fix: correct tool_result pairing after user rejection; add Anthropic repro; keep list serializer

examples/repro_reject_anthropic.py

@@ -0,0 +1,54 @@
+import os
+from pydantic import SecretStr
+
+from openhands.sdk import LLM, Agent, Conversation, Tool
+from openhands.sdk.security.confirmation_policy import AlwaysConfirm
+from openhands.tools.file_editor import FileEditorTool
+from openhands.tools.terminal import TerminalTool
+
+
+def main():
+    api_key = os.environ.get("LITELLM_API_KEY")
+    if not api_key:
+        raise SystemExit("LITELLM_API_KEY not set")
+
+    llm = LLM(
+        model="litellm_proxy/anthropic/claude-sonnet-4-5-20250929",
+        base_url="https://llm-proxy.eval.all-hands.dev",
+        api_key=SecretStr(api_key),
+        usage_id="repro-reject-anthropic",
+    )
+
+    agent = Agent(
+        llm=llm,
+        tools=[
+            Tool(name=TerminalTool.name),
+            Tool(name=FileEditorTool.name),
+        ],
+    )
+
+    conv = Conversation(agent=agent, workspace=os.getcwd())
+    conv.set_confirmation_policy(AlwaysConfirm())
+
+    # Intentionally trigger a file edit to get a tool_use
+    conv.send_message("Create a file TEST_REPRO.txt with content 'hello world'")
+    try:
+        conv.run()
+    except Exception as e:
+        print("First run error:", e)
+        raise
+
+    # Reject pending actions
+    conv.reject_pending_actions("Please explain first")
+
+    # Run again to send the tool_result (rejection) immediately after tool_use
+    try:
+        conv.run()
+        print("Second run completed without Anthropic tool_result error.")
+    except Exception as e:
+        print("Second run error:", e)
+        raise
+
+
+if __name__ == "__main__":
+    main()

tests/sdk/event/test_user_reject_tool_result_order.py

@@ -0,0 +1,52 @@
+from openhands.sdk.event import ActionEvent, MessageEvent
+from openhands.sdk.event.base import LLMConvertibleEvent
+from openhands.sdk.event.llm_convertible import UserRejectObservation
+from openhands.sdk.llm import Message, MessageToolCall, TextContent
+
+
+def test_reject_emits_tool_result_immediately_after_tool_use():
+    # 1) Simulate a user message that led to a tool call
+    user_msg = MessageEvent(
+        source="user",
+        llm_message=Message(role="user", content=[TextContent(text="do something")]),
+    )
+
+    # 2) Simulate assistant tool_use (ActionEvent)
+    tool_call = MessageToolCall(
+        id="toolu_01ABC",
+        name="str_replace_editor",
+        arguments="{}",
+        origin="completion",
+    )
+    action = ActionEvent(
+        source="agent",
+        thought=[TextContent(text="Let me do that")],
+        tool_name="str_replace_editor",
+        tool_call_id=tool_call.id,
+        tool_call=tool_call,
+        llm_response_id="resp_1",
+        action=None,  # not executed due to confirmation mode
+    )
+
+    # 3) Simulate a user rejection of that action
+    reject = UserRejectObservation(
+        source="environment",
+        action_id=action.id,
+        tool_name=action.tool_name,
+        tool_call_id=action.tool_call_id,
+        rejection_reason="Please explain first",
+    )
+
+    # Convert events to LLM messages using the core utility
+    messages = LLMConvertibleEvent.events_to_messages([user_msg, action, reject])
+
+    # Assert shape: assistant tool_use followed immediately by tool_result
+    # messages[0] is user
+    assert messages[1].role == "assistant"
+    assert messages[1].tool_calls and len(messages[1].tool_calls) == 1
+    assert messages[1].tool_calls[0].id == tool_call.id
+
+    assert messages[2].role == "tool"
+    assert messages[2].tool_call_id == tool_call.id
+    # Ensure serializer is forced to string to satisfy strict providers
+    assert getattr(messages[2], "force_string_serializer", False) is True