_path, 'r', encoding='utf-8') as f:
-                            schema = json.load(f)
-                            if 'properties' in schema:
-                                secret_fields = find_secret_fields(schema['properties'])
-                    except Exception as e:
-                        logger.debug("Error reading schema for secret detection: %s", e)
+                schema_mgr = api_v3.schema_manager
+                if not schema_mgr:
+                    return error_response(
+                        ErrorCode.SYSTEM_ERROR,
+                        'Schema manager not initialized',
+                        status_code=500
+                    )
+                schema = schema_mgr.load_schema(plugin_id, use_cache=False)
 
-                # Separate secrets from regular config (same logic as save_plugin_config)
-                regular_config, secrets_config = separate_secrets(plugin_config, secret_fields)
-                # The config form renders secrets masked, so every save posts
-                # them back blank. Without this the blank is merged over the
-                # stored value and the credential is destroyed by the act of
-                # changing an unrelated setting. A blank means "unchanged".
-                secrets_config = remove_empty_secrets(secrets_config)
-
-                # PRE-PROCESSING: Preserve 'enabled' state if not in regular_config
-                # This prevents overwriting the enabled state when saving config from a form that doesn't include the toggle
-                if 'enabled' not in regular_config:
-                    try:
-                        if plugin_id in current_config and 'enabled' in current_config[plugin_id]:
-                            regular_config['enabled'] = current_config[plugin_id]['enabled']
-                        elif api_v3.plugin_manager:
-                            # Fallback to plugin instance if config doesn't have it
-                            plugin_instance = api_v3.plugin_manager.get_plugin(plugin_id)
-                            if plugin_instance:
-                                regular_config['enabled'] = plugin_instance.enabled
-                        # Final fallback: default to True if plugin is loaded (matches BasePlugin default)
-                        if 'enabled' not in regular_config:
-                            regular_config['enabled'] = True
-                    except Exception as e:
-                        logger.debug("Error preserving enabled state: %s", e)
-                        # Default to True on error to avoid disabling plugins
-                        regular_config['enabled'] = True
-
-                # Get current secrets config
-                current_secrets = api_v3.config_manager.get_raw_file_content('secrets')
+                from web_interface.blueprints.api_v3.plugins import (
+                    _merge_onto_stored_plugin_config, _prepare_plugin_config_for_save,
+                )
+                plugin_config = _merge_onto_stored_plugin_config(
+                    plugin_id, submitted_config, current_config)
+                regular_config, secrets_config, error = _prepare_plugin_config_for_save(
+                    plugin_id, plugin_config, schema, schema_mgr, is_json=True)
+                if error:
+                    return error
 
                 # Deep merge regular config into main config
-                if plugin_id not in current_config:
-                    current_config[plugin_id] = {}
-                current_config[plugin_id] = deep_merge(current_config[plugin_id], regular_config)
-
-                # Deep merge secrets into secrets config
+                stored_section = current_config.get(plugin_id)
+                current_config[plugin_id] = deep_merge(
+                    stored_section if isinstance(stored_section, dict) else {}, regular_config)
                 if secrets_config:
-                    if plugin_id not in current_secrets:
-                        current_secrets[plugin_id] = {}
-                    # Lists merge by replacement, so deep_merge here wrote a
-                    # blanked array straight over the stored credentials.
-                    current_secrets[plugin_id] = merge_secrets(
-                        current_secrets[plugin_id], secrets_config)
-                    # Save secrets file
-                    api_v3.config_manager.save_raw_file_content('secrets', current_secrets)
+                    plugin_secrets_updates[plugin_id] = secrets_config
 
                 # Mark for removal from data dict (already processed)
                 plugin_keys_to_remove.append(key)
 
-                # Notify plugin of config change if loaded (with merged config including secrets)
-                try:
-                    if api_v3.plugin_manager:
-                        plugin_instance = api_v3.plugin_manager.get_plugin(plugin_id)
-                        if plugin_instance:
-                            # Reload merged config (includes secrets) and pass the plugin-specific section
-                            merged_config = api_v3.config_manager.load_config()
-                            plugin_full_config = merged_config.get(plugin_id, {})
-                            if hasattr(plugin_instance, 'on_config_change'):
-                                plugin_instance.on_config_change(plugin_full_config)
-                except Exception as hook_err:
-                    # Don't fail the save if hook fails
-                    logger.warning("on_config_change failed: %s", hook_err)
+        # Deep merge secrets into secrets config, once every plugin section
+        # has validated
+        if plugin_secrets_updates:
+            current_secrets = api_v3.config_manager.get_raw_file_content('secrets')
+            for plugin_id, secrets_config in plugin_secrets_updates.items():
+                if plugin_id not in current_secrets:
+                    current_secrets[plugin_id] = {}
+                # Lists merge by replacement, so deep_merge here wrote a
+                # blanked array straight over the stored credentials.
+                current_secrets[plugin_id] = merge_secrets(
+                    current_secrets[plugin_id], secrets_config)
+            # Save secrets file
+            api_v3.config_manager.save_raw_file_content('secrets', current_secrets)
 
         # Remove processed plugin keys from data (they're already in current_config)
         for key in plugin_keys_to_remove:
@@ -1152,6 +1124,19 @@ def save_main_config():
         except ImportError:
             pass
 
+        # Notify saved plugins of their new config (with secrets merged), now
+        # that it is on disk.
