feat(commands): /background, /btw slash commands + undo button + reasoning chip

Rebased onto master after #931 (aux title routing) to resolve streaming.py conflict.
All changes from both PRs are cleanly integrated.

2088 tests passing (2065 master + 23 from #931).

Co-authored-by: bergeouss <bergeouss@gmail.com>

CHANGELOG.md

@@ -29,10 +29,18 @@
   workspace subtree) and never enumerate blocked system roots. (`api/routes.py`,
   `api/workspace.py`, `static/panels.js`, `static/style.css`) (partial for #616)
 
-## [v0.50.182] — 2026-04-24
+## [v0.50.183] — 2026-04-24
+
+### Added
+- **`/btw` slash command** — ask an ephemeral side question using current session context without adding to history. Creates a hidden session, streams the answer in a visually distinct bubble, then discards the session. Includes `attachBtwStream()` SSE consumer and `POST /api/btw` route. (`api/routes.py`, `api/background.py`, `static/commands.js`, `static/messages.js`, `static/style.css`)
+- **`/background` slash command** — run a prompt in a parallel background agent without blocking the active conversation. Frontend polls `GET /api/background/status` for results and displays completed answers inline. Includes badge indicator in composer footer. (`api/routes.py`, `api/background.py`, `static/commands.js`, `static/messages.js`, `static/index.html`)
+- **Undo button on last assistant message** — surfaced as an ↩ icon on the last assistant message, calling the existing `/undo` command for discoverability. (`static/ui.js`)
+- **Reasoning effort chip in composer** — visual chip to set reasoning effort level from the composer footer without typing a command. (`static/ui.js`, `static/index.html`, `static/style.css`)
 
 ### Fixed
-- **Auxiliary title model now respected** — when `auxiliary.title_generation` is explicitly configured in `config.yaml`, the WebUI now routes title generation through that dedicated model instead of silently using the chat session's model. Adds `_aux_title_configured()` to detect meaningful auxiliary config, `_aux_title_timeout()` to respect the configured per-task timeout (was hardcoded to 15.0 s), and adds the missing `llm_invalid_aux` fallback path so invalid auxiliary outputs trigger the agent-model fallback. (`api/streaming.py`, `tests/test_title_aux_routing.py`) Co-authored by @starship-s.
+- **Ephemeral sessions correctly skip persistence** — added `return` after the ephemeral `done` event in `_run_agent_streaming()`, preventing ephemeral session state from being written to disk after stream completion. (`api/streaming.py`)
+
+Co-authored by @bergeouss.
 
