From 78678534f970a5a01dddfc979594367c58d04b44 Mon Sep 17 00:00:00 2001
From: "Guofang.Tang" <136770748@qq.com>
Date: Wed, 25 Feb 2026 07:12:49 +0800
Subject: [PATCH] fix(core): treat empty tool chunk ids as missing in merge
 (#35414)

---
 libs/core/langchain_core/utils/_merge.py    |  6 ++---
 libs/core/tests/unit_tests/test_messages.py | 28 +++++++++++++++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/libs/core/langchain_core/utils/_merge.py b/libs/core/langchain_core/utils/_merge.py
index ba7df2043cb..6a0cb38f078 100644
--- a/libs/core/langchain_core/utils/_merge.py
+++ b/libs/core/langchain_core/utils/_merge.py
@@ -121,9 +121,9 @@ def merge_lists(left: list | None, *others: list | None) -> list | None:
                             "index" in e_left
                             and e_left["index"] == e["index"]  # index matches
                             and (  # IDs not inconsistent
-                                e_left.get("id") is None
-                                or e.get("id") is None
-                                or e_left["id"] == e["id"]
+                                e_left.get("id") in (None, "")
+                                or e.get("id") in (None, "")
+                                or e_left.get("id") == e.get("id")
                             )
                         )
                     ]
diff --git a/libs/core/tests/unit_tests/test_messages.py b/libs/core/tests/unit_tests/test_messages.py
index 1955cef093b..a13fdc62a6f 100644
--- a/libs/core/tests/unit_tests/test_messages.py
+++ b/libs/core/tests/unit_tests/test_messages.py
@@ -122,6 +122,34 @@ def test_message_chunks() -> None:
         ],
     )
 
+    assert (
+        AIMessageChunk(
+            content="",
+            tool_call_chunks=[
+                create_tool_call_chunk(name="tool1", args="", id="1", index=0)
+            ],
+        )
+        + AIMessageChunk(
+            content="",
+            tool_call_chunks=[
+                create_tool_call_chunk(name=None, args='{"arg1": "val', id="", index=0)
+            ],
+        )
+        + AIMessageChunk(
+            content="",
+            tool_call_chunks=[
+                create_tool_call_chunk(name=None, args='ue"}', id="", index=0)
+            ],
+        )
+    ) == AIMessageChunk(
+        content="",
+        tool_call_chunks=[
+            create_tool_call_chunk(
+                name="tool1", args='{"arg1": "value"}', id="1", index=0
+            )
+        ],
+    )
+
     assert (
         AIMessageChunk(
             content="",