fix: repair orphaned toolUse in last session message during restore

_fix_broken_tool_use skipped the last message with an explicit guard,
relying on the agent-class fallback (_has_tool_use_in_latest_message).
That fallback only works within the same process. When a new process
restores a session that ended with an orphaned toolUse (e.g. after a
runtime timeout), the guard causes the broken history to be sent to
the model, producing a ValidationException.

Remove the guard and handle the last-message case by appending a
synthetic toolResult with status 'error'. The tool execution context
is already lost at restore time, so letting the model decide how to
proceed is the correct behavior.

Updates the test that asserted the old (incorrect) behavior.

Author: Zelys-DFKH

src/strands/session/repository_session_manager.py

@@ -273,37 +273,48 @@ def _fix_broken_tool_use(self, messages: list[Message]) -> list[Message]:
 
         # Then check for orphaned toolUse messages
         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 are 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})
+            if not any("toolUse" in content for content in message["content"]):
+                continue
+
+            tool_use_ids = [
+                content["toolUse"]["toolUseId"] for content in message["content"] if "toolUse" in content
+            ]
+
+            if index + 1 >= len(messages):
+                # The last message has an orphaned toolUse. The in-process fallback
+                # (_has_tool_use_in_latest_message) only works within the same process.
+                # On cross-process restore the tool execution context is lost, so report
+                # an error to the model and let it decide how to proceed.
+                logger.warning(
+                    "Session message history ends with an orphaned toolUse with no toolResult. "
+                    "Adding toolResult content blocks to create valid conversation."
+                )
+                missing_content_blocks = generate_missing_tool_result_content(tool_use_ids)
+                messages.append({"role": "user", "content": missing_content_blocks})
+            else:
+                # Check if the next message already has tool results
+                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 are 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:

tests/strands/session/test_repository_session_manager.py

@@ -416,8 +416,8 @@ def test_fix_broken_tool_use_handles_multiple_orphaned_tools(existing_session_ma
     assert tool_use_ids == {"orphaned-123", "orphaned-456"}
 
 
-def test_fix_broken_tool_use_ignores_last_message(session_manager):
-    """Test that orphaned toolUse in the last message is not fixed."""
+def test_fix_broken_tool_use_repairs_orphaned_last_message(session_manager):
+    """Test that orphaned toolUse in the last message is repaired with a synthetic toolResult."""
     messages = [
         {"role": "user", "content": [{"text": "Hello"}]},
         {
@@ -430,8 +430,13 @@ def test_fix_broken_tool_use_ignores_last_message(session_manager):
 
     fixed_messages = session_manager._fix_broken_tool_use(messages)
 
-    # Should remain unchanged since toolUse is in last message
-    assert fixed_messages == messages
+    # A synthetic toolResult should be appended for the orphaned toolUse
+    assert len(fixed_messages) == 3
+    assert fixed_messages[2]["role"] == "user"
+    tool_result_ids = [
+        c["toolResult"]["toolUseId"] for c in fixed_messages[2]["content"] if "toolResult" in c
+    ]
+    assert "last-message-123" in tool_result_ids
 
 
 def test_fix_broken_tool_use_does_not_change_valid_message(session_manager):