fix(agent): close tool-call sequence on all interrupt aborts, not just finalize_turn

#48879 closed the tool-call sequence on interrupt inside finalize_turn so a
/stop after a tool no longer persists a `tool` tail that the next user message
turns into a `tool -> user` role-alternation violation (which strict providers
like Gemini/Claude react to by hallucinating a continuation and ignoring prior
context — what users see as "lost context after stop").

But the retry-wait, error-handling, and post-error retry-wait interrupt aborts
in conversation_loop return early and never reach finalize_turn, so they still
persisted and returned a raw `tool` tail. Interrupting during provider
backoff/rate-limiting (common under heavy work) hit exactly this path.

Extract the close into a shared close_interrupted_tool_sequence helper and apply
it at every interrupt abort (finalize_turn + the three early returns) so the
whole bug class is fixed, not just the one site.

agent/conversation_loop.py

@@ -35,6 +35,7 @@ from agent.turn_context import build_turn_context
 from agent.turn_retry_state import TurnRetryState
 from agent.memory_manager import build_memory_context_block
 from agent.message_sanitization import (
+    close_interrupted_tool_sequence,
     _repair_tool_call_arguments,
     _sanitize_messages_non_ascii,
     _sanitize_messages_surrogates,
@@ -1396,10 +1397,12 @@ def run_conversation(
                     while time.time() < sleep_end:
                         if agent._interrupt_requested:
                             agent._vprint(f"{agent.log_prefix}⚡ Interrupt detected during retry wait, aborting.", force=True)
+                            _interrupt_text = f"Operation interrupted during retry ({_failure_hint}, attempt {retry_count}/{max_retries})."
+                            close_interrupted_tool_sequence(messages, _interrupt_text)
                             agent._persist_session(messages, conversation_history)
                             agent.clear_interrupt()
                             return {
-                                "final_response": f"Operation interrupted during retry ({_failure_hint}, attempt {retry_count}/{max_retries}).",
+                                "final_response": _interrupt_text,
                                 "messages": messages,
                                 "api_calls": api_call_count,
                                 "completed": False,
@@ -2663,10 +2666,12 @@ def run_conversation(
                 # Check for interrupt before deciding to retry
                 if agent._interrupt_requested:
                     agent._vprint(f"{agent.log_prefix}⚡ Interrupt detected during error handling, aborting retries.", force=True)
+                    _interrupt_text = f"Operation interrupted: handling API error ({error_type}: {agent._clean_error_message(str(api_error))})."
+                    close_interrupted_tool_sequence(messages, _interrupt_text)
                     agent._persist_session(messages, conversation_history)
                     agent.clear_interrupt()
                     return {
-                        "final_response": f"Operation interrupted: handling API error ({error_type}: {agent._clean_error_message(str(api_error))}).",
+                        "final_response": _interrupt_text,
                         "messages": messages,
                         "api_calls": api_call_count,
                         "completed": False,
@@ -3578,10 +3583,12 @@ def run_conversation(
                 while time.time() < sleep_end:
                     if agent._interrupt_requested:
                         agent._vprint(f"{agent.log_prefix}⚡ Interrupt detected during retry wait, aborting.", force=True)
+                        _interrupt_text = f"Operation interrupted: retrying API call after error (retry {retry_count}/{max_retries})."
+                        close_interrupted_tool_sequence(messages, _interrupt_text)
                         agent._persist_session(messages, conversation_history)
                         agent.clear_interrupt()
                         return {
-                            "final_response": f"Operation interrupted: retrying API call after error (retry {retry_count}/{max_retries}).",
+                            "final_response": _interrupt_text,
                             "messages": messages,
                             "api_calls": api_call_count,
                             "completed": False,

agent/message_sanitization.py

@@ -279,6 +279,38 @@ def _repair_tool_call_arguments(raw_args: str, tool_name: str = "?") -> str:
     return "{}"
 
 
+def close_interrupted_tool_sequence(messages: list, final_response: Any = None) -> bool:
+    """Append a synthetic assistant turn when an interrupted tail is a tool result.
+
+    A turn cut short by ``/stop`` can leave the transcript ending on a raw
+    ``tool`` message (a tool finished, or its execution was cancelled, but the
+    model never streamed a closing assistant turn). Persisting that tail means
+    the next user message lands as ``… tool → user`` — a role-alternation
+    violation that strict providers (Gemini, Claude) react to by hallucinating
+    a continuation of the user's message and ignoring prior context, which
+    reads to the user as "lost context" (#48879).
+
+    ``finalize_turn`` closes this on the happy interrupt path, but the
+    retry/backoff/error interrupt aborts in ``conversation_loop`` ``return``
+    early and never reach it — this shared helper closes the sequence on all of
+    them. ``final_response`` is usually empty on an interrupt, so an explicit
+    placeholder is used rather than an empty-content assistant turn.
+
+    Mutates ``messages`` in place. Returns True if a closing turn was appended.
+    """
+    if not messages:
+        return False
+    last = messages[-1]
+    if not isinstance(last, dict) or last.get("role") != "tool":
+        return False
+    text = final_response if isinstance(final_response, str) else ""
+    messages.append({
+        "role": "assistant",
+        "content": text.strip() or "Operation interrupted.",
+    })
+    return True
+
+
 def _strip_non_ascii(text: str) -> str:
     """Remove non-ASCII characters, replacing with closest ASCII equivalent or removing.
 
