diff --git a/src/graph/nodes.py b/src/graph/nodes.py
index cb107f0..af313d5 100644
--- a/src/graph/nodes.py
+++ b/src/graph/nodes.py
@@ -26,7 +26,7 @@ from src.tools import (
     python_repl_tool,
 )
 from src.tools.search import LoggedTavilySearch
-from src.utils.context_manager import ContextManager
+from src.utils.context_manager import ContextManager, validate_message_content
 from src.utils.json_utils import repair_json_output
 
 from ..config import SELECTED_SEARCH_ENGINE, SearchEngine
@@ -138,7 +138,8 @@ def background_investigation_node(state: State, config: RunnableConfig):
     logger.info("background investigation node is running.")
     configurable = Configuration.from_runnable_config(config)
     query = state.get("clarified_research_topic") or state.get("research_topic")
-    background_investigation_results = None
+    background_investigation_results = []
+    
     if SELECTED_SEARCH_ENGINE == SearchEngine.TAVILY.value:
         searched_content = LoggedTavilySearch(
             max_results=configurable.max_search_results
@@ -146,7 +147,27 @@ def background_investigation_node(state: State, config: RunnableConfig):
         # check if the searched_content is a tuple, then we need to unpack it
         if isinstance(searched_content, tuple):
             searched_content = searched_content[0]
-        if isinstance(searched_content, list):
+        
+        # Handle string JSON response (new format from fixed Tavily tool)
+        if isinstance(searched_content, str):
+            try:
+                parsed = json.loads(searched_content)
+                if isinstance(parsed, dict) and "error" in parsed:
+                    logger.error(f"Tavily search error: {parsed['error']}")
+                    background_investigation_results = []
+                elif isinstance(parsed, list):
+                    background_investigation_results = [
+                        f"## {elem.get('title', 'Untitled')}\n\n{elem.get('content', 'No content')}" 
+                        for elem in parsed
+                    ]
+                else:
+                    logger.error(f"Unexpected Tavily response format: {searched_content}")
+                    background_investigation_results = []
+            except json.JSONDecodeError:
+                logger.error(f"Failed to parse Tavily response as JSON: {searched_content}")
+                background_investigation_results = []
+        # Handle legacy list format
+        elif isinstance(searched_content, list):
             background_investigation_results = [
                 f"## {elem['title']}\n\n{elem['content']}" for elem in searched_content
             ]
@@ -159,10 +180,12 @@ def background_investigation_node(state: State, config: RunnableConfig):
             logger.error(
                 f"Tavily search returned malformed response: {searched_content}"
             )
+            background_investigation_results = []
     else:
         background_investigation_results = get_web_search_tool(
             configurable.max_search_results
         ).invoke(query)
+    
     return {
         "background_investigation_results": json.dumps(
             background_investigation_results, ensure_ascii=False
@@ -723,6 +746,14 @@ async def _execute_agent_step(
         recursion_limit = default_recursion_limit
 
     logger.info(f"Agent input: {agent_input}")
+    
+    # Validate message content before invoking agent
+    try:
+        validated_messages = validate_message_content(agent_input["messages"])
+        agent_input["messages"] = validated_messages
+    except Exception as validation_error:
+        logger.error(f"Error validating agent input messages: {validation_error}")
+    
     try:
         result = await agent.ainvoke(
             input=agent_input, config={"recursion_limit": recursion_limit}
@@ -734,6 +765,15 @@ async def _execute_agent_step(
         error_message = f"Error executing {agent_name} agent for step '{current_step.title}': {str(e)}"
         logger.exception(error_message)
         logger.error(f"Full traceback:\n{error_traceback}")
+        
+        # Enhanced error diagnostics for content-related errors
+        if "Field required" in str(e) and "content" in str(e):
+            logger.error(f"Message content validation error detected")
+            for i, msg in enumerate(agent_input.get('messages', [])):
+                logger.error(f"Message {i}: type={type(msg).__name__}, "
+                            f"has_content={hasattr(msg, 'content')}, "
+                            f"content_type={type(msg.content).__name__ if hasattr(msg, 'content') else 'N/A'}, "
+                            f"content_len={len(str(msg.content)) if hasattr(msg, 'content') and msg.content else 0}")
 
         detailed_error = f"[ERROR] {agent_name.capitalize()} Agent Error\n\nStep: {current_step.title}\n\nError Details:\n{str(e)}\n\nPlease check the logs for more information."
         current_step.execution_res = detailed_error
diff --git a/src/tools/tavily_search/tavily_search_results_with_images.py b/src/tools/tavily_search/tavily_search_results_with_images.py
index 7ecde9e..0fd83e6 100644
--- a/src/tools/tavily_search/tavily_search_results_with_images.py
+++ b/src/tools/tavily_search/tavily_search_results_with_images.py
@@ -129,12 +129,14 @@ class TavilySearchWithImages(TavilySearchResults):  # type: ignore[override, ove
             )
         except Exception as e:
             logger.error("Tavily search returned error: {}".format(e))
-            return repr(e), {}
+            error_result = json.dumps({"error": repr(e)}, ensure_ascii=False)
+            return error_result, {}
         cleaned_results = self.api_wrapper.clean_results_with_images(raw_results)
         logger.debug(
             "sync: %s", json.dumps(cleaned_results, indent=2, ensure_ascii=False)
         )
-        return cleaned_results, raw_results
+        result_json = json.dumps(cleaned_results, ensure_ascii=False)
+        return result_json, raw_results
 
     async def _arun(
         self,
@@ -156,9 +158,11 @@ class TavilySearchWithImages(TavilySearchResults):  # type: ignore[override, ove
             )
         except Exception as e:
             logger.error("Tavily search returned error: {}".format(e))
-            return repr(e), {}
+            error_result = json.dumps({"error": repr(e)}, ensure_ascii=False)
+            return error_result, {}
         cleaned_results = self.api_wrapper.clean_results_with_images(raw_results)
         logger.debug(
             "async: %s", json.dumps(cleaned_results, indent=2, ensure_ascii=False)
         )
-        return cleaned_results, raw_results
+        result_json = json.dumps(cleaned_results, ensure_ascii=False)
+        return result_json, raw_results
diff --git a/src/utils/context_manager.py b/src/utils/context_manager.py
index b56d6a6..d551cda 100644
--- a/src/utils/context_manager.py
+++ b/src/utils/context_manager.py
@@ -264,3 +264,54 @@ class ContextManager:
         """
         # TODO: summary implementation
         pass
+
+
+def validate_message_content(messages: List[BaseMessage]) -> List[BaseMessage]:
+    """
+    Validate and fix all messages to ensure they have valid content before sending to LLM.
+    
+    This function ensures:
+    1. All messages have a content field
+    2. No message has None or empty string content (except for legitimate empty responses)
+    3. Complex objects (lists, dicts) are converted to JSON strings
+    
+    Args:
+        messages: List of messages to validate
+    
+    Returns:
+        List of validated messages with fixed content
+    """
+    validated = []
+    for i, msg in enumerate(messages):
+        try:
+            # Check if message has content attribute
+            if not hasattr(msg, 'content'):
+                logger.warning(f"Message {i} ({type(msg).__name__}) has no content attribute")
+                msg.content = ""
+            
+            # Handle None content
+            elif msg.content is None:
+                logger.warning(f"Message {i} ({type(msg).__name__}) has None content, setting to empty string")
+                msg.content = ""
+            
+            # Handle complex content types (convert to JSON)
+            elif isinstance(msg.content, (list, dict)):
+                logger.debug(f"Message {i} ({type(msg).__name__}) has complex content type {type(msg.content).__name__}, converting to JSON")
+                msg.content = json.dumps(msg.content, ensure_ascii=False)
+            
+            # Handle other non-string types
+            elif not isinstance(msg.content, str):
+                logger.debug(f"Message {i} ({type(msg).__name__}) has non-string content type {type(msg.content).__name__}, converting to string")
+                msg.content = str(msg.content)
+            
+            validated.append(msg)
+        except Exception as e:
+            logger.error(f"Error validating message {i}: {e}")
+            # Create a safe fallback message
+            if isinstance(msg, ToolMessage):
+                msg.content = json.dumps({"error": str(e)}, ensure_ascii=False)
+            else:
+                msg.content = f"[Error processing message: {str(e)}]"
+            validated.append(msg)
+    
+    return validated
diff --git a/tests/integration/test_nodes.py b/tests/integration/test_nodes.py
index ef1e49c..7496019 100644
--- a/tests/integration/test_nodes.py
+++ b/tests/integration/test_nodes.py
@@ -136,7 +136,7 @@ def test_background_investigation_node_malformed_response(
 
         # Parse and verify the JSON content
         results = result["background_investigation_results"]
-        assert json.loads(results) is None
+        assert json.loads(results) == []
 
 
 @pytest.fixture
diff --git a/tests/unit/tools/test_tavily_search_results_with_images.py b/tests/unit/tools/test_tavily_search_results_with_images.py
index 1e24f04..e0f5301 100644
--- a/tests/unit/tools/test_tavily_search_results_with_images.py
+++ b/tests/unit/tools/test_tavily_search_results_with_images.py
@@ -1,6 +1,7 @@
 # Copyright (c) 2025 Bytedance Ltd. and/or its affiliates
 # SPDX-License-Identifier: MIT
 
+import json
 from unittest.mock import AsyncMock, Mock, patch
 
 import pytest
@@ -88,7 +89,7 @@ class TestTavilySearchWithImages:
 
         result, raw = search_tool._run("test query")
 
-        assert result == sample_cleaned_results
+        assert result == json.dumps(sample_cleaned_results, ensure_ascii=False)
         assert raw == sample_raw_results
 
         mock_api_wrapper.raw_results.assert_called_once_with(
@@ -113,7 +114,9 @@ class TestTavilySearchWithImages:
 
         result, raw = search_tool._run("test query")
 
-        assert "API Error" in result
+        result_dict = json.loads(result)
+        assert "error" in result_dict
+        assert "API Error" in result_dict["error"]
         assert raw == {}
         mock_api_wrapper.clean_results_with_images.assert_not_called()
 
@@ -131,7 +134,7 @@ class TestTavilySearchWithImages:
 
         result, raw = await search_tool._arun("test query")
 
-        assert result == sample_cleaned_results
+        assert result == json.dumps(sample_cleaned_results, ensure_ascii=False)
         assert raw == sample_raw_results
 
         mock_api_wrapper.raw_results_async.assert_called_once_with(
@@ -159,7 +162,9 @@ class TestTavilySearchWithImages:
 
         result, raw = await search_tool._arun("test query")
 
-        assert "Async API Error" in result
+        result_dict = json.loads(result)
+        assert "error" in result_dict
+        assert "Async API Error" in result_dict["error"]
         assert raw == {}
         mock_api_wrapper.clean_results_with_images.assert_not_called()
 
@@ -177,7 +182,7 @@ class TestTavilySearchWithImages:
 
         result, raw = search_tool._run("test query", run_manager=mock_run_manager)
 
-        assert result == sample_cleaned_results
+        assert result == json.dumps(sample_cleaned_results, ensure_ascii=False)
         assert raw == sample_raw_results
 
     @pytest.mark.asyncio
@@ -197,5 +202,5 @@ class TestTavilySearchWithImages:
             "test query", run_manager=mock_run_manager
         )
 
-        assert result == sample_cleaned_results
+        assert result == json.dumps(sample_cleaned_results, ensure_ascii=False)
         assert raw == sample_raw_results