+        for plugin_id in plugin_keys_to_remove:
+            try:
+                plugin_instance = api_v3.plugin_manager.get_plugin(plugin_id)
+                if plugin_instance and hasattr(plugin_instance, 'on_config_change'):
+                    merged_config = api_v3.config_manager.load_config()
+                    plugin_instance.on_config_change(_pkg._prepared_plugin_config(
+                        plugin_id, merged_config.get(plugin_id, {})))
+            except Exception as hook_err:
+                # Don't fail the save if hook fails
+                logger.warning("on_config_change failed: %s", hook_err)
+
         message = 'Configuration saved successfully'
         # Switching automatic updates on finishes their setup, which needs
         # the display service to restart (web_interface/auto_update.py).
diff --git a/web_interface/blueprints/api_v3/plugins.py b/web_interface/blueprints/api_v3/plugins.py
index c9643e54..493c6b79 100644
--- a/web_interface/blueprints/api_v3/plugins.py
+++ b/web_interface/blueprints/api_v3/plugins.py
@@ -792,12 +792,16 @@ def get_plugin_config():
         main_config = api_v3.config_manager.load_config()
         plugin_config = main_config.get(plugin_id, {})
 
-        # Merge with defaults from schema so form shows default values for missing fields
+        # Merge with defaults from schema so form shows default values for
+        # missing fields, reading legacy booleans as objects first: what the
+        # plugin runs with, and what posts back through the JSON save
         schema_mgr = api_v3.schema_manager
         if schema_mgr:
             try:
+                from src.plugin_system.schema_manager import prepare_plugin_config
                 defaults = schema_mgr.generate_default_config(plugin_id, use_cache=True)
-                plugin_config = schema_mgr.merge_with_defaults(plugin_config, defaults)
+                plugin_config = prepare_plugin_config(
+                    plugin_config, schema_mgr.load_schema(plugin_id, use_cache=True), defaults)
             except Exception as e:
                 # Log but don't fail - defaults merge is best effort
                 import logging
@@ -1725,7 +1729,14 @@ def save_plugin_config():
             if error:
                 return error
             plugin_id = data['plugin_id']
-            plugin_config = data.get('config', {})
+            submitted_config = data.get('config', {})
+            if not isinstance(submitted_config, dict):
+                return error_response(
+                    ErrorCode.INVALID_INPUT,
+                    'config must be a JSON object',
+                    status_code=400
+                )
+            plugin_config = _merge_onto_stored_plugin_config(plugin_id, submitted_config)
         else:
             # Form data (HTMX submission)
             # plugin_id comes from query string, config from form fields
@@ -2162,398 +2173,11 @@ def save_plugin_config():
         if 'application/json' in content_type:
             schema = schema_mgr.load_schema(plugin_id, use_cache=False)
 
-        # JSON path: fix numeric-keyed dicts that should be arrays.
-        # JS dotToNested() converts feeds.custom_feeds.0.name → {'0': {name:...}}
-        # instead of [{name:...}]. The form-data path has fix_array_structures for this;
-        # mirror that logic here for JSON submissions.
-        if 'application/json' in content_type and schema and 'properties' in schema:
-            def _fix_json_arrays(cfg, props):
-                for k, ps in props.items():
-                    if not isinstance(cfg, dict) or k not in cfg:
-                        continue
-                    pt = ps.get('type')
-                    val = cfg[k]
-                    if pt == 'array':
-                        items_schema = ps.get('items', {})
-                        item_type = items_schema.get('type')
-                        if isinstance(val, dict):
-                            keys = list(val.keys())
-                            if keys and all(str(x).isdigit() for x in keys):
-                                sorted_keys = sorted(keys, key=lambda x: int(str(x)))
-                                arr = [val[sk] for sk in sorted_keys]
-                                if item_type in ('integer', 'number'):
-                                    converted = []
-                                    for v in arr:
-                                        if isinstance(v, str):
-                                            try:
-                                                converted.append(int(v) if item_type == 'integer' else float(v))
-                                            except (ValueError, TypeError, OverflowError):
-                                                converted.append(v)
-                                        else:
-                                            converted.append(v)
-                                    arr = converted
-                                cfg[k] = arr
-                            elif not keys:
-                                cfg[k] = []
-                        # Recurse into each element when items are objects with properties,
-                        # covering both freshly-converted and already-list values.
