feat(core): zero-out token costs for cache hits (#32437)

libs/core/langchain_core/language_models/chat_models.py

@@ -666,6 +666,16 @@ class BaseChatModel(BaseLanguageModel[BaseMessage], ABC):
                 converted_generations.append(chat_gen)
             else:
                 # Already a ChatGeneration or other expected type
+                if hasattr(gen, "message") and isinstance(gen.message, AIMessage):
+                    # We zero out cost on cache hits
+                    gen.message = gen.message.model_copy(
+                        update={
+                            "usage_metadata": {
+                                **(gen.message.usage_metadata or {}),
+                                "total_cost": 0,
+                            }
+                        }
+                    )
                 converted_generations.append(gen)
         return converted_generations