fix(core): prevent tool call chunks from merging incorrectly in AIMes… (#8433)

Author: christian-bromann

.changeset/honest-vans-live.md

@@ -0,0 +1,5 @@
+---
+"@langchain/core": patch
+---
+
+fix(core): prevent tool call chunks from merging incorrectly in AIMes…

langchain-core/src/messages/base.ts

@@ -418,8 +418,12 @@ export function _mergeDicts(
       if (key === "type") {
         // Do not merge 'type' fields
         continue;
+      } else if (["id", "output_version", "model_provider"].includes(key)) {
+        // Keep the incoming value for these fields
+        merged[key] = value;
+      } else {
+        merged[key] += value;
       }
-      merged[key] += value;
     } else if (typeof merged[key] === "object" && !Array.isArray(merged[key])) {
       merged[key] = _mergeDicts(merged[key], value);
     } else if (Array.isArray(merged[key])) {
@@ -435,8 +439,10 @@ export function _mergeDicts(
   return merged;
 }
 
-// eslint-disable-next-line @typescript-eslint/no-explicit-any
-export function _mergeLists(left?: any[], right?: any[]) {
+export function _mergeLists<Content extends MessageContentComplex>(
+  left?: Content[],
+  right?: Content[]
+): Content[] | undefined {
   if (left === undefined && right === undefined) {
     return undefined;
   } else if (left === undefined || right === undefined) {
@@ -446,19 +452,36 @@ export function _mergeLists(left?: any[], right?: any[]) {
     for (const item of right) {
       if (
         typeof item === "object" &&
+        item !== null &&
         "index" in item &&
         typeof item.index === "number"
       ) {
         const toMerge = merged.findIndex(
-          (leftItem) => leftItem.index === item.index
+          (leftItem) =>
+            leftItem !== null &&
+            typeof leftItem === "object" &&
+            "index" in leftItem &&
+            leftItem.index === item.index &&
+            // Only merge if IDs match (or both are undefined)
+            ("id" in leftItem && "id" in item
+              ? leftItem.id === item.id
+              : !("id" in leftItem) && !("id" in item))
         );
-        if (toMerge !== -1) {
-          merged[toMerge] = _mergeDicts(merged[toMerge], item);
+        if (
+          toMerge !== -1 &&
+          typeof merged[toMerge] === "object" &&
+          merged[toMerge] !== null
+        ) {
+          merged[toMerge] = _mergeDicts(
+            merged[toMerge] as Record<string, unknown>,
+            item as Record<string, unknown>
+          ) as Content;
         } else {
           merged.push(item);
         }
       } else if (
         typeof item === "object" &&
+        item !== null &&
         "text" in item &&
         item.text === ""
       ) {

langchain-core/src/messages/tests/ai.test.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect } from "@jest/globals";
+import { AIMessageChunk } from "../ai.js";
+
+describe("AIMessageChunk", () => {
+  it("should properly merge tool call chunks", () => {
+    const chunk1 = new AIMessageChunk({
+      content: "",
+      tool_call_chunks: [
+        {
+          name: "add_new_task",
+          args: '{"tasks":["buy tomatoes","help child with math"]}',
+          type: "tool_call_chunk",
+          index: 0,
+          id: "9fb5c937-6944-4173-84be-ad1caee1cedd",
+        },
+      ],
+    });
+    const chunk2 = new AIMessageChunk({
+      content: "",
+      tool_call_chunks: [
+        {
+          name: "add_ideas",
+          args: '{"ideas":["read about Angular 19 updates"]}',
+          type: "tool_call_chunk",
+          index: 0,
+          id: "5abf542e-87f3-4899-87c6-8f7d9cb6a28d",
+        },
+      ],
+    });
+
+    const merged = chunk1.concat(chunk2);
+    expect(merged.tool_call_chunks).toHaveLength(2);
+
+    const firstCall = merged.tool_call_chunks?.[0];
+    expect(firstCall?.name).toBe("add_new_task");
+    expect(firstCall?.args).toBe(
+      '{"tasks":["buy tomatoes","help child with math"]}'
+    );
+    expect(firstCall?.id).toBe("9fb5c937-6944-4173-84be-ad1caee1cedd");
+
+    const secondCall = merged.tool_call_chunks?.[1];
+    expect(secondCall?.name).toBe("add_ideas");
+    expect(secondCall?.args).toBe(
+      '{"ideas":["read about Angular 19 updates"]}'
+    );
+    expect(secondCall?.id).toBe("5abf542e-87f3-4899-87c6-8f7d9cb6a28d");
+  });
+});