-                        if item_type == 'object' and 'properties' in items_schema:
-                            for elem in (cfg[k] if isinstance(cfg[k], list) else []):
-                                if isinstance(elem, dict):
-                                    _fix_json_arrays(elem, items_schema['properties'])
-                    elif pt == 'object' and 'properties' in ps and isinstance(val, dict):
-                        _fix_json_arrays(val, ps['properties'])
-            _fix_json_arrays(plugin_config, schema['properties'])
-
-        # PRE-PROCESSING: Preserve 'enabled' state if not in request
-        # This prevents overwriting the enabled state when saving config from a form that doesn't include the toggle
-        if 'enabled' not in plugin_config:
-            try:
-                current_config = api_v3.config_manager.load_config()
-                if plugin_id in current_config and 'enabled' in current_config[plugin_id]:
-                    plugin_config['enabled'] = current_config[plugin_id]['enabled']
-                    # logger.debug(f"Preserving enabled state for {plugin_id}: {plugin_config['enabled']}")
-                elif api_v3.plugin_manager:
-                    # Fallback to plugin instance if config doesn't have it
-                    plugin_instance = api_v3.plugin_manager.get_plugin(plugin_id)
-                    if plugin_instance:
-                        plugin_config['enabled'] = plugin_instance.enabled
-                # Final fallback: default to True if plugin is loaded (matches BasePlugin default)
-                if 'enabled' not in plugin_config:
-                    plugin_config['enabled'] = True
-            except Exception as e:
-                logger.debug("Error preserving enabled state: %s", e)
-                # Default to True on error to avoid disabling plugins
-                plugin_config['enabled'] = True
-
-        # Find secret fields (supports nested schemas and array-item secrets)
-        secret_fields = set()
-
-        if schema and 'properties' in schema:
-            secret_fields = find_secret_fields(schema['properties'])
-
-        # Apply defaults from schema to config BEFORE validation
-        # This ensures required fields with defaults are present before validation
-        # Store preserved enabled value before merge to protect it from defaults
-        preserved_enabled = None
-        if 'enabled' in plugin_config:
-            preserved_enabled = plugin_config['enabled']
-
-        if schema:
-            defaults = schema_mgr.generate_default_config(plugin_id, use_cache=True)
-            plugin_config = schema_mgr.merge_with_defaults(plugin_config, defaults)
-
-        # After merging defaults, replace any None array values with their schema defaults.
-        # merge_with_defaults gives user config higher priority, so a None submitted by
-        # the client can survive the merge — this pass cleans those up.
-        def _fix_none_arrays(cfg, props):
-            for k, pschema in props.items():
-                if pschema.get('type') == 'array':
-                    if isinstance(cfg, dict) and (k not in cfg or cfg[k] is None):
-                        cfg[k] = pschema.get('default', [])
-                elif pschema.get('type') == 'object' and 'properties' in pschema:
-                    if isinstance(cfg, dict) and isinstance(cfg.get(k), dict):
-                        _fix_none_arrays(cfg[k], pschema['properties'])
-
-        if schema and 'properties' in schema and isinstance(plugin_config, dict):
-            _fix_none_arrays(plugin_config, schema['properties'])
-
-        # Ensure enabled state is preserved after defaults merge
-        # Defaults should not overwrite an explicitly preserved enabled value
-        if preserved_enabled is not None:
-            # Restore preserved value if it was changed by defaults merge
-            if plugin_config.get('enabled') != preserved_enabled:
-                plugin_config['enabled'] = preserved_enabled
-
-        # Normalize config data: convert string numbers to integers/floats where schema expects numbers
-        # This handles form data which sends everything as strings
-        def normalize_config_values(config, schema_props, prefix=''):
-            """Recursively normalize config values based on schema types"""
-            if not isinstance(config, dict) or not isinstance(schema_props, dict):
-                return config
-
-            normalized = {}
-            for key, value in config.items():
-                field_path = f"{prefix}.{key}" if prefix else key
-
-                if key not in schema_props:
-                    # Field not in schema, keep as-is (will be caught by additionalProperties check if needed)
-                    normalized[key] = value
-                    continue
-
-                prop_schema = schema_props[key]
-                prop_type = prop_schema.get('type')
-
-                # Handle union types (e.g., ["integer", "null"])
-                if isinstance(prop_type, list):
-                    # Check if null is allowed and value is empty/null
-                    if 'null' in prop_type:
-                        # Handle various representations of null/empty
-                        if value is None:
-                            normalized[key] = None
-                            continue
-                        elif isinstance(value, str):
-                            # Strip whitespace and check for null representations
-                            value_stripped = value.strip()
-                            if value_stripped == '' or value_stripped.lower() in ('null', 'none', 'undefined'):
-                                normalized[key] = None
-                                continue
-
-                    # Try to normalize based on non-null types in the union
-                    # Check integer first (more specific than number)
-                    if 'integer' in prop_type:
-                        if isinstance(value, str):
-                            value_stripped = value.strip()
-                            if value_stripped == '':
-                                # Empty string with null allowed - already handled above, but double-check
-                                if 'null' in prop_type:
-                                    normalized[key] = None
-                                    continue
-                            try:
-                                normalized[key] = int(value_stripped)
-                                continue
-                            except (ValueError, TypeError, OverflowError):
-                                pass
-                        elif isinstance(value, (int, float)):
-                            normalized[key] = int(value)
-                            continue
-
-                    # Check number (less specific, but handles floats)
-                    if 'number' in prop_type:
-                        if isinstance(value, str):
-                            value_stripped = value.strip()
-                            if value_stripped == '':
-                                # Empty string with null allowed - already handled above, but double-check
-                                if 'null' in prop_type:
-                                    normalized[key] = None
-                                    continue
-                            try:
-                                normalized[key] = float(value_stripped)
-                                continue
-                            except (ValueError, TypeError, OverflowError):
-                                pass
-                        elif isinstance(value, (int, float)):
-                            normalized[key] = float(value)
-                            continue
-
-                    # Check boolean
-                    if 'boolean' in prop_type:
-                        if isinstance(value, str):
-                            normalized[key] = value.strip().lower() in ('true', '1', 'on', 'yes')
-                            continue
-
-                    # If no conversion worked and null is allowed, try to set to None
-                    # This handles cases where the value is an empty string or can't be converted
-                    if 'null' in prop_type:
