fix(run_agent): break permanent empty-response loop from orphan tool-tail (#21385)

When empty-response terminal scaffolding fires on a tool-result turn,
_drop_trailing_empty_response_scaffolding left the live history ending at
a bare 'tool' message. The next user input then landed as [...tool, user],
a protocol-invalid sequence that OpenRouter/Opus and other providers
silently fail on (returns empty content). That retriggered the empty-retry
recovery every turn, and recovery flags never hit SQLite (no column for
them), so history kept looking broken on every reload.

Two fixes:

1. Scaffolding strip rewinds the orphan assistant(tool_calls)+tool pair
   after popping sentinels. Only fires when scaffolding flags were
   actually present, so mid-iteration tool loops are untouched.

2. _repair_message_sequence runs right before every API call as a
   defensive belt: drops stray tool messages with unknown tool_call_ids,
   merges consecutive user messages so no user input is lost. Does NOT
   rewind assistant(tool_calls)+tool+user — that pattern is valid when
   the user redirected before the model got its continuation turn.

Repro: session 20260507_044111_fa7e65. Opus-4.7/OpenRouter returned
content-less response after a 42KB execute_code output, nudge+retry
chain exhausted (no fallback configured), terminal sentinel appended,
scaffolding stripped leaving bare tool tail, user typed 'wtf happened..'
and landed as tool→user violation. Every subsequent turn collapsed in
<50ms with the same 3-retry empty chain because the API request itself
was malformed.

Verified live via HTTP mock: pre-fix reproduced 5 api_calls/0.15s exit
'empty_response_exhausted'; post-fix 1 api_call/0.10s exit
'text_response(finish_reason=stop)'. Three-turn session flows cleanly
through the scenario. Full run_agent suite: 1242 passed (0 regressions,
2 pre-existing concurrent_interrupt failures unrelated).

run_agent.py

@@ -3836,7 +3836,17 @@ class AIAgent:
         self._flush_messages_to_session_db(messages, conversation_history)
 
