diff --git a/libs/langgraph/langgraph/graph/state.py b/libs/langgraph/langgraph/graph/state.py
index 8c95b9581..ae8941618 100644
--- a/libs/langgraph/langgraph/graph/state.py
+++ b/libs/langgraph/langgraph/graph/state.py
@@ -694,11 +694,16 @@ class CompiledStateGraph(CompiledGraph):
                 # if input is a Pydantic model, only update values
                 # for the keys that have been explicitly set by the users
                 # (this is needed to avoid sending updates for fields with None defaults)
-                output_keys_ = (
-                    [k for k in output_keys if k in input.model_fields_set]
-                    if hasattr(input, "model_fields_set")
-                    else output_keys
-                )
+                output_keys_ = output_keys
+                # Pydantic v2
+                if hasattr(input, "model_fields_set"):
+                    output_keys_ = [
+                        k for k in output_keys if k in input.model_fields_set
+                    ]
+                # Pydantic v1
+                elif hasattr(input, "__fields_set__"):
+                    output_keys_ = [k for k in output_keys if k in input.__fields_set__]
+
                 return [
                     (k, getattr(input, k))
                     for k in output_keys_