 ## [v0.50.181] — 2026-04-24
 

api/background.py

@@ -0,0 +1,87 @@
+"""Background and ephemeral task tracking for /background and /btw commands."""
+from __future__ import annotations
+
+import logging
+import threading
+import time
+from typing import Any
+
+logger = logging.getLogger(__name__)
+
+_lock = threading.Lock()
+
+# parent_session_id -> list of task dicts
+_BACKGROUND_TASKS: dict[str, list[dict[str, Any]]] = {}
+
+# btw ephemeral session tracking: parent_sid -> {ephemeral_sid, stream_id, question}
+_BTW_TRACKING: dict[str, dict[str, Any]] = {}
+
+
+def track_background(parent_sid: str, bg_sid: str, stream_id: str,
+                     task_id: str, prompt: str) -> None:
+    with _lock:
+        _BACKGROUND_TASKS.setdefault(parent_sid, []).append({
+            "task_id": task_id,
+            "bg_session_id": bg_sid,
+            "stream_id": stream_id,
+            "prompt": prompt,
+            "status": "running",
+            "started_at": time.time(),
+            "answer": None,
+            "completed_at": None,
+        })
+
+
+def track_btw(parent_sid: str, ephemeral_sid: str, stream_id: str,
+              question: str) -> None:
+    with _lock:
+        _BTW_TRACKING[parent_sid] = {
+            "ephemeral_session_id": ephemeral_sid,
+            "stream_id": stream_id,
+            "question": question,
+        }
+
+
+def complete_background(parent_sid: str, task_id: str, answer: str) -> None:
+    with _lock:
+        for t in _BACKGROUND_TASKS.get(parent_sid, []):
+            if t["task_id"] == task_id and t["status"] == "running":
+                t["status"] = "done"
+                t["answer"] = answer
+                t["completed_at"] = time.time()
+                break
+
+
+def get_results(parent_sid: str) -> list[dict[str, Any]]:
+    """Return completed background task results and remove only the done ones
+    from tracking.  Tasks still in ``status="running"`` MUST stay in the list
+    so that ``complete_background()`` can still find them when the worker
+    thread finishes — otherwise the first poll during a long-running task
+    silently drops it and the result is lost forever.
+    """
+    with _lock:
+        tasks = _BACKGROUND_TASKS.get(parent_sid, [])
+        done = [t for t in tasks if t["status"] == "done"]
+        still_running = [t for t in tasks if t["status"] != "done"]
+        if still_running:
+            _BACKGROUND_TASKS[parent_sid] = still_running
+        else:
+            _BACKGROUND_TASKS.pop(parent_sid, None)
+        return [{
+            "task_id": t["task_id"],
+            "prompt": t["prompt"],
+            "answer": t["answer"],
+            "completed_at": t["completed_at"],
+        } for t in done]
+
+
+def get_background_tasks(parent_sid: str) -> list[dict[str, Any]]:
+    """Return all background tasks (running and done) for a parent session."""
+    with _lock:
+        return list(_BACKGROUND_TASKS.get(parent_sid, []))
+
+
+def cleanup_btw(parent_sid: str) -> dict[str, Any] | None:
+    """Remove and return btw tracking for a parent session."""
+    with _lock:
+        return _BTW_TRACKING.pop(parent_sid, None)

api/routes.py

@@ -741,6 +741,13 @@ def handle_get(handler, parsed) -> bool:
         except KeyError:
             return bad(handler, "Session not found", 404)
 
+    if parsed.path == "/api/background/status":
+        sid = parse_qs(parsed.query).get("session_id", [""])[0]
+        if not sid:
+            return bad(handler, "Missing session_id")
+        from api.background import get_results
+        return j(handler, {"results": get_results(sid)})
+
     if parsed.path == "/api/sessions":
         webui_sessions = all_sessions()
         settings = load_settings()
@@ -1258,6 +1265,12 @@ def handle_post(handler, parsed) -> bool:
         except ValueError as e:
             return j(handler, {"error": str(e)})
 
+    if parsed.path == "/api/btw":
+        return _handle_btw(handler, body)
+
+    if parsed.path == "/api/background":
+        return _handle_background(handler, body)
+
     if parsed.path == "/api/chat/start":
         return _handle_chat_start(handler, body)
 
@@ -2457,6 +2470,133 @@ def _handle_sessions_cleanup(handler, body, zero_only=False):
     return j(handler, {"ok": True, "cleaned": cleaned})
 