-                        if isinstance(value, str):
-                            value_stripped = value.strip()
-                            if value_stripped == '' or value_stripped.lower() in ('null', 'none', 'undefined'):
-                                normalized[key] = None
-                                continue
-                        # If it's already None, keep it
-                        if value is None:
-                            normalized[key] = None
-                            continue
-
-                    # If no conversion worked, keep original value (will fail validation, but that's expected)
-                    # Log a warning for debugging
-                    logger.warning(f"Could not normalize field {field_path}: value={repr(value)}, type={type(value)}, schema_type={prop_type}")
-                    normalized[key] = value
-                    continue
-
-                if isinstance(value, dict) and prop_type == 'object' and 'properties' in prop_schema:
-                    # Recursively normalize nested objects
-                    normalized[key] = normalize_config_values(value, prop_schema['properties'], field_path)
-                elif isinstance(value, list) and prop_type == 'array' and 'items' in prop_schema:
-                    # Normalize array items
-                    items_schema = prop_schema['items']
-                    item_type = items_schema.get('type')
-
-                    # Handle union types in array items
-                    if isinstance(item_type, list):
-                        normalized_array = []
-                        for v in value:
-                            # Check if null is allowed
-                            if 'null' in item_type:
-                                if v is None or v == '' or (isinstance(v, str) and v.lower() in ('null', 'none')):
-                                    normalized_array.append(None)
-                                    continue
-
-                            # Try to normalize based on non-null types
-                            if 'integer' in item_type:
-                                if isinstance(v, str):
-                                    try:
-                                        normalized_array.append(int(v))
-                                        continue
-                                    except (ValueError, TypeError, OverflowError):
-                                        pass
-                                elif isinstance(v, (int, float)):
-                                    normalized_array.append(int(v))
-                                    continue
-                            elif 'number' in item_type:
-                                if isinstance(v, str):
-                                    try:
-                                        normalized_array.append(float(v))
-                                        continue
-                                    except (ValueError, TypeError, OverflowError):
-                                        pass
-                                elif isinstance(v, (int, float)):
-                                    normalized_array.append(float(v))
-                                    continue
-
-                            # If no conversion worked, keep original value
-                            normalized_array.append(v)
-                        normalized[key] = normalized_array
-                    elif item_type == 'integer':
-                        # Convert string numbers to integers
-                        normalized_array = []
-                        for v in value:
-                            if isinstance(v, str):
-                                try:
-                                    normalized_array.append(int(v))
-                                except (ValueError, TypeError, OverflowError):
-                                    normalized_array.append(v)
-                            elif isinstance(v, (int, float)):
-                                normalized_array.append(int(v))
-                            else:
-                                normalized_array.append(v)
-                        normalized[key] = normalized_array
-                    elif item_type == 'number':
-                        # Convert string numbers to floats
-                        normalized_array = []
-                        for v in value:
-                            if isinstance(v, str):
-                                try:
-                                    normalized_array.append(float(v))
-                                except (ValueError, TypeError, OverflowError):
-                                    normalized_array.append(v)
-                            else:
-                                normalized_array.append(v)
-                        normalized[key] = normalized_array
-                    elif item_type == 'object' and 'properties' in items_schema:
-                        # Recursively normalize array of objects
-                        normalized_array = []
-                        for v in value:
-                            if isinstance(v, dict):
-                                normalized_array.append(
-                                    normalize_config_values(v, items_schema['properties'], f"{field_path}[]")
-                                )
-                            else:
-                                normalized_array.append(v)
-                        normalized[key] = normalized_array
-                    else:
-                        normalized[key] = value
-                elif prop_type == 'integer':
-                    # Convert string to integer
-                    if isinstance(value, str):
-                        try:
-                            normalized[key] = int(value)
-                        except (ValueError, TypeError, OverflowError):
-                            normalized[key] = value
-                    else:
-                        normalized[key] = value
-                elif prop_type == 'number':
-                    # Convert string to float
-                    if isinstance(value, str):
-                        try:
-                            normalized[key] = float(value)
-                        except (ValueError, TypeError, OverflowError):
-                            normalized[key] = value
-                    else:
-                        normalized[key] = value
-                elif prop_type == 'boolean':
-                    # Convert string booleans
-                    if isinstance(value, str):
-                        normalized[key] = value.lower() in ('true', '1', 'on', 'yes')
-                    else:
-                        normalized[key] = value
-                else:
-                    normalized[key] = value
-
-            return normalized
-
-        # Normalize config before validation
-        if schema and 'properties' in schema:
-            plugin_config = normalize_config_values(plugin_config, schema['properties'])
-
-        # Filter config to only include schema-defined fields (important when additionalProperties is false)
-        # Use enhanced schema with core properties to ensure core properties are preserved during filtering
-        if schema and 'properties' in schema:
-            enhanced_schema_for_filtering = _enhance_schema_with_core_properties(schema)
-            plugin_config = _filter_config_by_schema(plugin_config, enhanced_schema_for_filtering)
-
-        # Debug logging for union type fields (temporary)
-        if 'rotation_settings' in plugin_config and 'random_seed' in plugin_config.get('rotation_settings', {}):
-            seed_value = plugin_config['rotation_settings']['random_seed']
-            logger.debug(f"After normalization, random_seed value: {repr(seed_value)}, type: {type(seed_value)}")
-
-        # Validate configuration against schema before saving
-        if schema:
-            # Log what we're validating for debugging
-            logger.info(f"Validating config for {plugin_id}")
-            # Only the shape. plugin_config still holds the submitted secret
-            # values at this point -- separate_secrets does not run until
-            # below -- so logging it wrote live credentials to the journal.
