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>

src/podcast/graph/script_writer_node.py

@@ -1,13 +1,16 @@
 # Copyright (c) 2025 Bytedance Ltd. and/or its affiliates
 # SPDX-License-Identifier: MIT
 
+import json
 import logging
 
+import openai
 from langchain_core.messages import HumanMessage, SystemMessage
 
 from src.config.agents import AGENT_LLM_MAP
 from src.llms.llm import get_llm_by_type
 from src.prompts.template import get_prompt_template
+from src.utils.json_utils import repair_json_output
 
 from ..types import Script
 from .state import PodcastState
@@ -17,14 +20,39 @@ logger = logging.getLogger(__name__)
 
 def script_writer_node(state: PodcastState):
     logger.info("Generating script for podcast...")
-    model = get_llm_by_type(
-        AGENT_LLM_MAP["podcast_script_writer"]
-    ).with_structured_output(Script, method="json_mode")
-    script = model.invoke(
-        [
-            SystemMessage(content=get_prompt_template("podcast/podcast_script_writer")),
-            HumanMessage(content=state["input"]),
-        ],
-    )
-    print(script)
+    base_model = get_llm_by_type(AGENT_LLM_MAP["podcast_script_writer"])
+
+    messages = [
+        SystemMessage(content=get_prompt_template("podcast/podcast_script_writer")),
+        HumanMessage(content=state["input"]),
+    ]
+
+    try:
+        # Try structured output with json_mode first
+        model = base_model.with_structured_output(Script, method="json_mode")
+        script = model.invoke(messages)
+    except openai.BadRequestError as e:
+        # Fall back for models that don't support json_object (e.g., Kimi K2)
+        if "json_object" in str(e).lower():
+            logger.warning(
+                f"Model doesn't support json_mode, falling back to prompting: {e}"
+            )
+            response = base_model.invoke(messages)
+            content = response.content if hasattr(response, "content") else str(response)
+            try:
+                repaired = repair_json_output(content)
+                script_dict = json.loads(repaired)
+            except json.JSONDecodeError as json_err:
+                logger.error(
+                    "Failed to parse JSON from podcast script writer fallback "
+                    "response: %s; content: %r",
+                    json_err,
+                    content,
+                )
+                raise
+            script = Script.model_validate(script_dict)
+        else:
+            raise
+
+    logger.debug("Generated podcast script: %s", script)
     return {"script": script, "audio_chunks": []}