 
+def _handle_btw(handler, body):
+    """POST /api/btw — ephemeral side question using session context.
+
+    Creates a temporary hidden session, streams the answer via SSE, then
+    discards the session. The parent session is not modified.
+    """
+    try:
+        require(body, "session_id")
+        require(body, "question")
+    except ValueError as e:
+        return bad(handler, str(e))
+    try:
+        s = get_session(body["session_id"])
+    except KeyError:
+        return bad(handler, "Session not found", 404)
+    question = str(body["question"]).strip()
+    if not question:
+        return bad(handler, "question is required")
+    # Duplicate-stream guard (same pattern as chat/start)
+    current_stream_id = getattr(s, "active_stream_id", None)
+    if current_stream_id:
+        with STREAMS_LOCK:
+            if current_stream_id in STREAMS:
+                return j(handler, {"error": "session already has an active stream"}, status=409)
+        s.active_stream_id = None
+    # Create ephemeral hidden session inheriting context
+    from api.models import new_session as _new_session
+    ephemeral = _new_session(workspace=s.workspace, model=s.model, profile=getattr(s, 'profile', None))
+    # Copy conversation history for context (agent reads from messages)
+    ephemeral.messages = list(s.messages or [])
+    ephemeral.title = f"btw: {question[:60]}"
+    ephemeral.save()
+    stream_id = uuid.uuid4().hex
+    ephemeral.active_stream_id = stream_id
+    ephemeral.save()
+    q = queue.Queue()
+    with STREAMS_LOCK:
+        STREAMS[stream_id] = q
+    from api.background import track_btw
+    track_btw(body["session_id"], ephemeral.session_id, stream_id, question)
+    thr = threading.Thread(
+        target=_run_agent_streaming,
+        args=(ephemeral.session_id, question, s.model, s.workspace, stream_id, None),
+        kwargs={"ephemeral": True},
+        daemon=True,
+    )
+    thr.start()
+    return j(handler, {"stream_id": stream_id, "session_id": ephemeral.session_id, "parent_session_id": body["session_id"]})
+
+
+def _handle_background(handler, body):
+    """POST /api/background — run prompt in parallel background agent.
+
+    Creates a hidden session, starts streaming in a daemon thread.
+    Frontend polls /api/background/status for completed results.
+    """
+    try:
+        require(body, "session_id")
+        require(body, "prompt")
+    except ValueError as e:
+        return bad(handler, str(e))
+    try:
+        s = get_session(body["session_id"])
+    except KeyError:
+        return bad(handler, "Session not found", 404)
+    prompt = str(body["prompt"]).strip()
+    if not prompt:
+        return bad(handler, "prompt is required")
+    from api.models import new_session as _new_session
+    bg = _new_session(workspace=s.workspace, model=s.model, profile=getattr(s, 'profile', None))
+    bg.title = f"bg: {prompt[:60]}"
+    bg.save()
+    stream_id = uuid.uuid4().hex
+    bg.active_stream_id = stream_id
+    bg.save()
+    q = queue.Queue()
+    with STREAMS_LOCK:
+        STREAMS[stream_id] = q
+    task_id = uuid.uuid4().hex[:8]
+    from api.background import track_background, complete_background
+    parent_sid = body["session_id"]
+    bg_sid = bg.session_id
+    track_background(parent_sid, bg_sid, stream_id, task_id, prompt)
+
+    def _run_bg_and_notify():
+        """Run the background agent, then mark the tracked task `done` with the
+        last assistant reply so `/api/background/status` can surface it.  Without
+        this, `complete_background()` is never called and the result is lost —
+        `get_results()` would see a forever-`running` task and return nothing.
+        """
+        try:
+            _run_agent_streaming(bg_sid, prompt, s.model, s.workspace, stream_id, None)
+            # Reload the bg session from disk and extract the final assistant reply.
+            try:
+                from api.models import Session as _Session
+                reloaded = _Session.load(bg_sid)
+                _answer = ""
+                for _m in reversed((reloaded.messages if reloaded else None) or []):
+                    if not isinstance(_m, dict) or _m.get("role") != "assistant":
+                        continue
+                    if _m.get("_error"):
+                        continue
+                    _content = str(_m.get("content") or "").strip()
+                    if _content:
+                        _answer = _content
+                        break
+                complete_background(parent_sid, task_id, _answer or "(no answer produced)")
+            except Exception:
+                complete_background(parent_sid, task_id, "(background task failed)")
+            # Best-effort cleanup of the hidden bg session file so it doesn't
+            # clutter the sidebar or SESSION_DIR. The index is pruned on the
+            # next rebuild via _index_entry_exists().
+            try:
+                (SESSION_DIR / f"{bg_sid}.json").unlink(missing_ok=True)
+            except Exception:
+                pass
+        except Exception:
+            try:
+                complete_background(parent_sid, task_id, "(background task failed)")
+            except Exception:
+                pass
+
+    thr = threading.Thread(target=_run_bg_and_notify, daemon=True)
+    thr.start()
+    return j(handler, {"task_id": task_id, "stream_id": stream_id, "session_id": bg.session_id})
+
+
 def _handle_chat_start(handler, body):
     try:
         require(body, "session_id")

api/streaming.py

@@ -866,8 +866,12 @@ def _sse(handler, event, data):
     handler.wfile.flush()
 
 
-def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, attachments=None):
-    """Run agent in background thread, writing SSE events to STREAMS[stream_id]."""
+def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, attachments=None, *, ephemeral=False):
+    """Run agent in background thread, writing SSE events to STREAMS[stream_id].
+
+    When ephemeral=True, session mutations are skipped — used by /btw to get
+    a streaming answer without persisting to the parent session.
+    """
     q = STREAMS.get(stream_id)
     if q is None:
         return
@@ -1336,6 +1340,27 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
                 task_id=session_id,
                 persist_user_message=msg_text,
             )
+            # ── Ephemeral mode (/btw): deliver answer, skip persistence, cleanup ──
+            if ephemeral:
+                _answer = ''
+                for _m in reversed(result.get('messages') or []):
+                    if isinstance(_m, dict) and _m.get('role') == 'assistant':
+                        _answer = str(_m.get('content', ''))
+                        break
+                put('done', {
+                    'session': {'session_id': session_id, 'messages': result.get('messages', [])},
+                    'usage': {'input_tokens': 0, 'output_tokens': 0},
+                    'ephemeral': True,
+                    'answer': _answer,
+                })
+                if _checkpoint_stop is not None:
+                    _checkpoint_stop.set()
+                try:
+                    import pathlib
+                    pathlib.Path(s.path).unlink(missing_ok=True)
+                except Exception:
+                    pass
+                return  # skip all normal persistence for ephemeral sessions
             if _checkpoint_stop is not None:
                 _checkpoint_stop.set()
             if _ckpt_thread is not None:

static/commands.js

@@ -20,6 +20,8 @@ const COMMANDS=[
   {name:'title',     desc:t('cmd_title'),    fn:cmdTitle,    arg:'[title]'},
   {name:'retry',     desc:t('cmd_retry'),    fn:cmdRetry,     noEcho:true},
   {name:'undo',      desc:t('cmd_undo'),     fn:cmdUndo,      noEcho:true},
+  {name:'btw',       desc:t('cmd_btw'),      fn:cmdBtw,       arg:'question', noEcho:true},
+  {name:'background',desc:t('cmd_background'),fn:cmdBackground,arg:'prompt',  noEcho:true},
   {name:'status',    desc:t('cmd_status'),   fn:cmdStatus},
   {name:'voice',     desc:t('cmd_voice'),    fn:cmdVoice,     noEcho:true},
   {name:'reasoning', desc:t('cmd_reasoning'), fn:cmdReasoning, arg:'show|hide|none|minimal|low|medium|high|xhigh', subArgs:['show','hide','none','minimal','low','medium','high','xhigh'], noEcho:true},
@@ -573,6 +575,36 @@ async function cmdUndo(){
     showToast(`↩ ${t('undid_n_messages')} ${r.removed_count} ${t('undid_messages_suffix')}`);
   }catch(e){showToast(t('undo_failed')+e.message);}
 }
+async function undoLastExchange(){await cmdUndo();}
+async function cmdBtw(args){
+  if(!S.session){showToast(t('no_active_session'));return;}
+  const question=(args||'').trim();
+  if(!question){showToast(t('cmd_btw_usage'));return;}
+  showToast(t('btw_asking'));
+  const activeSid=S.session.session_id;
+  try{
+    const r=await api('/api/btw',{method:'POST',body:JSON.stringify({session_id:activeSid,question})});
+    if(r&&r.error){showToast(r.error);return;}
+    // Connect to the ephemeral SSE stream
+    const streamId=r.stream_id;
+    const parentSid=r.parent_session_id;
+    if(typeof attachBtwStream==='function') attachBtwStream(parentSid,streamId,question);
+  }catch(e){showToast(t('btw_failed')+e.message);}
+}
+async function cmdBackground(args){
+  if(!S.session){showToast(t('no_active_session'));return;}
+  const prompt=(args||'').trim();
+  if(!prompt){showToast(t('cmd_background_usage'));return;}
+  showToast(t('bg_running'));
+  const activeSid=S.session.session_id;
+  try{
+    const r=await api('/api/background',{method:'POST',body:JSON.stringify({session_id:activeSid,prompt})});
+    if(r&&r.error){showToast(r.error);return;}
+    // Show background badge and start polling
+    if(typeof showBackgroundBadge==='function') showBackgroundBadge(r.task_id);
+    if(typeof startBackgroundPolling==='function') startBackgroundPolling(activeSid,r.task_id,prompt);
+  }catch(e){showToast(t('bg_failed')+e.message);}
+}
 async function cmdStatus(){
   if(!S.session){showToast(t('no_active_session'));return;}
   try{
@@ -623,6 +655,7 @@ function cmdReasoning(args){
       .then(function(st){
         const eff=(st && st.reasoning_effort)||arg;
         showToast(BRAIN+' Reasoning effort set to '+eff+' (saved; applies to next turn)');
+        if(typeof syncReasoningChip==='function') syncReasoningChip();
       })
       .catch(function(e){
         showToast(BRAIN+' Failed to set effort: '+(e && e.message ? e.message : arg));

static/i18n.js

@@ -102,6 +102,21 @@ const LOCALES = {
   cmd_title:'Get or set the session title',
   cmd_retry:'Resend the last message',
   cmd_undo:'Remove the last exchange',
+  cmd_btw:'Ask a side question (ephemeral)',
+  cmd_btw_usage:'/btw <question> — ask a side question using session context',
+  cmd_background:'Run a prompt in background',
+  cmd_background_usage:'/background <prompt> — run in parallel without blocking',
+  btw_asking:'Asking side question...',
+  btw_label:'Side question — not in history',
+  btw_done:'Side question answered',
+  btw_no_answer:'No answer received.',
+  btw_failed:'Side question failed: ',
+  bg_running:'Running in background...',
+  bg_complete:'Background task complete',
+  bg_label:'Background result:',
+  bg_no_answer:'(no answer)',
+  bg_failed:'Background task failed: ',
+  undo_exchange:'Undo last exchange',
   cmd_status:'Show session info',
   cmd_voice:'Toggle microphone input',
   stream_stopped:'Response stopped.',
@@ -199,7 +214,7 @@ const LOCALES = {
     settings_label_language: 'Language',
     settings_label_token_usage: 'Show token usage',
     settings_label_sidebar_density: 'Sidebar density',
-    cmd_reasoning: 'Toggle thinking block visibility (show/hide) or set effort level',
+    cmd_reasoning: 'Toggle thinking visibility (show/hide), set effort level, or check current status',
     settings_label_cli_sessions: 'Show agent sessions',
     settings_label_sync_insights: 'Sync to insights',
     settings_label_check_updates: 'Check for updates',
@@ -655,7 +670,7 @@ const LOCALES = {
     settings_label_language: 'Язык',
     settings_label_token_usage: 'Показывать использование токенов',
     settings_label_sidebar_density: 'Плотность боковой панели',
-    cmd_reasoning: 'Toggle thinking block visibility (show/hide) or set effort level',
+    cmd_reasoning: 'Toggle thinking visibility (show/hide), set effort level, or check current status',
     settings_label_cli_sessions: 'Показывать сеансы агента',
     settings_label_sync_insights: 'Синхронизировать с Insights',
     settings_label_check_updates: 'Проверять обновления',
@@ -1139,7 +1154,7 @@ const LOCALES = {
     settings_label_language: 'Idioma',
     settings_label_token_usage: 'Mostrar uso de tokens',
     settings_label_sidebar_density: 'Densidad de la barra lateral',
-    cmd_reasoning: 'Toggle thinking block visibility (show/hide) or set effort level',
+    cmd_reasoning: 'Toggle thinking visibility (show/hide), set effort level, or check current status',
     settings_label_cli_sessions: 'Mostrar sesiones de CLI',
     settings_label_sync_insights: 'Sincronizar con insights',
     settings_label_check_updates: 'Buscar actualizaciones',
@@ -1595,7 +1610,7 @@ const LOCALES = {
     settings_label_language: 'Sprache',
     settings_label_token_usage: 'Token-Verbrauch anzeigen',
     settings_label_sidebar_density: 'Seitenleistendichte',
-    cmd_reasoning: 'Toggle thinking block visibility (show/hide) or set effort level',
+    cmd_reasoning: 'Toggle thinking visibility (show/hide), set effort level, or check current status',
     settings_label_cli_sessions: 'Agent-Sitzungen anzeigen',
     settings_label_sync_insights: 'Mit Insights synchronisieren',
     settings_label_check_updates: 'Nach Updates suchen',
@@ -1852,7 +1867,7 @@ const LOCALES = {
     settings_label_language: '\u8bed\u8a00',
     settings_label_token_usage: '\u663e\u793a token \u7528\u91cf',
     settings_label_sidebar_density: '侧边栏密度',
-    cmd_reasoning: 'Toggle thinking block visibility (show/hide) or set effort level',
+    cmd_reasoning: 'Toggle thinking visibility (show/hide), set effort level, or check current status',
     settings_label_cli_sessions: '\u663e\u793a CLI \u4f1a\u8bdd',
     settings_label_sync_insights: '\u540c\u6b65\u5230 insights',
     settings_label_check_updates: '\u68c0\u67e5\u66f4\u65b0',
@@ -2292,7 +2307,7 @@ const LOCALES = {
     settings_label_language: '\u8a9d\u8a00',
     settings_label_token_usage: '\u986f\u793a token \u7528\u91cf',
     settings_label_sidebar_density: '側邊欄密度',
-    cmd_reasoning: 'Toggle thinking block visibility (show/hide) or set effort level',
+    cmd_reasoning: 'Toggle thinking visibility (show/hide), set effort level, or check current status',
     settings_label_cli_sessions: '\u986f\u793a CLI \u6703\u8a71',
     settings_label_sync_insights: '\u540c\u6b65\u5230 insights',
     settings_label_check_updates: '\u6aa2\u67e5\u66f4\u65b0',