-            logger.info(f"Config keys being validated: {list(plugin_config.keys())}")
-
-            # Get enhanced schema keys (including injected core properties)
-            # We need to create an enhanced schema to get the actual allowed keys
-            import copy
-            enhanced_schema = copy.deepcopy(schema)
-            if "properties" not in enhanced_schema:
-                enhanced_schema["properties"] = {}
-
-            # Core properties that are always injected during validation
-            core_properties = ["enabled", "display_duration", "live_priority"]
-            for prop_name in core_properties:
-                if prop_name not in enhanced_schema["properties"]:
-                    # Add placeholder to get the full list of allowed keys
-                    enhanced_schema["properties"][prop_name] = {"type": "any"}
-
-            is_valid, validation_errors = schema_mgr.validate_config_against_schema(
-                plugin_config, schema, plugin_id
-            )
-            if not is_valid:
-                # Log validation errors for debugging
-                logger.error(f"Config validation failed for {plugin_id}")
-                logger.error(f"Validation errors: {validation_errors}")
-                # Keys only, for the same reason as above.
-                logger.error(f"Config keys that failed: {list(plugin_config.keys())}")
-                logger.error(f"Schema properties: {list(enhanced_schema.get('properties', {}).keys())}")
-
-                # Also print to console for immediate visibility
-                import json
-                logger.warning("Config validation failed for plugin (see debug logs)")
-
-                # Log raw form data if this was a form submission
-                if 'application/json' not in (request.content_type or ''):
-                    form_data = request.form.to_dict()
-                return error_response(
-                    ErrorCode.CONFIG_VALIDATION_FAILED,
-                    'Configuration validation failed',
-                    details='; '.join(validation_errors) if validation_errors else 'Unknown validation error',
-                    context={
-                        'plugin_id': plugin_id,
-                        'validation_errors': validation_errors,
-                        'config_keys': list(plugin_config.keys()),
-                        'schema_keys': list(enhanced_schema.get('properties', {}).keys())
-                    },
-                    suggested_fixes=[
-                        'Review validation errors above',
-                        'Check config against schema',
-                        'Verify all required fields are present'
-                    ],
-                    status_code=400
-                )
-
-        # Separate secrets from regular config (handles nested configs and
-        # array-item secrets — see src/web_interface/secret_helpers.py)
-        regular_config, secrets_config = separate_secrets(plugin_config, secret_fields)
-        # The config form renders secrets masked, so every save posts
-        # them back blank. Without this the blank is merged over the
-        # stored value and the credential is destroyed by the act of
-        # changing an unrelated setting. A blank means "unchanged".
-        secrets_config = remove_empty_secrets(secrets_config)
+        regular_config, secrets_config, error = _prepare_plugin_config_for_save(
+            plugin_id, plugin_config, schema, schema_mgr,
+            is_json='application/json' in content_type)
+        if error:
+            return error
 
         # Get current configs
         current_config = api_v3.config_manager.load_config()
@@ -2626,7 +2250,8 @@ def save_plugin_config():
                 if plugin_instance:
                     # Reload merged config (includes secrets) and pass the plugin-specific section
                     merged_config = api_v3.config_manager.load_config()
-                    plugin_full_config = merged_config.get(plugin_id, {})
+                    plugin_full_config = _pkg._prepared_plugin_config(
+                        plugin_id, merged_config.get(plugin_id, {}))
                     if hasattr(plugin_instance, 'on_config_change'):
                         plugin_instance.on_config_change(plugin_full_config)
 
@@ -2677,6 +2302,419 @@ def save_plugin_config():
             context=error.context,
             status_code=500
         )
+def _merge_onto_stored_plugin_config(plugin_id, submitted_config, current_config=None):
+    """A JSON plugin-config body merged onto the plugin's stored section.
+
+    A JSON body carries the settings being changed, as a form post does: the
+    rest keep their stored values. Built from defaults alone, a body such as
+    ``{"enabled": true}`` reset every unsent setting of the plugin.
+    ``current_config`` is the loaded main config when the caller has one.
+    """
+    import copy
+    if current_config is None:
+        current_config = api_v3.config_manager.load_config()
+    stored = current_config.get(plugin_id)
+    base = copy.deepcopy(stored) if isinstance(stored, dict) else {}
+    return deep_merge(base, submitted_config)
+
+
+def _prepare_plugin_config_for_save(plugin_id, plugin_config, schema, schema_mgr, is_json):
+    """Turn a submitted plugin config into what gets stored.
+
+    Fixes array shapes, keeps the enabled state, reads legacy booleans and
+    applies schema defaults, normalizes value types, filters to the schema plus
+    the core-owned per-plugin properties, validates, and splits out secrets.
+    Shared by POST /plugins/config and plugin sections posted to
+    POST /config/main, so both store the same thing.
