fix: concat text blocks for system_prompt

Author: dbschmigelski

src/strands/agent/agent.py

@@ -213,7 +213,7 @@ def _find_normalized_tool_name(self, name: str) -> str:
 
     def __init__(
         self,
-        model: Model | str | None = None,
+        model: Union[Model, str, None] = None,
         messages: Optional[Messages] = None,
         tools: Optional[list[Union[str, dict[str, str], "ToolProvider", Any]]] = None,
         system_prompt: Optional[str | list[SystemContentBlock]] = None,
@@ -224,7 +224,7 @@ def __init__(
         conversation_manager: Optional[ConversationManager] = None,
         record_direct_tool_call: bool = True,
         load_tools_from_directory: bool = False,
-        trace_attributes: Mapping[str, AttributeValue] | None = None,
+        trace_attributes: Optional[Mapping[str, AttributeValue]] = None,
         *,
         agent_id: Optional[str] = None,
         name: Optional[str] = None,
@@ -970,27 +970,27 @@ def _filter_tool_parameters_for_recording(self, tool_name: str, input_params: di
 
     def _initialize_system_prompt(
         self, system_prompt: str | list[SystemContentBlock] | None
-    ) -> tuple[str | None, list[SystemContentBlock]]:
+    ) -> tuple[str | None, list[SystemContentBlock] | None]:
         """Initialize system prompt fields from constructor input.
 
         Maintains backwards compatibility by keeping system_prompt as str when string input
         provided, avoiding breaking existing consumers.
 
         Maps system_prompt input to both string and content block representations:
         - If string: system_prompt=string, _system_prompt_content=[{text: string}]
-        - If list with single text element: system_prompt=text, _system_prompt_content=list
-        - If list (other cases): system_prompt=None, _system_prompt_content=list
-        - If None: system_prompt=None, _system_prompt_content=[]
+        - If list with text elements: system_prompt=concatenated_text, _system_prompt_content=list
+        - If list without text elements: system_prompt=None, _system_prompt_content=list
+        - If None: system_prompt=None, _system_prompt_content=None
         """
         if isinstance(system_prompt, str):
             return system_prompt, [{"text": system_prompt}]
         elif isinstance(system_prompt, list):
-            # If list has single element with text, also set system_prompt for backwards compatibility
-            if len(system_prompt) == 1 and "text" in system_prompt[0]:
-                return system_prompt[0]["text"], system_prompt
-            return None, system_prompt
+            # Concatenate all text elements for backwards compatibility, None if no text found
+            text_parts = [block["text"] for block in system_prompt if "text" in block]
+            system_prompt_str = "\n".join(text_parts) if text_parts else None
+            return system_prompt_str, system_prompt
         else:
-            return None, []
+            return None, None
 
     def _append_message(self, message: Message) -> None:
         """Appends a message to the agent's list of messages and invokes the callbacks for the MessageCreatedEvent."""

src/strands/event_loop/streaming.py

@@ -427,12 +427,11 @@ async def stream_messages(
 
     Args:
         model: Model provider.
+        system_prompt: The system prompt string, used for backwards compatibility with models that expect it.
         messages: List of messages to send.
         tool_specs: The list of tool specs.
-        system_prompt: Optional system prompt string for backwards compatibility.
         tool_choice: Optional tool choice constraint for forcing specific tool usage.
-        system_prompt_content: The system prompt content blocks to send.
-
+        system_prompt_content: The authoritative system prompt content blocks that always contains the system prompt data.
     Yields:
         The reason for stopping, the final message, and the usage metrics
     """
@@ -449,5 +448,5 @@ async def stream_messages(
         system_prompt_content=system_prompt_content,
     )
 
-    async for event in process_stream(chunks):
+    async for event in process_stream(chunks, start_time):
         yield event

src/strands/models/bedrock.py

@@ -191,9 +191,8 @@ def _format_request(
         self,
         messages: Messages,
         tool_specs: Optional[list[ToolSpec]] = None,
-        system_prompt: Optional[str] = None,
-        tool_choice: ToolChoice | None = None,
         system_prompt_content: Optional[list[SystemContentBlock]] = None,
+        tool_choice: ToolChoice | None = None,
     ) -> dict[str, Any]:
         """Format a Bedrock converse stream request.
 
@@ -632,7 +631,7 @@ def callback(event: Optional[StreamEvent] = None) -> None:
         loop = asyncio.get_event_loop()
         queue: asyncio.Queue[Optional[StreamEvent]] = asyncio.Queue()
 
-        thread = asyncio.to_thread(self._stream, callback, messages, tool_specs, system_prompt, tool_choice, system_prompt_content)
+        thread = asyncio.to_thread(self._stream, callback, messages, tool_specs, system_prompt_content, tool_choice)
         task = asyncio.create_task(thread)
 
         while True:
@@ -649,9 +648,8 @@ def _stream(
         callback: Callable[..., None],
         messages: Messages,
         tool_specs: Optional[list[ToolSpec]] = None,
-        system_prompt: Optional[str] = None,
-        tool_choice: ToolChoice | None = None,
         system_prompt_content: Optional[list[SystemContentBlock]] = None,
+        tool_choice: ToolChoice | None = None,
     ) -> None:
         """Stream conversation with the Bedrock model.
 
@@ -662,16 +660,16 @@ def _stream(
             callback: Function to send events to the main thread.
             messages: List of message objects to be processed by the model.
             tool_specs: List of tool specifications to make available to the model.
-            tool_choice: Selection strategy for tool invocation.
             system_prompt_content: System prompt content blocks to provide context to the model.
+            tool_choice: Selection strategy for tool invocation.
 
         Raises:
             ContextWindowOverflowException: If the input exceeds the model's context window.
             ModelThrottledException: If the model service is throttling requests.
         """
         try:
             logger.debug("formatting request")
-            request = self._format_request(messages, tool_specs, None, tool_choice, system_prompt_content)
+            request = self._format_request(messages, tool_specs, system_prompt_content, tool_choice)
             logger.debug("request=<%s>", request)
 
             logger.debug("invoking model")

tests/strands/agent/test_agent.py

@@ -2193,7 +2193,7 @@ def test_agent_multiple_blocks_system_prompt():
     ]
     agent = Agent(system_prompt=system_prompt_content)
 
-    assert agent.system_prompt is None
+    assert agent.system_prompt == "You are a helpful assistant.\nAdditional instructions."
     assert agent._system_prompt_content == system_prompt_content
 
 
@@ -2211,7 +2211,7 @@ def test_agent_none_system_prompt():
     agent = Agent(system_prompt=None)
 
     assert agent.system_prompt is None
-    assert agent._system_prompt_content == []
+    assert agent._system_prompt_content == None
 
 
 def test_agent_empty_list_system_prompt():
@@ -2270,7 +2270,7 @@ def test_agent_initialize_system_prompt_multiple_blocks_input():
     ]
     result = agent._initialize_system_prompt(input_blocks)
 
-    assert result == (None, input_blocks)
+    assert result == ("First block\nSecond block", input_blocks)
 
 
 def test_agent_initialize_system_prompt_single_non_text_block_input():
@@ -2287,7 +2287,7 @@ def test_agent_initialize_system_prompt_none_input():
     agent = Agent()
     result = agent._initialize_system_prompt(None)
 
-    assert result == (None, [])
+    assert result == (None, None)
 
 
 def test_agent_initialize_system_prompt_empty_list_input():