fix(podcast): add fallback for models without json_object support (#747) (#785)

* fix(podcast): add fallback for models without json_object support (#747)

Models like Kimi K2 don't support response_format.type: json_object.
Add try-except to fall back to regular prompting with JSON parsing
when BadRequestError mentions json_object not supported.

- Add fallback to prompting + repair_json_output parsing
- Re-raise other BadRequestError types
- Add unit tests for script_writer_node with 100% coverage

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fixes: the unit test error of test_script_writer_node.py

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

tests/unit/podcast/__init__.py

@@ -0,0 +1,2 @@
+# Copyright (c) 2025 Bytedance Ltd. and/or its affiliates
+# SPDX-License-Identifier: MIT

tests/unit/podcast/test_script_writer_node.py

@@ -0,0 +1,214 @@
+# Copyright (c) 2025 Bytedance Ltd. and/or its affiliates
+# SPDX-License-Identifier: MIT
+
+import json
+from unittest.mock import MagicMock, patch
+
+import openai
+import pytest
+
+from src.podcast.graph.script_writer_node import script_writer_node
+from src.podcast.types import Script, ScriptLine
+
+
+class TestScriptWriterNode:
+    """Tests for script_writer_node function."""
+
+    @pytest.fixture
+    def sample_state(self):
+        """Create a sample podcast state."""
+        return {"input": "Test content for podcast generation"}
+
+    @pytest.fixture
+    def sample_script(self):
+        """Create a sample Script object."""
+        return Script(
+            locale="en",
+            lines=[
+                ScriptLine(speaker="male", paragraph="Hello, welcome to our podcast."),
+                ScriptLine(speaker="female", paragraph="Today we discuss testing."),
+            ],
+        )
+
+    @pytest.fixture
+    def sample_script_json(self, sample_script):
+        """Create JSON representation of sample script."""
+        return sample_script.model_dump_json()
+
+    @patch("src.podcast.graph.script_writer_node.get_prompt_template")
+    @patch("src.podcast.graph.script_writer_node.get_llm_by_type")
+    def test_script_writer_with_json_mode_success(
+        self, mock_get_llm, mock_get_template, sample_state, sample_script
+    ):
+        """Test successful script generation using json_mode."""
+        mock_get_template.return_value = "Generate a podcast script."
+
+        mock_model = MagicMock()
+        mock_structured_model = MagicMock()
+        mock_model.with_structured_output.return_value = mock_structured_model
+        mock_structured_model.invoke.return_value = sample_script
+        mock_get_llm.return_value = mock_model
+
+        result = script_writer_node(sample_state)
+
+        assert result["script"] == sample_script
+        assert result["audio_chunks"] == []
+        mock_model.with_structured_output.assert_called_once_with(
+            Script, method="json_mode"
+        )
+
+    @patch("src.podcast.graph.script_writer_node.get_prompt_template")
+    @patch("src.podcast.graph.script_writer_node.get_llm_by_type")
+    def test_script_writer_fallback_on_json_object_not_supported(
+        self, mock_get_llm, mock_get_template, sample_state, sample_script_json
+    ):
+        """Test fallback to prompting when model doesn't support json_object."""
+        mock_get_template.return_value = "Generate a podcast script."
+
+        mock_model = MagicMock()
+        mock_structured_model = MagicMock()
+        mock_model.with_structured_output.return_value = mock_structured_model
+
+        # Simulate json_object not supported error
+        mock_structured_model.invoke.side_effect = openai.BadRequestError(
+            message="json_object is not supported by this model",
+            response=MagicMock(status_code=400),
+            body={"error": {"message": "json_object is not supported"}},
+        )
+
+        # Mock the fallback response
+        mock_response = MagicMock()
+        mock_response.content = sample_script_json
+        mock_model.invoke.return_value = mock_response
+
+        mock_get_llm.return_value = mock_model
+
+        result = script_writer_node(sample_state)
+
+        assert result["script"].locale == "en"
+        assert len(result["script"].lines) == 2
+        assert result["audio_chunks"] == []
+        # Verify fallback was used
+        mock_model.invoke.assert_called_once()
+
+    @patch("src.podcast.graph.script_writer_node.get_prompt_template")
+    @patch("src.podcast.graph.script_writer_node.get_llm_by_type")
+    def test_script_writer_reraises_other_bad_request_errors(
+        self, mock_get_llm, mock_get_template, sample_state
+    ):
+        """Test that other BadRequestError types are re-raised."""