     def _drop_trailing_empty_response_scaffolding(self, messages: List[Dict]) -> None:
-        """Remove private empty-response retry/failure scaffolding from transcript tails."""
+        """Remove private empty-response retry/failure scaffolding from transcript tails.
+
+        Also rewinds past any trailing tool-result / assistant(tool_calls) pair
+        that the failed iteration left hanging. Without this, the tail ends at
+        a raw ``tool`` message and the next user turn lands as
+        ``...tool, user, user`` — a protocol-invalid sequence that most
+        providers silently reject (returns empty content), causing the
+        empty-retry loop to fire forever. See #<TBD>.
+        """
+        # Pass 1: strip the flagged scaffolding messages themselves.
+        dropped_scaffolding = False
         while (
             messages
             and isinstance(messages[-1], dict)
@@ -3846,6 +3856,137 @@ class AIAgent:
             )
         ):
             messages.pop()
+            dropped_scaffolding = True
+
+        # Pass 2: if we stripped scaffolding, rewind through any trailing
+        # tool-result messages plus the assistant(tool_calls) message that
+        # produced them. This preserves role alternation so the next user
+        # message follows a user or assistant message, not an orphan tool
+        # result. Only runs when scaffolding was actually present — normal
+        # conversation tails (real tool loops mid-progress) are untouched.
+        if not dropped_scaffolding:
+            return
+
+        # Drop any trailing tool-result messages
+        while (
+            messages
+            and isinstance(messages[-1], dict)
+            and messages[-1].get("role") == "tool"
+        ):
+            messages.pop()
+
+        # Drop the assistant message that issued the tool calls, if the tail
+        # now ends in an assistant-with-tool_calls (the pair that owned the
+        # just-popped tool results). Without this, the tail is
+        # ``assistant(tool_calls=...)`` with no tool answers, which some
+        # providers also reject.
+        if (
+            messages
+            and isinstance(messages[-1], dict)
+            and messages[-1].get("role") == "assistant"
+            and messages[-1].get("tool_calls")
+        ):
+            messages.pop()
+
+    def _repair_message_sequence(self, messages: List[Dict]) -> int:
+        """Collapse malformed role-alternation left in the live history.
+
+        Providers (OpenAI, OpenRouter, Anthropic) expect strict alternation:
+        after the system message, user/tool alternates with assistant, with
+        no two consecutive user messages and no tool-result that doesn't
+        follow an assistant-with-tool_calls. Violations cause silent empty
+        responses on most providers, which triggers the empty-retry loop.
+
+        This runs right before the API call as a defensive belt — by the
+        time it fires, the scaffolding strip should already have prevented
+        most shapes, but external callers (gateway multi-queue replay,
+        session resume, cron, explicit conversation_history passed in by
+        host code) can feed in already-broken histories.
+
+        Repairs applied:
+          1. Stray ``tool`` messages whose ``tool_call_id`` doesn't match
+             any preceding assistant tool_call — dropped.
+          2. Consecutive ``user`` messages — merged with newline separator
+             so no user input is lost.
+
+        Deliberately does NOT rewind orphan ``assistant(tool_calls)+tool``
+        pairs that precede a user message — that pattern IS valid when the
+        previous turn completed normally and the user jumped in to redirect
+        before the model got a continuation turn (the ongoing dialog
+        pattern). The empty-response scaffolding stripper handles the
+        genuinely-broken variant via its flag-gated rewind.
+
+        Returns the number of repairs made (for logging/telemetry).
+        """
+        if not messages:
+            return 0
+
+        repairs = 0
+
+        # Pass 1: drop stray tool messages that don't follow a known
+        # assistant tool_call_id. Uses a rolling set of known ids refreshed
+        # on each assistant message.
+        known_tool_ids: set = set()
+        filtered: List[Dict] = []
+        for msg in messages:
+            if not isinstance(msg, dict):
+                filtered.append(msg)
+                continue
+            role = msg.get("role")
+            if role == "assistant":
+                known_tool_ids = set()
+                for tc in (msg.get("tool_calls") or []):
+                    tc_id = tc.get("id") if isinstance(tc, dict) else None
+                    if tc_id:
+                        known_tool_ids.add(tc_id)
+                filtered.append(msg)
+            elif role == "tool":
+                tc_id = msg.get("tool_call_id")
+                if tc_id and tc_id in known_tool_ids:
+                    filtered.append(msg)
+                else:
+                    repairs += 1
+            else:
+                if role == "user":
+                    # A user turn closes the tool-result run; subsequent
+                    # tool messages without a fresh assistant tool_call
+                    # are orphans.
+                    known_tool_ids = set()
+                filtered.append(msg)
+
+        # Pass 2: merge consecutive user messages. Preserves all user input
+        # so nothing the user typed is lost.
+        merged: List[Dict] = []
+        for msg in filtered:
+            if (
+                merged
+                and isinstance(msg, dict)
+                and msg.get("role") == "user"
+                and isinstance(merged[-1], dict)
+                and merged[-1].get("role") == "user"
+            ):
+                prev = merged[-1]
+                prev_content = prev.get("content", "")
+                new_content = msg.get("content", "")
+                # Only merge plain-text content; leave multimodal (list)
+                # content alone — collapsing image/audio blocks risks
+                # mangling the attachment structure.
+                if isinstance(prev_content, str) and isinstance(new_content, str):
+                    prev["content"] = (
+                        (prev_content + "\n\n" + new_content)
+                        if prev_content and new_content
+                        else (prev_content or new_content)
+                    )
+                    repairs += 1
+                    continue
+            merged.append(msg)
+
+        if repairs > 0:
+            # Rewrite in place so downstream paths (persistence, return
+            # value, session DB flush) see the repaired sequence.
+            messages[:] = merged
+
+        return repairs
 
     def _flush_messages_to_session_db(self, messages: List[Dict], conversation_history: List[Dict] = None):
         """Persist any un-flushed messages to the SQLite session store.
@@ -11133,6 +11274,21 @@ class AIAgent:
                     self.session_id or "-",
                 )
 
