fix(core,openai): fix v1 streaming tool calls dropped for chat completions

libs/core/langchain_core/messages/ai.py

@@ -247,7 +247,11 @@ def content_blocks(self) -> list[types.ContentBlock]:
         first before falling back to best-effort parsing. For details, see the property
         on `BaseMessage`.
         """
-        if self.response_metadata.get("output_version") == "v1":
+        if self.response_metadata.get("output_version") == "v1" and isinstance(
+            self.content, list
+        ):
+            # Only short-circuit when content is already a list of ContentBlock
+            # dicts. See AIMessageChunk.content_blocks for full rationale.
             return cast("list[types.ContentBlock]", self.content)
 
         model_provider = self.response_metadata.get("model_provider")
@@ -440,7 +444,16 @@ def lc_attributes(self) -> dict:
     @property
     def content_blocks(self) -> list[types.ContentBlock]:
         """Return standard, typed `ContentBlock` dicts from the message."""
-        if self.response_metadata.get("output_version") == "v1":
+        if self.response_metadata.get("output_version") == "v1" and isinstance(
+            self.content, list
+        ):
+            # Only short-circuit when content is already a list of ContentBlock
+            # dicts. Some streaming implementations keep content as a string
+            # even when output_version="v1" is set (e.g., OpenAI Chat
+            # Completions), so it must fall through to the model_provider
+            # translator which builds ContentBlock dicts from tool_calls /
+            # tool_call_chunks. Without this guard, string content would be
+            # returned directly, silently dropping tool calls.
             return cast("list[types.ContentBlock]", self.content)
 
         model_provider = self.response_metadata.get("model_provider")

libs/core/tests/unit_tests/messages/test_ai.py

@@ -482,6 +482,49 @@ def test_content_blocks() -> None:
     ]
 
 
+def test_content_blocks_v1_string_content_falls_through() -> None:
+    """Test that content_blocks falls through to translator when content is a string.
+
+    When output_version="v1" is set but content is a string (as in Chat
+    Completions streaming), content_blocks must not short-circuit. It should
+    fall through to the model_provider translator so tool calls are included.
+    """
+    # AIMessage with string content + tool_calls + v1 metadata
+    msg = AIMessage(
+        content="Hello",
+        tool_calls=[
+            create_tool_call(name="foo", args={"a": 1}, id="tc_1"),
+        ],
+        response_metadata={
+            "output_version": "v1",
+            "model_provider": "openai",
+        },
+    )
+    blocks = msg.content_blocks
+    assert isinstance(blocks, list)
+    # Should contain a text block and a tool_call block, not the raw string
+    types_found = {b["type"] for b in blocks}
+    assert "text" in types_found
+    assert "tool_call" in types_found
+
+    # AIMessageChunk with string content + tool_call_chunks + v1 metadata
+    chunk = AIMessageChunk(
+        content="Hello",
+        tool_call_chunks=[
+            create_tool_call_chunk(name="foo", args='{"a": 1}', id="tc_1", index=0),
+        ],
+        response_metadata={
+            "output_version": "v1",
+            "model_provider": "openai",
+        },
+    )
+    blocks = chunk.content_blocks
+    assert isinstance(blocks, list)
+    types_found = {b["type"] for b in blocks}
+    assert "text" in types_found
+    assert "tool_call_chunk" in types_found
+
+
 def test_content_blocks_reasoning_extraction() -> None:
     """Test best-effort reasoning extraction from `additional_kwargs`."""
     message = AIMessage(

libs/partners/openai/langchain_openai/chat_models/base.py

@@ -1185,8 +1185,13 @@ def _convert_chunk_to_generation_chunk(
                 message=default_chunk_class(content="", usage_metadata=usage_metadata),
                 generation_info=base_generation_info,
             )
+            # Keep content as "" (the default) rather than converting to [].
+            # All Chat Completions content chunks arrive as strings. Starting
+            # with [] causes merge_content to silently drop string content
+            # (empty list is falsy, so no merge branch applies). The empty
+            # list also triggers the content_blocks isinstance(list) short-
+            # circuit, which would return [] and miss tool_call_chunks.
             if self.output_version == "v1":
-                generation_chunk.message.content = []
                 generation_chunk.message.response_metadata["output_version"] = "v1"
 
             return generation_chunk
@@ -1217,6 +1222,9 @@ def _convert_chunk_to_generation_chunk(
             message_chunk.usage_metadata = usage_metadata
 
         message_chunk.response_metadata["model_provider"] = "openai"
+        # Propagate output_version so content_blocks can detect v1 mode.
+        if self.output_version == "v1":
+            message_chunk.response_metadata["output_version"] = "v1"
         return ChatGenerationChunk(
             message=message_chunk, generation_info=generation_info or None
         )

libs/partners/openai/tests/unit_tests/chat_models/test_base.py

@@ -1347,6 +1347,178 @@ def test_output_version_compat() -> None:
     assert llm._use_responses_api({}) is True
 
