fix(core): fix merge_lists incorrectly merging parallel tool calls (#35281)

libs/core/langchain_core/utils/_merge.py

@@ -117,7 +117,15 @@ def merge_lists(left: list | None, *others: list | None) -> list | None:
                     to_merge = [
                         i
                         for i, e_left in enumerate(merged)
-                        if "index" in e_left and e_left["index"] == e["index"]
+                        if (
+                            "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"]
+                            )
+                        )
                     ]
                     if to_merge:
                         # TODO: Remove this once merge_dict is updated with special

libs/core/tests/unit_tests/test_messages.py

@@ -916,6 +916,47 @@ def test_merge_tool_calls() -> None:
     assert len(merged) == 2
 
 
+def test_merge_tool_calls_parallel_same_index() -> None:
+    """Test parallel tool calls with same index but different IDs."""
+    # Two parallel tool calls with the same index but different IDs
+    left = create_tool_call_chunk(
+        name="read_file", args='{"path": "foo.txt"}', id="tooluse_ABC", index=0
+    )
+    right = create_tool_call_chunk(
+        name="search_text", args='{"query": "bar"}', id="tooluse_DEF", index=0
+    )
+    merged = merge_lists([left], [right])
+    assert merged is not None
+    assert len(merged) == 2
+    assert merged[0]["name"] == "read_file"
+    assert merged[0]["id"] == "tooluse_ABC"
+    assert merged[1]["name"] == "search_text"
+    assert merged[1]["id"] == "tooluse_DEF"
+
+    # Streaming continuation: same index, id=None on continuation chunk
+    # should still merge correctly with the original chunk
+    first = create_tool_call_chunk(name="tool1", args="", id="id1", index=0)
+    continuation = create_tool_call_chunk(
+        name=None, args='{"key": "value"}', id=None, index=0
+    )
+    merged = merge_lists([first], [continuation])
+    assert merged is not None
+    assert len(merged) == 1
+    assert merged[0]["name"] == "tool1"
+    assert merged[0]["args"] == '{"key": "value"}'
+    assert merged[0]["id"] == "id1"
+
+    # Three parallel tool calls all with the same index
+    tc1 = create_tool_call_chunk(name="tool_a", args="{}", id="id_a", index=0)
+    tc2 = create_tool_call_chunk(name="tool_b", args="{}", id="id_b", index=0)
+    tc3 = create_tool_call_chunk(name="tool_c", args="{}", id="id_c", index=0)
+    merged = merge_lists([tc1], [tc2], [tc3])
+    assert merged is not None
+    assert len(merged) == 3
+    assert [m["name"] for m in merged] == ["tool_a", "tool_b", "tool_c"]
+    assert [m["id"] for m in merged] == ["id_a", "id_b", "id_c"]
+
+
 def test_tool_message_serdes() -> None:
     message = ToolMessage(
         "foo", artifact={"bar": {"baz": 123}}, tool_call_id="1", status="error"