+            # Defensive: repair malformed role-alternation before API call.
+            # Catches cases where the history got wedged into a
+            # ``tool → user`` or ``user → user`` tail (e.g. after empty-
+            # response scaffolding was stripped and a new user message
+            # landed after an orphan tool result). Most providers return
+            # empty content on malformed sequences, which would otherwise
+            # retrigger the empty-retry loop indefinitely.
+            repaired_seq = self._repair_message_sequence(messages)
+            if repaired_seq > 0:
+                request_logger.info(
+                    "Repaired %s message-alternation violations before request (session=%s)",
+                    repaired_seq,
+                    self.session_id or "-",
+                )
+
             api_messages = []
             for idx, msg in enumerate(messages):
                 api_msg = msg.copy()

tests/run_agent/test_empty_response_recovery_persistence.py

@@ -21,9 +21,21 @@ def _agent_with_stubbed_persistence():
 
 
 def test_persist_session_strips_trailing_empty_recovery_scaffolding():
+    """After stripping scaffolding, also rewind past orphan trailing tool-result
+    messages that the failed iteration left behind. Otherwise the next user
+    message lands after a bare ``tool`` and produces a protocol-invalid
+    sequence that most providers silently fail on, retriggering the empty-
+    retry loop indefinitely.
+    """
     agent = _agent_with_stubbed_persistence()
     messages = [
         {"role": "user", "content": "run the task"},
+        {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "call_1", "type": "function",
+                            "function": {"name": "x", "arguments": "{}"}}],
+        },
         {"role": "tool", "content": "{}", "tool_call_id": "call_1"},
         {
             "role": "assistant",
@@ -42,9 +54,11 @@ def test_persist_session_strips_trailing_empty_recovery_scaffolding():
 
     AIAgent._persist_session(agent, messages, conversation_history=[])
 
+    # After strip + rewind, only the original user message remains. The
+    # assistant(tool_calls) + tool pair is dropped because its iteration
+    # never produced a real response.
     assert messages == [
         {"role": "user", "content": "run the task"},
-        {"role": "tool", "content": "{}", "tool_call_id": "call_1"},
     ]
     assert agent.saved_session_logs[-1] == messages
     assert all(not msg.get("_empty_recovery_synthetic") for msg in messages)

