diff --git a/.changeset/spotty-ghosts-fly.md b/.changeset/spotty-ghosts-fly.md
new file mode 100644
index 000000000..454a27456
--- /dev/null
+++ b/.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
diff --git a/libs/sdk/src/react/manager.ts b/libs/sdk/src/react/manager.ts
index 5bd66bb4c..a0c2d976d 100644
--- a/libs/sdk/src/react/manager.ts
+++ b/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);
     };