fix(sdk): mutate function receiving incorrect previous value (#1669)

.changeset/spotty-ghosts-fly.md

@@ -0,0 +1,5 @@
+---
+"@langchain/langgraph-sdk": patch
+---
+
+Fix mutate function in `onCustomEvent` and in `onUpdateEvent` receiving incorrect previous value

libs/sdk/src/react/manager.ts

@@ -163,7 +163,10 @@ export class StreamManager<
     return (
       update: Partial<StateType> | ((prev: StateType) => Partial<StateType>)
     ) => {
-      const prev = { ...historyValues, ...this.state.values };
+      const prev = {
+        ...historyValues,
+        ...(this.state.values ?? [null, "stream"])[0],
+      };
       const next = typeof update === "function" ? update(prev) : update;
       this.setStreamValues({ ...prev, ...next }, kind);
     };