+
+    Returns ``(regular_config, secrets_config, None)``, or
+    ``(None, None, error_response)`` when validation fails.
+    """
+    # JSON path: fix numeric-keyed dicts that should be arrays.
+    # JS dotToNested() converts feeds.custom_feeds.0.name → {'0': {name:...}}
+    # instead of [{name:...}]. The form-data path has fix_array_structures for this;
+    # mirror that logic here for JSON submissions.
+    if is_json and schema and 'properties' in schema:
+        def _fix_json_arrays(cfg, props):
+            for k, ps in props.items():
+                if not isinstance(cfg, dict) or k not in cfg:
+                    continue
+                pt = ps.get('type')
+                val = cfg[k]
+                if pt == 'array':
+                    items_schema = ps.get('items', {})
+                    item_type = items_schema.get('type')
+                    if isinstance(val, dict):
+                        keys = list(val.keys())
+                        if keys and all(str(x).isdigit() for x in keys):
+                            sorted_keys = sorted(keys, key=lambda x: int(str(x)))
+                            arr = [val[sk] for sk in sorted_keys]
+                            if item_type in ('integer', 'number'):
+                                converted = []
+                                for v in arr:
+                                    if isinstance(v, str):
+                                        try:
+                                            converted.append(int(v) if item_type == 'integer' else float(v))
+                                        except (ValueError, TypeError, OverflowError):
+                                            converted.append(v)
+                                    else:
+                                        converted.append(v)
+                                arr = converted
+                            cfg[k] = arr
+                        elif not keys:
+                            cfg[k] = []
+                    # Recurse into each element when items are objects with properties,
+                    # covering both freshly-converted and already-list values.
+                    if item_type == 'object' and 'properties' in items_schema:
+                        for elem in (cfg[k] if isinstance(cfg[k], list) else []):
+                            if isinstance(elem, dict):
+                                _fix_json_arrays(elem, items_schema['properties'])
+                elif pt == 'object' and 'properties' in ps and isinstance(val, dict):
+                    _fix_json_arrays(val, ps['properties'])
+        _fix_json_arrays(plugin_config, schema['properties'])
+
+    # PRE-PROCESSING: Preserve 'enabled' state if not in request
+    # This prevents overwriting the enabled state when saving config from a form that doesn't include the toggle
+    if 'enabled' not in plugin_config:
+        try:
+            current_config = api_v3.config_manager.load_config()
+            if plugin_id in current_config and 'enabled' in current_config[plugin_id]:
+                plugin_config['enabled'] = current_config[plugin_id]['enabled']
+                # logger.debug(f"Preserving enabled state for {plugin_id}: {plugin_config['enabled']}")
+            elif api_v3.plugin_manager:
+                # Fallback to plugin instance if config doesn't have it
+                plugin_instance = api_v3.plugin_manager.get_plugin(plugin_id)
+                if plugin_instance:
+                    plugin_config['enabled'] = plugin_instance.enabled
+            # Final fallback: default to True if plugin is loaded (matches BasePlugin default)
+            if 'enabled' not in plugin_config:
+                plugin_config['enabled'] = True
+        except Exception as e:
+            logger.debug("Error preserving enabled state: %s", e)
+            # Default to True on error to avoid disabling plugins
+            plugin_config['enabled'] = True
+
+    # Find secret fields (supports nested schemas and array-item secrets)
+    secret_fields = set()
+
+    if schema and 'properties' in schema:
+        secret_fields = find_secret_fields(schema['properties'])
+
+    # Apply defaults from schema to config BEFORE validation
+    # This ensures required fields with defaults are present before validation
+    # Store preserved enabled value before merge to protect it from defaults
+    preserved_enabled = None
+    if 'enabled' in plugin_config:
+        preserved_enabled = plugin_config['enabled']
+
+    if schema:
+        # Legacy booleans read as {"enabled": ...} objects first (#588), the
+        # same preparation loading applies, so a config the device reads
+        # without complaint also saves.
+        from src.plugin_system.schema_manager import prepare_plugin_config
+        defaults = schema_mgr.generate_default_config(plugin_id, use_cache=True)
+        plugin_config = prepare_plugin_config(plugin_config, schema, defaults)
+
+    # After merging defaults, replace any None array values with their schema defaults.
+    # merge_with_defaults gives user config higher priority, so a None submitted by
+    # the client can survive the merge — this pass cleans those up.
