diff --git a/src/textual/app.py b/src/textual/app.py
index ae92bc9ca..0d900822b 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -36,6 +36,7 @@ from .devtools.client import DevtoolsClient, DevtoolsConnectionError, DevtoolsLo
 from .devtools.redirect_output import StdoutRedirector
 from .dom import DOMNode
 from .driver import Driver
+from .features import parse_features, FeatureFlag
 from .file_monitor import FileMonitor
 from .geometry import Offset, Region, Size
 from .layouts.dock import Dock
@@ -89,7 +90,6 @@ class App(Generic[ReturnType], DOMNode):
 
     def __init__(
         self,
-        screen: bool = True,
         driver_class: Type[Driver] | None = None,
         log: str = "",
         log_verbosity: int = 1,
@@ -162,11 +162,7 @@ class App(Generic[ReturnType], DOMNode):
         if css is not None:
             self.css = css
 
-        self.features = set(
-            feature.strip().lower()
-            for feature in os.getenv("TEXTUAL", "").split(",")
-            if feature.strip()
-        )
+        self.features: frozenset[FeatureFlag] = parse_features(os.getenv("TEXTUAL", ""))
 
         self.registry: set[MessagePump] = set()
 
diff --git a/src/textual/features.py b/src/textual/features.py
new file mode 100644
index 000000000..fe4ebab62
--- /dev/null
+++ b/src/textual/features.py
@@ -0,0 +1,32 @@
+import sys
+
+from typing import cast
+
+if sys.version_info >= (3, 8):
+    from typing import Final, Literal
+else:
+    from typing_extensions import Final, Literal
+
+FEATURES: Final = {"devtools", "debug"}
+
+FeatureFlag = Literal["devtools", "debug"]
+
+
+def parse_features(features: str) -> frozenset[FeatureFlag]:
+    """Parse features env var
+
+    Args:
+        features (str): Comma seprated feature flags
+
+    Returns:
+        frozenset[FeatureFlag]: A frozen set of known features.
+    """
+
+    features_set = frozenset(
+        set(
+            feature.strip().lower()
+            for feature in features.split(",")
+            if feature.strip()
+        ).intersection(FEATURES)
+    )
+    return cast("frozenset[FeatureFlag]", features_set)