fix: Fix broken converstaion with orphaned toolUse

src/strands/agent/agent.py

@@ -33,6 +33,7 @@
 from .. import _identifier
 from .._async import run_async
 from ..event_loop.event_loop import event_loop_cycle
+from ..tools._tool_helpers import generate_missing_tool_result_content
 
 if TYPE_CHECKING:
     from ..experimental.tools import ToolProvider
@@ -280,7 +281,7 @@ def __init__(
                 Defaults to None.
             session_manager: Manager for handling agent sessions including conversation history and state.
                 If provided, enables session-based persistence and state management.
-            tool_executor: Definition of tool execution stragety (e.g., sequential, concurrent, etc.).
+            tool_executor: Definition of tool execution strategy (e.g., sequential, concurrent, etc.).
 
         Raises:
             ValueError: If agent id contains path separators.
@@ -816,6 +817,21 @@ def _convert_prompt_to_messages(self, prompt: AgentInput) -> Messages:
 
         messages: Messages | None = None
         if prompt is not None:
+            # Check if the latest message is toolUse
+            if len(self.messages) > 0 and any("toolUse" in content for content in self.messages[-1]["content"]):
+                # Add toolResult message after to have a valid conversation
+                logger.info(
+                    "Agents latest message is toolUse, appending a toolResult message to have valid conversation."
+                )
+                tool_use_ids = [
+                    content["toolUse"]["toolUseId"] for content in self.messages[-1]["content"] if "toolUse" in content
+                ]
+                self._append_message(
+                    {
+                        "role": "user",
+                        "content": generate_missing_tool_result_content(tool_use_ids),
+                    }
+                )
             if isinstance(prompt, str):
                 # String input - convert to user message
                 messages = [{"role": "user", "content": [{"text": prompt}]}]

src/strands/session/repository_session_manager.py

@@ -4,6 +4,7 @@
 from typing import TYPE_CHECKING, Any, Optional
 
 from ..agent.state import AgentState
+from ..tools._tool_helpers import generate_missing_tool_result_content
 from ..types.content import Message
 from ..types.exceptions import SessionException
 from ..types.session import (
@@ -159,6 +160,50 @@ def initialize(self, agent: "Agent", **kwargs: Any) -> None:
             # Restore the agents messages array including the optional prepend messages
             agent.messages = prepend_messages + [session_message.to_message() for session_message in session_messages]
 
+            # Fix broken session histories: https://github.com/strands-agents/sdk-python/issues/859
+            agent.messages = self._fix_broken_tool_use(agent.messages)
+
+    def _fix_broken_tool_use(self, messages: list[Message]) -> list[Message]:
+        """Add tool_result after orphaned tool_use messages.
+
+        Before 1.15.0, strands had a bug where they persisted sessions with a potentially broken messages array.
+        This method retroactively fixes that issue by adding a tool_result outside of session management. After 1.15.0,
+        this bug is no longer present.
+        """
+        for index, message in enumerate(messages):
+            # Check all but the latest message in the messages array
+            # The latest message being orphaned is handled in the agent class
+            if index + 1 < len(messages):
+                if any("toolUse" in content for content in message["content"]):
+                    tool_use_ids = [
+                        content["toolUse"]["toolUseId"] for content in message["content"] if "toolUse" in content
+                    ]
+
+                    # Check if there are more messages after the current toolUse message
+                    tool_result_ids = [
+                        content["toolResult"]["toolUseId"]
+                        for content in messages[index + 1]["content"]
+                        if "toolResult" in content
+                    ]
+
+                    missing_tool_use_ids = list(set(tool_use_ids) - set(tool_result_ids))
+                    # If there area missing tool use ids, that means the messages history is broken
+                    if missing_tool_use_ids:
+                        logger.warning(
+                            "Session message history has an orphaned toolUse with no toolResult. "
+                            "Adding toolResult content blocks to create valid conversation."
+                        )
+                        # Create the missing toolResult content blocks
+                        missing_content_blocks = generate_missing_tool_result_content(missing_tool_use_ids)
+
+                        if tool_result_ids:
+                            # If there were any toolResult ids, that means only some of the content blocks are missing
+                            messages[index + 1]["content"].extend(missing_content_blocks)
+                        else:
+                            # The message following the toolUse was not a toolResult, so lets insert it
+                            messages.insert(index + 1, {"role": "user", "content": missing_content_blocks})
+        return messages
+
     def sync_multi_agent(self, source: "MultiAgentBase", **kwargs: Any) -> None:
         """Serialize and update the multi-agent state into the session repository.
 

src/strands/tools/_tool_helpers.py

@@ -1,6 +1,7 @@
 """Helpers for tools."""
 