+    def _fix_none_arrays(cfg, props):
+        for k, pschema in props.items():
+            if pschema.get('type') == 'array':
+                if isinstance(cfg, dict) and (k not in cfg or cfg[k] is None):
+                    cfg[k] = pschema.get('default', [])
+            elif pschema.get('type') == 'object' and 'properties' in pschema:
+                if isinstance(cfg, dict) and isinstance(cfg.get(k), dict):
+                    _fix_none_arrays(cfg[k], pschema['properties'])
+
+    if schema and 'properties' in schema and isinstance(plugin_config, dict):
+        _fix_none_arrays(plugin_config, schema['properties'])
+
+    # Ensure enabled state is preserved after defaults merge
+    # Defaults should not overwrite an explicitly preserved enabled value
+    if preserved_enabled is not None:
+        # Restore preserved value if it was changed by defaults merge
+        if plugin_config.get('enabled') != preserved_enabled:
+            plugin_config['enabled'] = preserved_enabled
+
+    # Normalize config data: convert string numbers to integers/floats where schema expects numbers
+    # This handles form data which sends everything as strings
+    def normalize_config_values(config, schema_props, prefix=''):
+        """Recursively normalize config values based on schema types"""
+        if not isinstance(config, dict) or not isinstance(schema_props, dict):
+            return config
+
+        normalized = {}
+        for key, value in config.items():
+            field_path = f"{prefix}.{key}" if prefix else key
+
+            if key not in schema_props:
+                # Field not in schema, keep as-is (will be caught by additionalProperties check if needed)
+                normalized[key] = value
+                continue
+
+            prop_schema = schema_props[key]
+            prop_type = prop_schema.get('type')
+
+            # Handle union types (e.g., ["integer", "null"])
+            if isinstance(prop_type, list):
+                # Check if null is allowed and value is empty/null
+                if 'null' in prop_type:
+                    # Handle various representations of null/empty
+                    if value is None:
+                        normalized[key] = None
+                        continue
+                    elif isinstance(value, str):
+                        # Strip whitespace and check for null representations
+                        value_stripped = value.strip()
+                        if value_stripped == '' or value_stripped.lower() in ('null', 'none', 'undefined'):
+                            normalized[key] = None
+                            continue
+
+                # Try to normalize based on non-null types in the union
+                # Check integer first (more specific than number)
+                if 'integer' in prop_type:
+                    if isinstance(value, str):
+                        value_stripped = value.strip()
+                        if value_stripped == '':
+                            # Empty string with null allowed - already handled above, but double-check
+                            if 'null' in prop_type:
+                                normalized[key] = None
+                                continue
+                        try:
+                            normalized[key] = int(value_stripped)
+                            continue
+                        except (ValueError, TypeError, OverflowError):
+                            pass
+                    elif isinstance(value, (int, float)):
+                        normalized[key] = int(value)
+                        continue
+
+                # Check number (less specific, but handles floats)
+                if 'number' in prop_type:
+                    if isinstance(value, str):
+                        value_stripped = value.strip()
+                        if value_stripped == '':
+                            # Empty string with null allowed - already handled above, but double-check
+                            if 'null' in prop_type:
+                                normalized[key] = None
+                                continue
+                        try:
+                            normalized[key] = float(value_stripped)
+                            continue
+                        except (ValueError, TypeError, OverflowError):
+                            pass
+                    elif isinstance(value, (int, float)):
+                        normalized[key] = float(value)
+                        continue
+
+                # Check boolean
+                if 'boolean' in prop_type:
+                    if isinstance(value, str):
+                        normalized[key] = value.strip().lower() in ('true', '1', 'on', 'yes')
+                        continue
+
+                # If no conversion worked and null is allowed, try to set to None
+                # This handles cases where the value is an empty string or can't be converted
+                if 'null' in prop_type:
+                    if isinstance(value, str):
+                        value_stripped = value.strip()
+                        if value_stripped == '' or value_stripped.lower() in ('null', 'none', 'undefined'):
+                            normalized[key] = None
+                            continue
+                    # If it's already None, keep it
+                    if value is None:
+                        normalized[key] = None
+                        continue
+
+                # If no conversion worked, keep original value (will fail validation, but that's expected)
+                # Log a warning for debugging
+                logger.warning(f"Could not normalize field {field_path}: value={repr(value)}, type={type(value)}, schema_type={prop_type}")
+                normalized[key] = value
+                continue
+
+            if isinstance(value, dict) and prop_type == 'object' and 'properties' in prop_schema:
+                # Recursively normalize nested objects
+                normalized[key] = normalize_config_values(value, prop_schema['properties'], field_path)
+            elif isinstance(value, list) and prop_type == 'array' and 'items' in prop_schema:
+                # Normalize array items
+                items_schema = prop_schema['items']
+                item_type = items_schema.get('type')
+
+                # Handle union types in array items
+                if isinstance(item_type, list):
+                    normalized_array = []
+                    for v in value:
+                        # Check if null is allowed
+                        if 'null' in item_type:
+                            if v is None or v == '' or (isinstance(v, str) and v.lower() in ('null', 'none')):
+                                normalized_array.append(None)
+                                continue
+
+                        # Try to normalize based on non-null types
+                        if 'integer' in item_type:
+                            if isinstance(v, str):
+                                try:
+                                    normalized_array.append(int(v))
+                                    continue
+                                except (ValueError, TypeError, OverflowError):
+                                    pass
+                            elif isinstance(v, (int, float)):
+                                normalized_array.append(int(v))
+                                continue
+                        elif 'number' in item_type:
+                            if isinstance(v, str):
+                                try:
