fix(core): update chunk concat logic to match on missing ID fields

.changeset/afraid-paws-give.md

@@ -0,0 +1,5 @@
+---
+"@langchain/core": patch
+---
+
+update chunk concat logic to match on missing ID fields

langchain-core/src/messages/base.ts

@@ -456,17 +456,19 @@ export function _mergeLists<Content extends MessageContentComplex>(
         "index" in item &&
         typeof item.index === "number"
       ) {
-        const toMerge = merged.findIndex(
-          (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))
-        );
+        const toMerge = merged.findIndex((leftItem) => {
+          const isObject = typeof leftItem === "object";
+          const indiciesMatch =
+            "index" in leftItem && leftItem.index === item.index;
+          const idsMatch =
+            "id" in leftItem && "id" in item && leftItem?.id === item?.id;
+          const eitherItemMissingID =
+            !("id" in leftItem) ||
+            !leftItem?.id ||
+            !("id" in item) ||
+            !item?.id;
+          return isObject && indiciesMatch && (idsMatch || eitherItemMissingID);
+        });
         if (
           toMerge !== -1 &&
           typeof merged[toMerge] === "object" &&

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

@@ -2,7 +2,7 @@ import { describe, it, expect } from "@jest/globals";
 import { AIMessageChunk } from "../ai.js";
 
 describe("AIMessageChunk", () => {
-  it("should properly merge tool call chunks", () => {
+  it("should properly merge tool call chunks that have matching indices and ids", () => {
     const chunk1 = new AIMessageChunk({
       content: "",
       tool_call_chunks: [
@@ -45,4 +45,71 @@ describe("AIMessageChunk", () => {
     );
     expect(secondCall?.id).toBe("5abf542e-87f3-4899-87c6-8f7d9cb6a28d");
   });
+
+  it("should properly merge tool call chunks that have matching indices and at least one id is blank", () => {
+    const chunk1 = new AIMessageChunk({
+      content: "",
+      tool_call_chunks: [
+        {
+          name: "add_new_task",
+          type: "tool_call_chunk",
+          index: 0,
+          id: "9fb5c937-6944-4173-84be-ad1caee1cedd",
+        },
+      ],
+    });
+    const chunk2 = new AIMessageChunk({
+      content: "",
+      tool_call_chunks: [
+        {
+          args: '{"tasks":["buy tomatoes","help child with math"]}',
+          type: "tool_call_chunk",
+          index: 0,
+        },
+      ],
+    });
+
+    const merged = chunk1.concat(chunk2);
+    expect(merged.tool_call_chunks).toHaveLength(1);
+
+    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");
+  });
+
+  it("should properly merge tool call chunks that have matching indices no IDs at all", () => {
+    const chunk1 = new AIMessageChunk({
+      content: "",
+      tool_call_chunks: [
+        {
+          name: "add_new_task",
+          type: "tool_call_chunk",
+          index: 0,
+        },
+      ],
+    });
+    const chunk2 = new AIMessageChunk({
+      content: "",
+      tool_call_chunks: [
+        {
+          args: '{"tasks":["buy tomatoes","help child with math"]}',
+          type: "tool_call_chunk",
+          index: 0,
+        },
+      ],
+    });
+
+    const merged = chunk1.concat(chunk2);
+    expect(merged.tool_call_chunks).toHaveLength(1);
+
+    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).toBeUndefined();
+  });
 });