diff --git a/reflex/constants/__init__.py b/reflex/constants/__init__.py
index e816da0f7..f5946bf5e 100644
--- a/reflex/constants/__init__.py
+++ b/reflex/constants/__init__.py
@@ -1,6 +1,7 @@
 """The constants package."""
 
 from .base import (
+    APP_HARNESS_FLAG,
     COOKIES,
     IS_LINUX,
     IS_MACOS,
diff --git a/reflex/constants/base.py b/reflex/constants/base.py
index af96583ad..f737858c0 100644
--- a/reflex/constants/base.py
+++ b/reflex/constants/base.py
@@ -257,6 +257,7 @@ SESSION_STORAGE = "session_storage"
 # Testing variables.
 # Testing os env set by pytest when running a test case.
 PYTEST_CURRENT_TEST = "PYTEST_CURRENT_TEST"
+APP_HARNESS_FLAG = "APP_HARNESS_FLAG"
 
 REFLEX_VAR_OPENING_TAG = "<reflex.Var>"
 REFLEX_VAR_CLOSING_TAG = "</reflex.Var>"
diff --git a/reflex/testing.py b/reflex/testing.py
index 582d7f8c9..65f81385e 100644
--- a/reflex/testing.py
+++ b/reflex/testing.py
@@ -283,6 +283,7 @@ class AppHarness:
             before_decorated_pages = reflex.app.DECORATED_PAGES[self.app_name].copy()
             # Ensure the AppHarness test does not skip State assignment due to running via pytest
             os.environ.pop(reflex.constants.PYTEST_CURRENT_TEST, None)
+            os.environ[reflex.constants.APP_HARNESS_FLAG] = "true"
             self.app_module = reflex.utils.prerequisites.get_compiled_app(
                 # Do not reload the module for pre-existing apps (only apps generated from source)
                 reload=self.app_source is not None
diff --git a/reflex/utils/build.py b/reflex/utils/build.py
index aeb8afd50..9ea941792 100644
--- a/reflex/utils/build.py
+++ b/reflex/utils/build.py
@@ -13,7 +13,7 @@ from rich.progress import MofNCompleteColumn, Progress, TimeElapsedColumn
 from reflex import constants
 from reflex.config import get_config
 from reflex.utils import console, path_ops, prerequisites, processes
-from reflex.utils.exec import is_testing_env
+from reflex.utils.exec import is_in_app_harness
 
 
 def set_env_json():
@@ -22,7 +22,7 @@ def set_env_json():
         str(prerequisites.get_web_dir() / constants.Dirs.ENV_JSON),
         {
             **{endpoint.name: endpoint.get_url() for endpoint in constants.Endpoint},
-            "TEST_MODE": is_testing_env(),
+            "TEST_MODE": is_in_app_harness(),
         },
     )
 
diff --git a/reflex/utils/exec.py b/reflex/utils/exec.py
index 621c4a608..1c777b854 100644
--- a/reflex/utils/exec.py
+++ b/reflex/utils/exec.py
@@ -487,6 +487,15 @@ def is_testing_env() -> bool:
     return constants.PYTEST_CURRENT_TEST in os.environ
 
 
+def is_in_app_harness() -> bool:
+    """Whether the app is running in the app harness.
+
+    Returns:
+        True if the app is running in the app harness.
+    """
+    return constants.APP_HARNESS_FLAG in os.environ
+
+
 def is_prod_mode() -> bool:
     """Check if the app is running in production mode.