diff --git a/src/debugpy/_vendored/force_pydevd.py b/src/debugpy/_vendored/force_pydevd.py
index b4b2f904f..980948fda 100644
--- a/src/debugpy/_vendored/force_pydevd.py
+++ b/src/debugpy/_vendored/force_pydevd.py
@@ -22,6 +22,10 @@
     os.environ[str("PYDEVD_DEBUG")] = str("True")
     os.environ[str("PYDEVD_DEBUG_FILE")] = os.environ["DEBUGPY_LOG_DIR"] + str("/debugpy.pydevd.log")
 
+# Disable pydevd frame-eval optimizations only if unset, to allow opt-in.
+if "PYDEVD_USE_FRAME_EVAL" not in os.environ:
+    os.environ[str("PYDEVD_USE_FRAME_EVAL")] = str("NO")
+
 # Constants must be set before importing any other pydevd module
 # # due to heavy use of "from" in them.
 with vendored('pydevd'):
diff --git a/tests/debugpy/test_run.py b/tests/debugpy/test_run.py
index 280bb5cee..3502ab8da 100644
--- a/tests/debugpy/test_run.py
+++ b/tests/debugpy/test_run.py
@@ -325,7 +325,7 @@ def code_to_debug():
             pass
 
         using_frame_eval = backchannel.receive()
-        assert using_frame_eval == (frame_eval in ("yes", ""))
+        assert using_frame_eval == (frame_eval == "yes")
 
 
 @pytest.mark.parametrize("run", [runners.all_launch[0]])