+                                    normalized_array.append(float(v))
+                                    continue
+                                except (ValueError, TypeError, OverflowError):
+                                    pass
+                            elif isinstance(v, (int, float)):
+                                normalized_array.append(float(v))
+                                continue
+
+                        # If no conversion worked, keep original value
+                        normalized_array.append(v)
+                    normalized[key] = normalized_array
+                elif item_type == 'integer':
+                    # Convert string numbers to integers
+                    normalized_array = []
+                    for v in value:
+                        if isinstance(v, str):
+                            try:
+                                normalized_array.append(int(v))
+                            except (ValueError, TypeError, OverflowError):
+                                normalized_array.append(v)
+                        elif isinstance(v, (int, float)):
+                            normalized_array.append(int(v))
+                        else:
+                            normalized_array.append(v)
+                    normalized[key] = normalized_array
+                elif item_type == 'number':
+                    # Convert string numbers to floats
+                    normalized_array = []
+                    for v in value:
+                        if isinstance(v, str):
+                            try:
+                                normalized_array.append(float(v))
+                            except (ValueError, TypeError, OverflowError):
+                                normalized_array.append(v)
+                        else:
+                            normalized_array.append(v)
+                    normalized[key] = normalized_array
+                elif item_type == 'object' and 'properties' in items_schema:
+                    # Recursively normalize array of objects
+                    normalized_array = []
+                    for v in value:
+                        if isinstance(v, dict):
+                            normalized_array.append(
+                                normalize_config_values(v, items_schema['properties'], f"{field_path}[]")
+                            )
+                        else:
+                            normalized_array.append(v)
+                    normalized[key] = normalized_array
+                else:
+                    normalized[key] = value
+            elif prop_type == 'integer':
+                # Convert string to integer
+                if isinstance(value, str):
+                    try:
+                        normalized[key] = int(value)
+                    except (ValueError, TypeError, OverflowError):
+                        normalized[key] = value
+                else:
+                    normalized[key] = value
+            elif prop_type == 'number':
+                # Convert string to float
+                if isinstance(value, str):
+                    try:
+                        normalized[key] = float(value)
+                    except (ValueError, TypeError, OverflowError):
+                        normalized[key] = value
+                else:
+                    normalized[key] = value
+            elif prop_type == 'boolean':
+                # Convert string booleans
+                if isinstance(value, str):
+                    normalized[key] = value.lower() in ('true', '1', 'on', 'yes')
+                else:
+                    normalized[key] = value
+            else:
+                normalized[key] = value
+
+        return normalized
+
+    # Normalize config before validation
+    if schema and 'properties' in schema:
+        plugin_config = normalize_config_values(plugin_config, schema['properties'])
+
+    # Filter config to only include schema-defined fields (important when additionalProperties is false)
+    # Use enhanced schema with core properties to ensure core properties are preserved during filtering
+    if schema and 'properties' in schema:
+        enhanced_schema_for_filtering = _enhance_schema_with_core_properties(schema)
+        plugin_config = _filter_config_by_schema(plugin_config, enhanced_schema_for_filtering)
+
+    # Debug logging for union type fields (temporary)
+    if 'rotation_settings' in plugin_config and 'random_seed' in plugin_config.get('rotation_settings', {}):
+        seed_value = plugin_config['rotation_settings']['random_seed']
+        logger.debug(f"After normalization, random_seed value: {repr(seed_value)}, type: {type(seed_value)}")
+
+    # Validate configuration against schema before saving
+    if schema:
+        # Log what we're validating for debugging
+        logger.info(f"Validating config for {plugin_id}")
+        # Only the shape. plugin_config still holds the submitted secret
+        # values at this point -- separate_secrets does not run until
+        # below -- so logging it wrote live credentials to the journal.
+        logger.info(f"Config keys being validated: {list(plugin_config.keys())}")
+
+        # Schema keys including the injected core properties, for the error
+        enhanced_schema = _enhance_schema_with_core_properties(schema)
+
+        is_valid, validation_errors = schema_mgr.validate_config_against_schema(
+            plugin_config, schema, plugin_id
+        )
+        if not is_valid:
+            # Log validation errors for debugging
+            logger.error(f"Config validation failed for {plugin_id}")
+            logger.error(f"Validation errors: {validation_errors}")
+            # Keys only, for the same reason as above.
+            logger.error(f"Config keys that failed: {list(plugin_config.keys())}")
+            logger.error(f"Schema properties: {list(enhanced_schema.get('properties', {}).keys())}")
+
+            # Also print to console for immediate visibility
+            logger.warning("Config validation failed for plugin (see debug logs)")
+
+            return None, None, error_response(
+                ErrorCode.CONFIG_VALIDATION_FAILED,
+                'Configuration validation failed',
+                details='; '.join(validation_errors) if validation_errors else 'Unknown validation error',
+                context={
+                    'plugin_id': plugin_id,
+                    'validation_errors': validation_errors,
+                    'config_keys': list(plugin_config.keys()),
+                    'schema_keys': list(enhanced_schema.get('properties', {}).keys())
+                },
+                suggested_fixes=[
+                    'Review validation errors above',
+                    'Check config against schema',
+                    'Verify all required fields are present'
+                ],
+                status_code=400
+            )
+
+    # Separate secrets from regular config (handles nested configs and
+    # array-item secrets — see src/web_interface/secret_helpers.py)
+    regular_config, secrets_config = separate_secrets(plugin_config, secret_fields)
+    # The config form renders secrets masked, so every save posts
+    # them back blank. Without this the blank is merged over the
+    # stored value and the credential is destroyed by the act of
+    # changing an unrelated setting. A blank means "unchanged".
+    secrets_config = remove_empty_secrets(secrets_config)
+
+    return regular_config, secrets_config, None
+
+
 @api_v3.route('/plugins/schema', methods=['GET'])
 def get_plugin_schema():
     """Get plugin configuration schema"""