diff --git a/src/plugin_system/testing/harness.py b/src/plugin_system/testing/harness.py
index 0e16f584..d46c11c7 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
+from .loading import load_config_defaults, load_manifest, merge_config
 from .sizes import DEFAULT_TEST_SIZES, safe_mode_filename, size_label
 
 logger = get_logger("[Plugin Harness]")
@@ -294,7 +294,9 @@ 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 = {"enabled": True, **load_config_defaults(plugin_dir), **(config or {})}
+    config = merge_config(
+        merge_config({"enabled": True}, load_config_defaults(plugin_dir)),
+        config or {})
     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 061e95a0..326b3e73 100644
--- a/src/plugin_system/testing/loading.py
+++ b/src/plugin_system/testing/loading.py
@@ -33,6 +33,45 @@ 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.
+
+    A shallow merge would let `-c '{"nhl": {"enabled": true}}'` replace the whole
+    nhl subtree and silently discard every other nhl default -- the same class of
+    bug this function exists to fix.
+    """
+    merged = dict(base)
+    for key, value in (override or {}).items():
+        if isinstance(value, dict) and isinstance(merged.get(key), dict):
+            merged[key] = merge_config(merged[key], value)
+        else:
+            merged[key] = value
+    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)."""
     schema_path = Path(plugin_dir) / 'config_schema.json'
@@ -40,11 +79,7 @@ def load_config_defaults(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
         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 isinstance(prop, dict) and 'default' in prop:
-            defaults[key] = prop['default']
-    return defaults
+    return _defaults_from_properties(schema.get('properties', {}))
 
 
 def load_harness_spec(plugin_dir: Union[str, Path]) -> Dict[str, Any]: