fix: conditionally set stop sequences based on LLM support

- Fix issue #3317 where CrewAI injects stop sequences causing Bedrock GPT-OSS 400 errors
- Only set stop sequences when LLM supports them via use_stop_words flag
- Preserve existing stop sequences for unsupported models
- Add comprehensive tests covering the fix
- Add reproduction script to verify the issue and fix

Co-Authored-By: João <joao@crewai.com>

tests/test_bedrock_stop_sequence_fix.py

@@ -0,0 +1,192 @@
+import pytest
+from unittest.mock import Mock, patch
+from crewai.agents.crew_agent_executor import CrewAgentExecutor
+from crewai.llm import LLM
+from crewai.agents.agent_builder.base_agent import BaseAgent
+from crewai.agents.tools_handler import ToolsHandler
+
+
+class TestBedrockStopSequenceFix:
+    """Test cases for issue #3317 - Bedrock stop sequence fix"""
+
+    def test_stop_sequences_set_when_supported(self):
+        """Test that stop sequences are set when LLM supports them"""
+        mock_llm = Mock(spec=LLM)
+        mock_llm.supports_stop_words.return_value = True
+        mock_llm.stop = []
+        
+        mock_agent = Mock(spec=BaseAgent)
+        mock_task = Mock()
+        mock_crew = Mock()
+        mock_tools_handler = Mock(spec=ToolsHandler)
+        
+        stop_words = ["\nObservation:", "\nThought:"]
+        executor = CrewAgentExecutor(
+            llm=mock_llm,
+            task=mock_task,
+            crew=mock_crew,
+            agent=mock_agent,
+            prompt={"prompt": "test"},
+            max_iter=5,
+            tools=[],
+            tools_names="",
+            stop_words=stop_words,
+            tools_description="",
+            tools_handler=mock_tools_handler
+        )
+        
+        assert executor.use_stop_words is True
+        assert mock_llm.stop == stop_words
+
+    def test_stop_sequences_not_set_when_unsupported(self):
+        """Test that stop sequences are not set when LLM doesn't support them"""
+        mock_llm = Mock(spec=LLM)
+        mock_llm.supports_stop_words.return_value = False
+        mock_llm.stop = []
+        
+        mock_agent = Mock(spec=BaseAgent)
+        mock_task = Mock()
+        mock_crew = Mock()
+        mock_tools_handler = Mock(spec=ToolsHandler)
+        
+        stop_words = ["\nObservation:", "\nThought:"]
+        executor = CrewAgentExecutor(
+            llm=mock_llm,
+            task=mock_task,
+            crew=mock_crew,
+            agent=mock_agent,
+            prompt={"prompt": "test"},
+            max_iter=5,
+            tools=[],
+            tools_names="",
+            stop_words=stop_words,
+            tools_description="",
+            tools_handler=mock_tools_handler
+        )
+        
+        assert executor.use_stop_words is False
+        assert mock_llm.stop == []
+
+    def test_existing_stop_sequences_preserved_when_supported(self):
+        """Test that existing stop sequences are preserved when adding new ones"""
+        mock_llm = Mock(spec=LLM)
+        mock_llm.supports_stop_words.return_value = True
+        mock_llm.stop = ["existing_stop"]
+        
+        mock_agent = Mock(spec=BaseAgent)
+        mock_task = Mock()
+        mock_crew = Mock()
+        mock_tools_handler = Mock(spec=ToolsHandler)
+        
+        stop_words = ["\nObservation:"]
+        executor = CrewAgentExecutor(
+            llm=mock_llm,
+            task=mock_task,
+            crew=mock_crew,
+            agent=mock_agent,
+            prompt={"prompt": "test"},
+            max_iter=5,
+            tools=[],
+            tools_names="",
+            stop_words=stop_words,
+            tools_description="",
+            tools_handler=mock_tools_handler
+        )
+        
+        assert executor.use_stop_words is True
+        assert "existing_stop" in mock_llm.stop
+        assert "\nObservation:" in mock_llm.stop
+
+    def test_existing_stop_sequences_preserved_when_unsupported(self):
+        """Test that existing stop sequences are preserved when LLM doesn't support new ones"""
+        mock_llm = Mock(spec=LLM)
+        mock_llm.supports_stop_words.return_value = False
+        mock_llm.stop = ["existing_stop"]
+        
+        mock_agent = Mock(spec=BaseAgent)
+        mock_task = Mock()
+        mock_crew = Mock()
+        mock_tools_handler = Mock(spec=ToolsHandler)
+        
+        stop_words = ["\nObservation:"]
+        executor = CrewAgentExecutor(
+            llm=mock_llm,
+            task=mock_task,
+            crew=mock_crew,
+            agent=mock_agent,
+            prompt={"prompt": "test"},
+            max_iter=5,
+            tools=[],
+            tools_names="",
+            stop_words=stop_words,
+            tools_description="",
+            tools_handler=mock_tools_handler
+        )
+        
+        assert executor.use_stop_words is False
+        assert mock_llm.stop == ["existing_stop"]
+
+    @patch('crewai.llm.get_supported_openai_params')
+    def test_bedrock_model_stop_words_support(self, mock_get_params):
+        """Test that Bedrock models correctly report stop word support"""
+        mock_get_params.return_value = ['model', 'messages', 'temperature']  # No 'stop'
+        
+        llm = LLM(model="bedrock/converse/openai.gpt-oss-20b-1:0")
+        
+        assert llm.supports_stop_words() is False
+
+    @patch('crewai.llm.get_supported_openai_params')
+    def test_openai_model_stop_words_support(self, mock_get_params):
+        """Test that OpenAI models correctly report stop word support"""
+        mock_get_params.return_value = ['model', 'messages', 'temperature', 'stop']
+        
+        llm = LLM(model="gpt-4")
+        
+        assert llm.supports_stop_words() is True
+
+    def test_use_stop_words_flag_consistency(self):
+        """Test that use_stop_words flag is consistent with LLM support"""
+        mock_llm_supporting = Mock(spec=LLM)
+        mock_llm_supporting.supports_stop_words.return_value = True
+        mock_llm_supporting.stop = []
+        
+        mock_agent = Mock(spec=BaseAgent)
+        mock_task = Mock()
+        mock_crew = Mock()
+        mock_tools_handler = Mock(spec=ToolsHandler)
+        
+        executor_supporting = CrewAgentExecutor(
+            llm=mock_llm_supporting,
+            task=mock_task,
+            crew=mock_crew,
+            agent=mock_agent,
+            prompt={"prompt": "test"},
+            max_iter=5,
+            tools=[],
+            tools_names="",
+            stop_words=["\nObservation:"],
+            tools_description="",
+            tools_handler=mock_tools_handler
+        )
+        
+        assert executor_supporting.use_stop_words is True
+        
+        mock_llm_non_supporting = Mock(spec=LLM)
+        mock_llm_non_supporting.supports_stop_words.return_value = False
+        mock_llm_non_supporting.stop = []
+        
+        executor_non_supporting = CrewAgentExecutor(
+            llm=mock_llm_non_supporting,
+            task=mock_task,
+            crew=mock_crew,
+            agent=mock_agent,
+            prompt={"prompt": "test"},
+            max_iter=5,
+            tools=[],
+            tools_names="",
+            stop_words=["\nObservation:"],
+            tools_description="",
+            tools_handler=mock_tools_handler
+        )
+        
+        assert executor_non_supporting.use_stop_words is False