static/index.html

@@ -346,6 +346,21 @@
                 <span class="composer-workspace-chevron" aria-hidden="true"><svg width="10" height="10" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><polyline points="6 9 12 15 18 9"/></svg></span>
               </button>
             </div>
+            <div class="composer-reasoning-wrap" id="composerReasoningWrap" style="display:none">
+              <button class="composer-reasoning-chip" id="composerReasoningChip" type="button" onclick="toggleReasoningDropdown()" title="Reasoning effort level">
+                <span class="composer-reasoning-icon" aria-hidden="true"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><path d="M12 2a7 7 0 0 1 7 7c0 2.38-1.19 4.47-3 5.74V17a1 1 0 0 1-1 1H9a1 1 0 0 1-1-1v-2.26C6.19 13.47 5 11.38 5 9a7 7 0 0 1 7-7z"/><line x1="9" y1="21" x2="15" y2="21"/></svg></span>
+                <span class="composer-reasoning-label" id="composerReasoningLabel"></span>
+                <span class="composer-reasoning-chevron" aria-hidden="true"><svg width="10" height="10" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><polyline points="6 9 12 15 18 9"/></svg></span>
+              </button>
+              <div class="composer-reasoning-dropdown" id="composerReasoningDropdown">
+                <div class="reasoning-option" data-effort="none">None</div>
+                <div class="reasoning-option" data-effort="minimal">Minimal</div>
+                <div class="reasoning-option" data-effort="low">Low</div>
+                <div class="reasoning-option" data-effort="medium">Medium</div>
+                <div class="reasoning-option" data-effort="high">High</div>
+                <div class="reasoning-option" data-effort="xhigh">Extra High</div>
+              </div>
+            </div>
             <div class="composer-model-wrap">
               <button class="composer-model-chip" id="composerModelChip" type="button" onclick="toggleModelDropdown()" title="Conversation model">
                 <span class="composer-model-icon" aria-hidden="true"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><rect x="4" y="4" width="16" height="16" rx="2"/><rect x="9" y="9" width="6" height="6"/><path d="M15 2v2"/><path d="M15 20v2"/><path d="M2 15h2"/><path d="M2 9h2"/><path d="M20 15h2"/><path d="M20 9h2"/><path d="M9 2v2"/><path d="M9 20v2"/></svg></span>
@@ -396,6 +411,7 @@
             <button class="cancel-btn" id="btnCancel" onclick="cancelStream()" style="display:none" title="Stop generation" aria-label="Stop generation">
               <svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="5" y="5" width="14" height="14" rx="2"></rect></svg>
             </button>
+            <span class="bg-badge" id="bgBadge" style="display:none" title="Background tasks running">0</span>
             <button class="send-btn" id="btnSend" title="Send message" disabled>
               <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.5" stroke-linecap="round" stroke-linejoin="round"><line x1="12" y1="19" x2="12" y2="5"/><polyline points="5 12 12 5 19 12"/></svg>
             </button>