 
+def test_convert_chunk_to_generation_chunk_v1_keeps_string_content() -> None:
+    """Verify _convert_chunk_to_generation_chunk with output_version='v1'."""
+    llm = ChatOpenAI(model="gpt-4o", output_version="v1")
+
+    # Empty-choices chunk (usage-only)
+    empty_chunk: dict[str, Any] = {
+        "id": "chatcmpl-test",
+        "object": "chat.completion.chunk",
+        "created": 0,
+        "model": "gpt-4o",
+        "choices": [],
+        "usage": {"prompt_tokens": 5, "completion_tokens": 3, "total_tokens": 8},
+    }
+    gen = llm._convert_chunk_to_generation_chunk(empty_chunk, AIMessageChunk, None)
+    assert gen is not None
+    assert gen.message.content == ""  # NOT []
+    assert gen.message.response_metadata.get("output_version") == "v1"
+
+    # Content-bearing chunk with tool_call delta
+    tool_chunk: dict[str, Any] = {
+        "id": "chatcmpl-test",
+        "object": "chat.completion.chunk",
+        "created": 0,
+        "model": "gpt-4o",
+        "choices": [
+            {
+                "index": 0,
+                "delta": {
+                    "role": "assistant",
+                    "content": "",
+                    "tool_calls": [
+                        {
+                            "index": 0,
+                            "id": "call_abc",
+                            "function": {"name": "get_weather", "arguments": ""},
+                        }
+                    ],
+                },
+                "logprobs": None,
+                "finish_reason": None,
+            }
+        ],
+        "usage": None,
+    }
+    gen = llm._convert_chunk_to_generation_chunk(tool_chunk, AIMessageChunk, None)
+    assert gen is not None
+    assert isinstance(gen.message.content, str)
+    assert gen.message.response_metadata.get("output_version") == "v1"
+    assert gen.message.response_metadata.get("model_provider") == "openai"
+
+
+def test_v1_streaming_tool_calls_in_content_blocks() -> None:
+    """End-to-end: streaming chunks with tool calls produce correct content_blocks."""
+    stream_chunks: list[dict[str, Any]] = [
+        # Initial empty-choices chunk
+        {
+            "id": "chatcmpl-test",
+            "object": "chat.completion.chunk",
+            "created": 0,
+            "model": "gpt-4o",
+            "choices": [
+                {
+                    "index": 0,
+                    "delta": {"role": "assistant", "content": ""},
+                    "logprobs": None,
+                    "finish_reason": None,
+                }
+            ],
+            "usage": None,
+        },
+        # Tool call start
+        {
+            "id": "chatcmpl-test",
+            "object": "chat.completion.chunk",
+            "created": 0,
+            "model": "gpt-4o",
+            "choices": [
+                {
+                    "index": 0,
+                    "delta": {
+                        "tool_calls": [
+                            {
+                                "index": 0,
+                                "id": "call_abc",
+                                "function": {
+                                    "name": "get_weather",
+                                    "arguments": '{"loc',
+                                },
+                            }
+                        ]
+                    },
+                    "logprobs": None,
+                    "finish_reason": None,
+                }
+            ],
+            "usage": None,
+        },
+        # Tool call args continuation
+        {
+            "id": "chatcmpl-test",
+            "object": "chat.completion.chunk",
+            "created": 0,
+            "model": "gpt-4o",
+            "choices": [
+                {
+                    "index": 0,
+                    "delta": {
+                        "tool_calls": [
+                            {
+                                "index": 0,
+                                "function": {"arguments": 'ation": "SF"}'},
+                            }
+                        ]
+                    },
+                    "logprobs": None,
+                    "finish_reason": None,
+                }
+            ],
+            "usage": None,
+        },
+        # Finish
+        {
+            "id": "chatcmpl-test",
+            "object": "chat.completion.chunk",
+            "created": 0,
+            "model": "gpt-4o",
+            "choices": [
+                {
+                    "index": 0,
+                    "delta": {},
+                    "logprobs": None,
+                    "finish_reason": "tool_calls",
+                }
+            ],
+            "usage": None,
+        },
+        # Usage chunk
+        {
+            "id": "chatcmpl-test",
+            "object": "chat.completion.chunk",
+            "created": 0,
+            "model": "gpt-4o",
+            "choices": [],
+            "usage": {
+                "prompt_tokens": 10,
+                "completion_tokens": 5,
+                "total_tokens": 15,
+            },
+        },
+    ]
+
+    llm = ChatOpenAI(model="gpt-4o", output_version="v1")
+
+    aggregated: AIMessageChunk | None = None
+    for raw_chunk in stream_chunks:
+        gen = llm._convert_chunk_to_generation_chunk(raw_chunk, AIMessageChunk, None)
+        if gen is None:
+            continue
+        chunk = cast(AIMessageChunk, gen.message)
+        aggregated = chunk if aggregated is None else aggregated + chunk
+
+    assert aggregated is not None
+    # Tool calls should be present
+    assert len(aggregated.tool_call_chunks) == 1
+    assert aggregated.tool_call_chunks[0]["name"] == "get_weather"
+
+    # content_blocks should include tool_call_chunk blocks
+    blocks = aggregated.content_blocks
+    block_types = {b["type"] for b in blocks}
+    assert "tool_call_chunk" in block_types
+
+
 def test_verbosity_parameter_payload() -> None:
     """Test verbosity parameter is included in request payload for Responses API."""
     llm = ChatOpenAI(model="gpt-5", verbosity="high", use_responses_api=True)