langchain_anthropic: add stop_reason in ChatAnthropic stream result (#23689)

`ChatAnthropic` can get `stop_reason` from the resulting `AIMessage` in
`invoke` and `ainvoke`, but not in `stream` and `astream`.
This is a different behavior from `ChatOpenAI`.
It is possible to get `stop_reason` from `stream` as well, since it is
needed to determine the next action after the LLM call. This would be
easier to handle in situations where only `stop_reason` is needed.

- Issue: NA
- Dependencies: NA
- Twitter handle: https://x.com/kiarina37

libs/partners/anthropic/langchain_anthropic/chat_models.py

@@ -518,7 +518,7 @@ class ChatAnthropic(BaseChatModel):
 
     anthropic_api_url: Optional[str] = Field(None, alias="base_url")
     """Base URL for API requests. Only specify if using a proxy or service emulator.
-    
+
     If a value isn't passed in and environment variable ANTHROPIC_BASE_URL is set, value
     will be read from there.
     """
@@ -1139,6 +1139,10 @@ def _make_message_chunk_from_anthropic_event(
                 output_tokens=output_tokens,
                 total_tokens=output_tokens,
             ),
+            response_metadata={
+                "stop_reason": event.delta.stop_reason,
+                "stop_sequence": event.delta.stop_sequence,
+            },
         )
     else:
         pass

libs/partners/anthropic/tests/integration_tests/test_chat_models.py

@@ -57,6 +57,8 @@ def test_stream() -> None:
         full.usage_metadata["input_tokens"] + full.usage_metadata["output_tokens"]
         == full.usage_metadata["total_tokens"]
     )
+    assert "stop_reason" in full.response_metadata
+    assert "stop_sequence" in full.response_metadata
 
 
 async def test_astream() -> None:
@@ -91,6 +93,8 @@ async def test_astream() -> None:
         full.usage_metadata["input_tokens"] + full.usage_metadata["output_tokens"]
         == full.usage_metadata["total_tokens"]
     )
+    assert "stop_reason" in full.response_metadata
+    assert "stop_sequence" in full.response_metadata
 
     # test usage metadata can be excluded
     model = ChatAnthropic(model_name=MODEL_NAME, stream_usage=False)  # type: ignore[call-arg]