static/messages.js

@@ -1323,4 +1323,111 @@ function sendBrowserNotification(title,body){
   }
 }
 
+// ── /btw ephemeral stream ────────────────────────────────────────────────────
+// Connects to the ephemeral SSE stream from /api/btw and renders the answer
+// in a visually distinct bubble that is NOT persisted to session history.
+
+function attachBtwStream(parentSid, streamId, question){
+  if(!parentSid||!streamId) return;
+  const src=new EventSource('/api/stream?stream_id='+encodeURIComponent(streamId));
+  let answer='';
+  let btwRow=null;
+  function _ensureBtwRow(){
+    if(btwRow&&btwRow.isConnected) return;
+    const inner=$('msgInner');
+    if(!inner) return;
+    btwRow=document.createElement('div');
+    btwRow.className='msg-row msg-row-btw';
+    btwRow.dataset.role='assistant';
+    btwRow.dataset.btw='1';
+    const labelEl=document.createElement('div');
+    labelEl.className='msg-btw-label';
+    labelEl.textContent=t('btw_label');
+    const qEl=document.createElement('div');
+    qEl.className='msg-body';
+    qEl.textContent=question;
+    const ansEl=document.createElement('div');
+    ansEl.className='msg-body msg-btw-answer';
+    ansEl.textContent='...';
+    btwRow.appendChild(labelEl);
+    btwRow.appendChild(qEl);
+    btwRow.appendChild(ansEl);
+    inner.appendChild(btwRow);
+    btwRow.scrollIntoView({behavior:'smooth',block:'end'});
+  }
+  src.addEventListener('token',e=>{
+    try{answer+=JSON.parse(e.data).text||'';}catch(_){}
+    _ensureBtwRow();
+    const ansEl=btwRow&&btwRow.querySelector('.msg-btw-answer');
+    if(ansEl) ansEl.innerHTML=renderMd(answer);
+  });
+  src.addEventListener('done',e=>{
+    src.close();
+    try{
+      const d=JSON.parse(e.data);
+      if(d.answer&&!answer) answer=d.answer;
+    }catch(_){}
+    if(btwRow&&btwRow.isConnected){
+      const ansEl=btwRow.querySelector('.msg-btw-answer');
+      if(ansEl) ansEl.innerHTML=renderMd(answer||t('btw_no_answer'));
+    }
+    showToast(t('btw_done'));
+  });
+  src.addEventListener('apperror',e=>{
+    src.close();
+    try{
+      const d=JSON.parse(e.data);
+      showToast(t('btw_failed')+(d.message||''));
+    }catch(_){showToast(t('btw_failed'));}
+    if(btwRow&&btwRow.isConnected) btwRow.remove();
+  });
+  src.addEventListener('stream_end',()=>{src.close();});
+  src.onerror=()=>{src.close();if(btwRow&&btwRow.isConnected) btwRow.remove();};
+}
+
+// ── /background task tracking ────────────────────────────────────────────────
+
+let _bgPollTimers={};
+let _bgActiveTasks=new Set();
+
+function showBackgroundBadge(taskId){
+  _bgActiveTasks.add(taskId);
+  const badge=$('bgBadge');
+  if(badge){
+    badge.textContent=String(_bgActiveTasks.size);
+    badge.style.display=_bgActiveTasks.size?'':'none';
+  }
+}
+function hideBackgroundBadge(taskId){
+  _bgActiveTasks.delete(taskId);
+  const badge=$('bgBadge');
+  if(badge){
+    badge.textContent=String(_bgActiveTasks.size);
+    badge.style.display=_bgActiveTasks.size?'':'none';
+  }
+}
+function startBackgroundPolling(parentSid, taskId, prompt){
+  if(_bgPollTimers[taskId]) return;
+  async function _poll(){
+    try{
+      const r=await api('/api/background/status?session_id='+encodeURIComponent(parentSid));
+      if(r&&r.results){
+        for(const res of r.results){
+          if(res.task_id===taskId){
+            hideBackgroundBadge(taskId);
+            delete _bgPollTimers[taskId];
+            const msg={role:'assistant',content:`**${t('bg_label')}** ${prompt.slice(0,80)}\n\n${res.answer||t('bg_no_answer')}`,'_background':true,_ts:Date.now()/1000};
+            S.messages.push(msg);
+            renderMessages();
+            showToast(t('bg_complete'));
+            return;
+          }
+        }
+      }
+    }catch(_){}
+    _bgPollTimers[taskId]=setTimeout(_poll,3000);
+  }
+  _poll();
+}
+
 // ── Panel navigation (Chat / Tasks / Skills / Memory) ──

