fix(langchain-classic): fix init_chat_model for HuggingFace models (#33943)

libs/partners/huggingface/tests/unit_tests/test_chat_models.py

@@ -337,3 +337,55 @@ def test_profile() -> None:
         llm=empty_llm,
     )
     assert model.profile
+
+
+def test_init_chat_model_huggingface() -> None:
+    """Test that init_chat_model works with HuggingFace models.
+
+    This test verifies that the fix for issue #28226 works correctly.
+    The issue was that init_chat_model didn't properly handle HuggingFace
+    model initialization, particularly the required 'task' parameter and
+    parameter separation between HuggingFacePipeline and ChatHuggingFace.
+    """
+    from langchain.chat_models.base import init_chat_model
+
+    # Test basic initialization with default task
+    # Note: This test may skip in CI if model download fails, but it verifies
+    # that the initialization code path works correctly
+    try:
+        llm = init_chat_model(
+            model="microsoft/Phi-3-mini-4k-instruct",
+            model_provider="huggingface",
+            temperature=0,
+            max_tokens=1024,
+        )
+
+        # Verify that ChatHuggingFace was created successfully
+        assert llm is not None
+        from langchain_huggingface import ChatHuggingFace
+
+        assert isinstance(llm, ChatHuggingFace)
+
+        # Verify that the llm attribute is set (this was the bug - it was missing)
+        assert hasattr(llm, "llm")
+        assert llm.llm is not None
+
+        # Test with explicit task parameter
+        llm2 = init_chat_model(
+            model="microsoft/Phi-3-mini-4k-instruct",
+            model_provider="huggingface",
+            task="text-generation",
+            temperature=0.5,
+        )
+        assert isinstance(llm2, ChatHuggingFace)
+        assert llm2.llm is not None
+    except (
+        ImportError,
+        OSError,
+        RuntimeError,
+        ValueError,
+    ) as e:
+        # If model download fails in CI, skip the test rather than failing
+        # The important part is that the code path doesn't raise ValidationError
+        # about missing 'llm' field, which was the original bug
+        pytest.skip(f"Skipping test due to model download/initialization error: {e}")