@@ -431,6 +463,7 @@ def _sanitize_structure_non_ascii(payload: Any) -> bool:
 
 __all__ = [
     "_SURROGATE_RE",
+    "close_interrupted_tool_sequence",
     "_sanitize_surrogates",
     "_sanitize_structure_surrogates",
     "_sanitize_messages_surrogates",

agent/turn_finalizer.py

@@ -181,13 +181,9 @@ def finalize_turn(
         # here instead. On an interrupt ``final_response`` is typically
         # empty, so fall back to an explicit placeholder rather than
         # persisting an empty-content assistant turn.
-        if interrupted and messages and messages[-1].get("role") == "tool":
-            messages.append(
-                {
-                    "role": "assistant",
-                    "content": (final_response or "").strip() or "Operation interrupted.",
-                }
-            )
+        if interrupted:
+            from agent.message_sanitization import close_interrupted_tool_sequence
+            close_interrupted_tool_sequence(messages, final_response)
 
         agent._persist_session(messages, conversation_history)
     except Exception as _persist_err:

tests/agent/test_close_interrupted_tool_sequence.py

@@ -0,0 +1,86 @@
+"""Regression tests for ``close_interrupted_tool_sequence`` (#48879 follow-up).
+
+#48879 closed the tool-call sequence on interrupt inside ``finalize_turn``,
+but the retry/backoff/error interrupt aborts in ``conversation_loop`` ``return``
+early and never reach it — so they persisted a raw ``tool`` tail. The next user
+message then lands as ``... tool → user``, the role-alternation violation that
+makes strict providers (Gemini, Claude) hallucinate a continuation and ignore
+prior context (what the user perceives as "lost context").
+
+The fix routes every interrupt abort through this one shared helper. These tests
+pin the helper's contract and prove the post-interrupt + next-user-message
+transcript is alternation-safe.
+"""
+
+from agent.message_sanitization import close_interrupted_tool_sequence
+
+
+def _tool_tail():
+    return [
+        {"role": "user", "content": "edit the file"},
+        {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "c1", "function": {"name": "patch", "arguments": "{}"}}],
+        },
+        {"role": "tool", "tool_call_id": "c1", "content": "ok edited"},
+    ]
+
+
+def _assert_no_tool_then_user(messages):
+    for i in range(len(messages) - 1):
+        if messages[i].get("role") == "tool":
+            assert messages[i + 1].get("role") != "user", (
+                f"role-alternation violation: tool → user at index {i}"
+            )
+
+
+def test_tool_tail_is_closed_with_placeholder():
+    messages = _tool_tail()
+    assert close_interrupted_tool_sequence(messages, None) is True
+    assert messages[-1]["role"] == "assistant"
+    assert messages[-1]["content"] == "Operation interrupted."
+
+
+def test_tool_tail_keeps_interrupt_text_when_present():
+    messages = _tool_tail()
+    close_interrupted_tool_sequence(messages, "Operation interrupted during retry (attempt 2/3).")
+    assert messages[-1]["role"] == "assistant"
+    assert messages[-1]["content"] == "Operation interrupted during retry (attempt 2/3)."
+
+
+def test_blank_interrupt_text_falls_back_to_placeholder():
+    messages = _tool_tail()
+    close_interrupted_tool_sequence(messages, "   ")
+    assert messages[-1]["content"] == "Operation interrupted."
+
+
+def test_closing_makes_next_user_message_alternation_safe():
+    """The whole point: appending a user turn after the close must not
+    produce the ``tool → user`` shape strict providers choke on."""
+    messages = _tool_tail()
+    close_interrupted_tool_sequence(messages, None)
+    follow_on = messages + [{"role": "user", "content": "they do! increase the timing"}]
+    _assert_no_tool_then_user(follow_on)
+
+
+def test_assistant_tail_is_left_untouched():
+    messages = [
+        {"role": "user", "content": "hi"},
+        {"role": "assistant", "content": "partial reply"},
+    ]
+    before = [dict(m) for m in messages]
+    assert close_interrupted_tool_sequence(messages, "interrupted") is False
+    assert messages == before
+
+
+def test_user_tail_is_left_untouched():
+    messages = [{"role": "user", "content": "hi"}]
+    assert close_interrupted_tool_sequence(messages, None) is False
+    assert len(messages) == 1
+
+
+def test_empty_messages_is_noop():
+    messages = []
+    assert close_interrupted_tool_sequence(messages, "x") is False
+    assert messages == []