tests/run_agent/test_message_sequence_repair.py

@@ -0,0 +1,201 @@
+"""Tests for pre-API-call message-sequence repair.
+
+Covers ``_repair_message_sequence`` and the extended
+``_drop_trailing_empty_response_scaffolding`` behavior that rewinds past
+orphan tool-result tails. Together these prevent the self-reinforcing empty-
+response loop observed in session 20260507_044111_fa7e65, where a tool-result
+followed directly by a user message produced silent empty responses from
+providers (violating role alternation), which retriggered the empty-retry
+recovery every turn.
+"""
+
+from run_agent import AIAgent
+
+
+def _bare_agent():
+    return AIAgent.__new__(AIAgent)
+
+
+# ── _drop_trailing_empty_response_scaffolding ──────────────────────────────
+
+def test_drop_scaffolding_rewinds_orphan_tool_tail():
+    """When scaffolding is stripped, also rewind the orphan assistant+tool pair."""
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": "task"},
+        {"role": "assistant", "content": "",
+         "tool_calls": [{"id": "t1", "type": "function",
+                         "function": {"name": "f", "arguments": "{}"}}]},
+        {"role": "tool", "tool_call_id": "t1", "content": "out"},
+        {"role": "assistant", "content": "(empty)",
+         "_empty_terminal_sentinel": True},
+    ]
+
+    AIAgent._drop_trailing_empty_response_scaffolding(agent, messages)
+
+    assert messages == [{"role": "user", "content": "task"}]
+
+
+def test_drop_scaffolding_keeps_tail_when_no_scaffolding():
+    """Mid-iteration tool results must NOT be rewound — only if scaffolding fires."""
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": "task"},
+        {"role": "assistant", "content": "",
+         "tool_calls": [{"id": "t1", "type": "function",
+                         "function": {"name": "f", "arguments": "{}"}}]},
+        {"role": "tool", "tool_call_id": "t1", "content": "out"},
+    ]
+    original = [dict(m) for m in messages]
+
+    AIAgent._drop_trailing_empty_response_scaffolding(agent, messages)
+
+    assert messages == original
+
+
+def test_drop_scaffolding_handles_multiple_parallel_tool_results():
+    """Parallel tool calls (one assistant → many tool results) all rewound together."""
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": "task"},
+        {"role": "assistant", "content": "",
+         "tool_calls": [
+             {"id": "t1", "type": "function",
+              "function": {"name": "f", "arguments": "{}"}},
+             {"id": "t2", "type": "function",
+              "function": {"name": "g", "arguments": "{}"}},
+         ]},
+        {"role": "tool", "tool_call_id": "t1", "content": "out1"},
+        {"role": "tool", "tool_call_id": "t2", "content": "out2"},
+        {"role": "assistant", "content": "(empty)",
+         "_empty_terminal_sentinel": True},
+    ]
+
+    AIAgent._drop_trailing_empty_response_scaffolding(agent, messages)
+
+    assert messages == [{"role": "user", "content": "task"}]
+
+
+# ── _repair_message_sequence ───────────────────────────────────────────────
+
+def test_repair_merges_consecutive_user_messages():
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": "first"},
+        {"role": "user", "content": "second"},
+    ]
+
+    repairs = AIAgent._repair_message_sequence(agent, messages)
+
+    assert repairs == 1
+    assert len(messages) == 1
+    assert messages[0]["role"] == "user"
+    assert messages[0]["content"] == "first\n\nsecond"
+
+
+def test_repair_preserves_user_content_when_one_side_empty():
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": ""},
+        {"role": "user", "content": "real message"},
+    ]
+
+    AIAgent._repair_message_sequence(agent, messages)
+
+    assert messages == [{"role": "user", "content": "real message"}]
+
+
+def test_repair_does_not_rewind_ongoing_dialog_tool_pair():
+    """assistant(tool_calls) + tool + user is a VALID pattern (user redirect
+    before the model gets its continuation turn). Repair must not touch it —
+    only the flag-gated scaffolding strip rewinds, and only when the
+    empty-recovery scaffolding was actually present.
+    """
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": "Q1"},
+        {"role": "assistant", "content": "",
+         "tool_calls": [{"id": "t1", "type": "function",
+                         "function": {"name": "f", "arguments": "{}"}}]},
+        {"role": "tool", "tool_call_id": "t1", "content": "out"},
+        {"role": "user", "content": "Q2"},
+    ]
+    original = [dict(m) for m in messages]
+
+    repairs = AIAgent._repair_message_sequence(agent, messages)
+
+    assert repairs == 0
+    assert messages == original
+
+
+def test_repair_drops_stray_tool_with_unknown_tool_call_id():
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": "hi"},
+        {"role": "assistant", "content": "hello"},
+        {"role": "tool", "tool_call_id": "orphan", "content": "stray"},
+        {"role": "user", "content": "real"},
+    ]
+
+    repairs = AIAgent._repair_message_sequence(agent, messages)
+
+    assert repairs >= 1
+    assert all(m.get("role") != "tool" for m in messages)
+
+
+def test_repair_leaves_valid_conversation_unchanged():
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": "list files"},
+        {"role": "assistant", "content": "",
+         "tool_calls": [{"id": "t1", "type": "function",
+                         "function": {"name": "ls", "arguments": "{}"}}]},
+        {"role": "tool", "tool_call_id": "t1", "content": "a.txt b.txt"},
+        {"role": "assistant", "content": "Found 2 files"},
+        {"role": "user", "content": "more"},
+    ]
+    original = [dict(m) for m in messages]
+
+    repairs = AIAgent._repair_message_sequence(agent, messages)
+
+    assert repairs == 0
+    assert messages == original
+
+
+def test_repair_preserves_multimodal_user_content():
+    """Multimodal (list) content must NOT be merged — risks mangling attachments."""
+    agent = _bare_agent()
+    messages = [
+        {"role": "user", "content": [{"type": "text", "text": "hi"},
+                                     {"type": "image_url", "image_url": {"url": "..."}}]},
+        {"role": "user", "content": "follow-up"},
+    ]
+
+    AIAgent._repair_message_sequence(agent, messages)
+
+    # The multimodal user message stays as a distinct message — no merge
+    assert len(messages) == 2
+    assert isinstance(messages[0]["content"], list)
+
+
+def test_repair_empty_messages_returns_zero():
+    agent = _bare_agent()
+    messages = []
+
+    repairs = AIAgent._repair_message_sequence(agent, messages)
+
+    assert repairs == 0
+    assert messages == []
+
+
+def test_repair_preserves_system_messages():
+    agent = _bare_agent()
+    messages = [
+        {"role": "system", "content": "You are..."},
+        {"role": "user", "content": "hi"},
+    ]
+    original = [dict(m) for m in messages]
+
+    AIAgent._repair_message_sequence(agent, messages)
+
+    assert messages == original