+        mock_get_template.return_value = "Generate a podcast script."
+
+        mock_model = MagicMock()
+        mock_structured_model = MagicMock()
+        mock_model.with_structured_output.return_value = mock_structured_model
+
+        # Simulate a different BadRequestError (not json_object related)
+        mock_structured_model.invoke.side_effect = openai.BadRequestError(
+            message="Invalid model parameter",
+            response=MagicMock(status_code=400),
+            body={"error": {"message": "Invalid model parameter"}},
+        )
+
+        mock_get_llm.return_value = mock_model
+
+        with pytest.raises(openai.BadRequestError) as exc_info:
+            script_writer_node(sample_state)
+
+        assert "Invalid model parameter" in str(exc_info.value)
+
+    @patch("src.podcast.graph.script_writer_node.get_prompt_template")
+    @patch("src.podcast.graph.script_writer_node.get_llm_by_type")
+    def test_script_writer_fallback_with_markdown_wrapped_json(
+        self, mock_get_llm, mock_get_template, sample_state
+    ):
+        """Test fallback handles JSON wrapped in markdown code blocks."""
+        mock_get_template.return_value = "Generate a podcast script."
+
+        mock_model = MagicMock()
+        mock_structured_model = MagicMock()
+        mock_model.with_structured_output.return_value = mock_structured_model
+
+        mock_structured_model.invoke.side_effect = openai.BadRequestError(
+            message="json_object is not supported",
+            response=MagicMock(status_code=400),
+            body={},
+        )
+
+        # Mock response with markdown-wrapped JSON (common LLM output)
+        mock_response = MagicMock()
+        mock_response.content = """```json
+{
+    "locale": "zh",
+    "lines": [
+        {"speaker": "male", "paragraph": "欢迎收听播客。"}
+    ]
+}
+```"""
+        mock_model.invoke.return_value = mock_response
+
+        mock_get_llm.return_value = mock_model
+
+        result = script_writer_node(sample_state)
+
+        assert result["script"].locale == "zh"
+        assert len(result["script"].lines) == 1
+        assert result["script"].lines[0].speaker == "male"
+
+    @patch("src.podcast.graph.script_writer_node.get_prompt_template")
+    @patch("src.podcast.graph.script_writer_node.get_llm_by_type")
+    def test_script_writer_fallback_raises_on_invalid_json(
+        self, mock_get_llm, mock_get_template, sample_state
+    ):
+        """Test that fallback raises JSONDecodeError when response is not valid JSON."""
+        mock_get_template.return_value = "Generate a podcast script."
+
+        mock_model = MagicMock()
+        mock_structured_model = MagicMock()
+        mock_model.with_structured_output.return_value = mock_structured_model
+
+        mock_structured_model.invoke.side_effect = openai.BadRequestError(
+            message="json_object is not supported",
+            response=MagicMock(status_code=400),
+            body={},
+        )
+
+        # Mock response with completely invalid JSON
+        mock_response = MagicMock()
+        mock_response.content = "This is not JSON at all, just plain text response."
+        mock_model.invoke.return_value = mock_response
+
+        mock_get_llm.return_value = mock_model
+
+        with pytest.raises(json.JSONDecodeError):
+            script_writer_node(sample_state)
+
+    @patch("src.podcast.graph.script_writer_node.get_prompt_template")
+    @patch("src.podcast.graph.script_writer_node.get_llm_by_type")
+    def test_script_writer_fallback_raises_on_invalid_schema(
+        self, mock_get_llm, mock_get_template, sample_state
+    ):
+        """Test that fallback raises ValidationError when JSON doesn't match Script schema."""
+        mock_get_template.return_value = "Generate a podcast script."
+
+        mock_model = MagicMock()
+        mock_structured_model = MagicMock()
+        mock_model.with_structured_output.return_value = mock_structured_model
+
+        mock_structured_model.invoke.side_effect = openai.BadRequestError(
+            message="json_object is not supported",
+            response=MagicMock(status_code=400),
+            body={},
+        )
+
+        # Mock response with valid JSON but invalid schema (missing required fields, wrong types)
+        mock_response = MagicMock()
+        mock_response.content = '{"locale": "invalid_locale", "lines": "not_a_list"}'
+        mock_model.invoke.return_value = mock_response
+
+        mock_get_llm.return_value = mock_model
+
+        # Pydantic ValidationError is raised when schema validation fails
+        from pydantic import ValidationError
+        with pytest.raises(ValidationError):
+            script_writer_node(sample_state)