fix(langgraph): Update messagesStateReducer behavior to apply rightMe…

…ssages in order (#847)

libs/langgraph/src/graph/message.ts

@@ -26,7 +26,7 @@ export function messagesStateReducer(
   const leftMessages = (leftArray as BaseMessageLike[]).map(
     coerceMessageLikeToMessage
   );
-  let rightMessages = (rightArray as BaseMessageLike[]).map(
+  const rightMessages = (rightArray as BaseMessageLike[]).map(
     coerceMessageLikeToMessage
   );
   // assign missing ids
@@ -36,25 +36,23 @@ export function messagesStateReducer(
       m.lc_kwargs.id = m.id;
     }
   }
-  const dedupeMap = new Map();
   for (const m of rightMessages) {
     if (m.id === null || m.id === undefined) {
       m.id = v4();
       m.lc_kwargs.id = m.id;
     }
-    dedupeMap.set(m.id, m);
   }
-  rightMessages = Array.from(dedupeMap.values());
   // merge
-  const leftIdxById = new Map(leftMessages.map((m, i) => [m.id, i]));
   const merged = [...leftMessages];
+  const mergedById = new Map(merged.map((m, i) => [m.id, i]));
   const idsToRemove = new Set();
   for (const m of rightMessages) {
-    const existingIdx = leftIdxById.get(m.id);
+    const existingIdx = mergedById.get(m.id);
     if (existingIdx !== undefined) {
       if (m._getType() === "remove") {
         idsToRemove.add(m.id);
       } else {
+        idsToRemove.delete(m.id);
         merged[existingIdx] = m;
       }
     } else {
@@ -63,6 +61,7 @@ export function messagesStateReducer(
           `Attempting to delete a message with an ID that doesn't exist ('${m.id}')`
         );
       }
+      mergedById.set(m.id, merged.length);
       merged.push(m);
     }
   }

libs/langgraph/src/tests/prebuilt.test.ts

@@ -8,6 +8,7 @@ import {
   AIMessage,
   BaseMessage,
   HumanMessage,
+  RemoveMessage,
   SystemMessage,
   ToolMessage,
 } from "@langchain/core/messages";
@@ -811,4 +812,18 @@ describe("messagesStateReducer", () => {
     expect(deduped.length).toEqual(1);
     expect(deduped[0].content).toEqual("bar2");
   });
+
+  it("should apply right-side messages in order", () => {
+    const messages = [
+      new RemoveMessage({ id: "foo" }),
+      new HumanMessage({ id: "foo", content: "bar" }),
+      new HumanMessage({ id: "foo", content: "bar2" }),
+    ];
+    const deduped = messagesStateReducer(
+      [new HumanMessage({ id: "foo", content: "bar3" })],
+      messages
+    );
+    expect(deduped.length).toEqual(1);
+    expect(deduped[0].content).toEqual("bar2");
+  });
 });