-from strands.tools.decorator import tool
+from ..tools.decorator import tool
+from ..types.content import ContentBlock
 
 
 # https://github.com/strands-agents/sdk-python/issues/998
@@ -13,3 +14,17 @@ def noop_tool() -> None:
     summarization will fail. As a workaround, we register the no-op tool.
     """
     pass
+
+
+def generate_missing_tool_result_content(tool_use_ids: list[str]) -> list[ContentBlock]:
+    """Generate ToolResult content blocks for orphaned ToolUse message."""
+    return [
+        {
+            "toolResult": {
+                "toolUseId": tool_use_id,
+                "status": "error",
+                "content": [{"text": "Tool was interrupted."}],
+            }
+        }
+        for tool_use_id in tool_use_ids
+    ]

tests/strands/agent/test_agent.py

@@ -2215,3 +2215,143 @@ def test_redact_user_content(content, expected):
     agent = Agent()
     result = agent._redact_user_content(content, "REDACTED")
     assert result == expected
+
+
+def test_agent_fixes_orphaned_tool_use_on_new_prompt(mock_model, agenerator):
+    """Test that agent adds toolResult for orphaned toolUse when called with new prompt."""
+    mock_model.mock_stream.return_value = agenerator(
+        [
+            {"messageStart": {"role": "assistant"}},
+            {"contentBlockStart": {"start": {"text": ""}}},
+            {"contentBlockDelta": {"delta": {"text": "Fixed!"}}},
+            {"contentBlockStop": {}},
+            {"messageStop": {"stopReason": "end_turn"}},
+        ]
+    )
+
+    # Start with orphaned toolUse message
+    messages = [
+        {
+            "role": "assistant",
+            "content": [
+                {"toolUse": {"toolUseId": "orphaned-123", "name": "tool_decorated", "input": {"random_string": "test"}}}
+            ],
+        }
+    ]
+
+    agent = Agent(model=mock_model, messages=messages)
+
+    # Call with new prompt should fix orphaned toolUse
+    agent("Continue conversation")
+
+    # Should have added toolResult message
+    assert len(agent.messages) >= 3
+    assert agent.messages[1] == {
+        "role": "user",
+        "content": [
+            {
+                "toolResult": {
+                    "toolUseId": "orphaned-123",
+                    "status": "error",
+                    "content": [{"text": "Tool was interrupted."}],
+                }
+            }
+        ],
+    }
+
+
+def test_agent_fixes_multiple_orphaned_tool_uses(mock_model, agenerator):
+    """Test that agent handles multiple orphaned toolUse messages."""
+    mock_model.mock_stream.return_value = agenerator(
+        [
+            {"messageStart": {"role": "assistant"}},
+            {"contentBlockStart": {"start": {"text": ""}}},
+            {"contentBlockDelta": {"delta": {"text": "Fixed multiple!"}}},
+            {"contentBlockStop": {}},
+            {"messageStop": {"stopReason": "end_turn"}},
+        ]
+    )
+
+    messages = [
+        {
+            "role": "assistant",
+            "content": [
+                {
+                    "toolUse": {
+                        "toolUseId": "orphaned-123",
+                        "name": "tool_decorated",
+                        "input": {"random_string": "test1"},
+                    }
+                },
+                {
+                    "toolUse": {
+                        "toolUseId": "orphaned-456",
+                        "name": "tool_decorated",
+                        "input": {"random_string": "test2"},
+                    }
+                },
+            ],
+        }
+    ]
+
+    agent = Agent(model=mock_model, messages=messages)
+    agent("Continue")
+
+    # Should have toolResult for both toolUse IDs
+    assert agent.messages[1] == {
+        "role": "user",
+        "content": [
+            {
+                "toolResult": {
+                    "toolUseId": "orphaned-123",
+                    "status": "error",
+                    "content": [{"text": "Tool was interrupted."}],
+                }
+            },
+            {
+                "toolResult": {
+                    "toolUseId": "orphaned-456",
+                    "status": "error",
+                    "content": [{"text": "Tool was interrupted."}],
+                }
+            },
+        ],
+    }
+
+
+def test_agent_skips_fix_for_valid_conversation(mock_model, agenerator):
+    """Test that agent doesn't modify valid toolUse/toolResult pairs."""
+    mock_model.mock_stream.return_value = agenerator(
+        [
+            {"messageStart": {"role": "assistant"}},
+            {"contentBlockStart": {"start": {"text": ""}}},
+            {"contentBlockDelta": {"delta": {"text": "No fix needed!"}}},
+            {"contentBlockStop": {}},
+            {"messageStop": {"stopReason": "end_turn"}},
+        ]
+    )
+
+    # Valid conversation with toolUse followed by toolResult
+    messages = [
+        {
+            "role": "assistant",
+            "content": [
+                {"toolUse": {"toolUseId": "valid-123", "name": "tool_decorated", "input": {"random_string": "test"}}}
+            ],
+        },
+        {
+            "role": "user",
+            "content": [
+                {"toolResult": {"toolUseId": "valid-123", "status": "success", "content": [{"text": "result"}]}}
+            ],
+        },
+    ]
+
+    agent = Agent(model=mock_model, messages=messages)
+    original_length = len(agent.messages)
+
+    agent("Continue")
+
+    # Should not have added any toolResult messages
+    # Only the new user message and assistant response should be added
+    assert len(agent.messages) == original_length + 2