core[patch]: Treat type as a special field when merging lists (#22750)

Should we even log a warning? At least for Anthropic, it's expected to
get e.g. `text_block` followed by `text_delta`.

@ccurme @baskaryan @efriis

libs/core/langchain_core/messages/base.py

@@ -105,6 +105,9 @@ def merge_content(
         # Add the second content to the last element
         if isinstance(first_content[-1], str):
             return first_content[:-1] + [first_content[-1] + second_content]
+        # If second content is an empty string, treat as a no-op
+        elif second_content == "":
+            return first_content
         else:
             # Otherwise, add the second content as a new element of the list
             return first_content + [second_content]

libs/core/langchain_core/utils/_merge.py

@@ -60,6 +60,10 @@ def merge_lists(left: Optional[List], right: Optional[List]) -> Optional[List]:
                     if e_left["index"] == e["index"]
                 ]
                 if to_merge:
+                    # If a top-level "type" has been set for a chunk, it should no
+                    # longer be overridden by the "type" field in future chunks.
+                    if "type" in merged[to_merge[0]] and "type" in e:
+                        e.pop("type")
                     merged[to_merge[0]] = merge_dicts(merged[to_merge[0]], e)
                 else:
                     merged = merged + [e]

libs/core/tests/unit_tests/test_messages.py

@@ -203,6 +203,35 @@ def test_complex_ai_message_chunks() -> None:
         )
     ), "Concatenating when both content arrays are dicts with separate indexes should not merge"  # noqa: E501
 
+    assert (
+        AIMessageChunk(content=[{"index": 0, "text": "I am", "type": "text_block"}])
+        + AIMessageChunk(
+            content=[{"index": 0, "text": " indeed.", "type": "text_block"}]
+        )
+        == AIMessageChunk(
+            content=[{"index": 0, "text": "I am indeed.", "type": "text_block"}]
+        )
+    ), "Concatenating when both content arrays are dicts with the same index and type should merge"  # noqa: E501
+
+    assert (
+        AIMessageChunk(content=[{"index": 0, "text": "I am", "type": "text_block"}])
+        + AIMessageChunk(
+            content=[{"index": 0, "text": " indeed.", "type": "text_block_delta"}]
+        )
+        == AIMessageChunk(
+            content=[{"index": 0, "text": "I am indeed.", "type": "text_block"}]
+        )
+    ), "Concatenating when both content arrays are dicts with the same index and different types should merge without updating type"  # noqa: E501
+
+    assert (
+        AIMessageChunk(content=[{"index": 0, "text": "I am", "type": "text_block"}])
+        + AIMessageChunk(content="", response_metadata={"extra": "value"})
+        == AIMessageChunk(
+            content=[{"index": 0, "text": "I am", "type": "text_block"}],
+            response_metadata={"extra": "value"},
+        )
+    ), "Concatenating when one content is an array and one is an empty string should not add a new item, but should concat other fields"  # noqa: E501
+
 
 def test_function_message_chunks() -> None:
     assert FunctionMessageChunk(