diff --git a/lib/crewai/src/crewai/types/callback.py b/lib/crewai/src/crewai/types/callback.py
index 5fb9dd906..74cf8a3f6 100644
--- a/lib/crewai/src/crewai/types/callback.py
+++ b/lib/crewai/src/crewai/types/callback.py
@@ -21,10 +21,11 @@ from pydantic.functional_serializers import PlainSerializer
 def _is_non_roundtrippable(fn: object) -> bool:
     """Return ``True`` if *fn* cannot survive a serialize/deserialize round-trip.
 
-    Only plain module-level functions and built-in functions produce dotted
-    paths that :func:`_resolve_dotted_path` can reliably resolve.  Bound
-    methods, ``functools.partial`` objects, callable class instances, lambdas,
-    and closures all fail or silently change semantics during round-tripping.
+    Built-in functions, plain module-level functions, and classes produce
+    dotted paths that :func:`_resolve_dotted_path` can reliably resolve.
+    Bound methods, ``functools.partial`` objects, callable class instances,
+    lambdas, and closures all fail or silently change semantics during
+    round-tripping.
 
     Args:
         fn: The object to check.
@@ -32,7 +33,7 @@ def _is_non_roundtrippable(fn: object) -> bool:
     Returns:
         ``True`` if *fn* would not round-trip through JSON serialization.
     """
-    if inspect.isbuiltin(fn):
+    if inspect.isbuiltin(fn) or inspect.isclass(fn):
         return False
     if inspect.isfunction(fn):
         qualname = getattr(fn, "__qualname__", "")
@@ -44,9 +45,8 @@ def string_to_callable(value: Any) -> Callable[..., Any]:
     """Convert a dotted path string to the callable it references.
 
     If *value* is already callable it is returned as-is, with a warning if
-    it is a lambda, closure, or nested function.  Otherwise, it is treated
-    as ``"module.qualname"`` and
-    resolved via :func:`importlib.import_module`.
+    it cannot survive JSON round-tripping.  Otherwise, it is treated as
+    ``"module.qualname"`` and resolved via :func:`_resolve_dotted_path`.
 
     Args:
         value: A callable or a dotted-path string e.g. ``"builtins.print"``.
@@ -60,7 +60,7 @@ def string_to_callable(value: Any) -> Callable[..., Any]:
     if callable(value):
         if _is_non_roundtrippable(value):
             warnings.warn(
-                "Lambdas, closures, and nested functions cannot be serialized "
+                f"{type(value).__name__} callbacks cannot be serialized "
                 "and will prevent checkpointing. "
                 "Use a module-level named function instead.",
                 UserWarning,