diff --git a/marimo/_messaging/tracebacks.py b/marimo/_messaging/tracebacks.py
index 0e1a7019c4f..3e17617ad60 100644
--- a/marimo/_messaging/tracebacks.py
+++ b/marimo/_messaging/tracebacks.py
@@ -22,13 +22,14 @@ def _highlight_traceback(traceback: str) -> str:
 
 
 def write_traceback(traceback: str) -> None:
-    if isinstance(sys.stderr, Stderr):
-        sys.stderr._write_with_mimetype(
-            _highlight_traceback(_trim_traceback(traceback)),
-            mimetype="application/vnd.marimo+traceback",
-        )
-    else:
+    # Short-circuit: avoid unnecessary function call if not custom stderr
+    if not isinstance(sys.stderr, Stderr):
         sys.stderr.write(traceback)
+        return
+    sys.stderr._write_with_mimetype(
+        _highlight_traceback(_trim_traceback(traceback)),
+        mimetype="application/vnd.marimo+traceback",
+    )
 
 
 def _trim_traceback(traceback: str) -> str:
diff --git a/marimo/_runtime/reload/autoreload.py b/marimo/_runtime/reload/autoreload.py
index 61a1e4ced31..76c05afd9b2 100644
--- a/marimo/_runtime/reload/autoreload.py
+++ b/marimo/_runtime/reload/autoreload.py
@@ -402,10 +402,7 @@ def append_obj(
     name: str,
     obj: object,
 ) -> bool:
-    in_module = (
-        hasattr(obj, "__module__") and obj.__module__ == module.__name__
-    )
-    if not in_module:
+    if getattr(obj, "__module__", None) != module.__name__:
         return False
 
     key = (module.__name__, name)
@@ -431,15 +428,11 @@ def superreload(
     if old_objects is None:
         old_objects = {}
 
-    # collect old objects in the module
-    for name, obj in list(module.__dict__.items()):
-        if not append_obj(module, old_objects, name, obj):
-            continue
-        key = (module.__name__, name)
-        try:
-            old_objects.setdefault(key, []).append(weakref.ref(obj))
-        except TypeError:
-            pass
+    # Collect old objects in the module (append_obj already adds weakref, avoid duplication here)
+    for name, obj in module.__dict__.items():
+        append_obj(module, old_objects, name, obj)
+
+    # reload module
 
     # reload module
     old_dict: dict[str, Any] | None = None
@@ -449,7 +442,7 @@ def superreload(
         old_name = module.__name__
         module.__dict__.clear()
         module.__dict__["__name__"] = old_name
-        module.__dict__["__loader__"] = old_dict["__loader__"]
+        module.__dict__["__loader__"] = old_dict.get("__loader__")
     except (TypeError, AttributeError, KeyError):
         pass
 
@@ -472,13 +465,14 @@ def superreload(
         raise
 
     # iterate over all objects and update functions & classes
-    for name, new_obj in list(module.__dict__.items()):
+    for name, new_obj in module.__dict__.items():
         key = (module.__name__, name)
-        if key not in old_objects:
+        old_refs = old_objects.get(key)
+        if old_refs is None:
             continue
 
         new_refs = []
-        for old_ref in old_objects[key]:
+        for old_ref in old_refs:
             old_obj = old_ref()
             if old_obj is None:
                 continue