diff --git a/lib/crewai/src/crewai/types/callback.py b/lib/crewai/src/crewai/types/callback.py
index 162311c63..e8f2021ea 100644
--- a/lib/crewai/src/crewai/types/callback.py
+++ b/lib/crewai/src/crewai/types/callback.py
@@ -18,29 +18,34 @@ from pydantic import BeforeValidator, WithJsonSchema
 from pydantic.functional_serializers import PlainSerializer
 
 
-def _is_lambda(fn: object) -> bool:
-    """Return ``True`` if *fn* is a lambda expression.
+def _is_non_roundtrippable(fn: object) -> bool:
+    """Return ``True`` if *fn* cannot survive a serialize/deserialize round-trip.
 
-    Uses ``__qualname__`` ending with ``"<lambda>"`` for resilience against
-    ``__name__`` being reassigned.  ``inspect.isfunction`` gates the check
-    so non-function callables (classes, partials, etc.) are never flagged.
+    Detects lambdas via ``__qualname__`` ending with ``"<lambda>"`` and
+    closures or nested functions via ``"<locals>"`` appearing anywhere in
+    ``__qualname__``.  Both produce dotted paths that
+    :func:`string_to_callable` cannot resolve back to the original object.
+    ``inspect.isfunction`` gates the check so non-function callables
+    like classes and partials are never flagged.
 
     Args:
         fn: The object to check.
 
     Returns:
-        ``True`` if *fn* is a lambda, ``False`` otherwise.
+        ``True`` if *fn* is a lambda, closure, or nested function.
     """
-    return inspect.isfunction(fn) and getattr(fn, "__qualname__", "").endswith(
-        "<lambda>"
-    )
+    if not inspect.isfunction(fn):
+        return False
+    qualname = getattr(fn, "__qualname__", "")
+    return qualname.endswith("<lambda>") or "<locals>" in qualname
 
 
 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.  Otherwise, it is treated as ``"module.qualname"`` and
+    it is a lambda, closure, or nested function.  Otherwise, it is treated
+    as ``"module.qualname"`` and
     resolved via :func:`importlib.import_module`.
 
     Args:
@@ -54,9 +59,10 @@ def string_to_callable(value: Any) -> Callable[..., Any]:
         AttributeError: If the attribute cannot be found on the imported module.
     """
     if callable(value):
-        if _is_lambda(value):
+        if _is_non_roundtrippable(value):
             warnings.warn(
-                "Lambdas cannot be serialized and will prevent checkpointing. "
+                "Lambdas, closures, and nested functions cannot be serialized "
+                "and will prevent checkpointing. "
                 "Use a module-level named function instead.",
                 UserWarning,
                 stacklevel=2,