static/style.css

@@ -593,6 +593,17 @@
   .composer-workspace-chip.active{color:var(--text);background:var(--accent-bg);border-color:var(--accent-bg);}
   .composer-workspace-icon,.composer-workspace-chevron{display:inline-flex;align-items:center;justify-content:center;flex-shrink:0;line-height:1;}
   .composer-workspace-label{min-width:0;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;}
+  .composer-reasoning-wrap{position:relative;flex:0 1 auto;min-width:0;}
+  .composer-reasoning-chip{display:inline-flex;align-items:center;gap:8px;max-width:180px;padding:8px 10px 8px 12px;border-radius:999px;border:1px solid transparent;background-color:transparent;color:var(--muted);font-weight:500;cursor:pointer;transition:color .15s,background-color .15s,border-color .15s;}
+  .composer-reasoning-chip:hover{color:var(--text);background-color:var(--hover-bg);}
+  .composer-reasoning-chip.active{color:var(--text);background:var(--accent-bg);border-color:var(--accent-bg);}
+  .composer-reasoning-icon,.composer-reasoning-chevron{display:inline-flex;align-items:center;justify-content:center;flex-shrink:0;line-height:1;}
+  .composer-reasoning-label{min-width:0;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;}
+  .composer-reasoning-dropdown{display:none;position:absolute;bottom:calc(100% + 6px);left:0;background:var(--surface);border:1px solid var(--border);border-radius:10px;box-shadow:0 8px 24px rgba(0,0,0,.15);min-width:140px;z-index:100;padding:4px;animation:dropdown-in .12s ease-out;}
+  .composer-reasoning-dropdown.open{display:block;}
+  .reasoning-option{padding:8px 14px;border-radius:6px;cursor:pointer;font-size:13px;color:var(--text);white-space:nowrap;transition:background-color .1s;}
+  .reasoning-option:hover{background:var(--hover-bg);}
+  .reasoning-option.selected{color:var(--accent);font-weight:600;}
   .composer-model-wrap{position:relative;flex:0 1 auto;min-width:0;}
   .composer-model-chip{display:inline-flex;align-items:center;gap:8px;max-width:220px;padding:8px 10px 8px 12px;border-radius:999px;border:1px solid transparent;background-color:transparent;color:var(--muted);font-weight:500;cursor:pointer;transition:color .15s,background-color .15s,border-color .15s;}
   .composer-model-chip:hover{color:var(--text);background-color:var(--hover-bg);}
@@ -769,12 +780,15 @@
     .composer-profile-label,
     .composer-workspace-label,
     .composer-model-label,
+    .composer-reasoning-label,
     .composer-profile-chevron,
     .composer-workspace-chevron,
-    .composer-model-chevron{display:none;}
+    .composer-model-chevron,
+    .composer-reasoning-chevron{display:none;}
     .composer-profile-chip,
     .composer-workspace-chip,
-    .composer-model-chip{max-width:44px;min-width:44px;min-height:44px;padding:6px;justify-content:center;gap:0;font-size:11px;}
+    .composer-model-chip,
+    .composer-reasoning-chip{max-width:44px;min-width:44px;min-height:44px;padding:6px;justify-content:center;gap:0;font-size:11px;}
     .composer-divider{display:none;}
     .composer-status{max-width:96px;font-size:10px;}
     .send-btn{width:32px;height:32px;}
@@ -1909,3 +1923,40 @@ body.resizing{user-select:none;cursor:col-resize;}
   .assistant-turn .msg-foot{opacity:1;}
   .msg-actions{opacity:1;}
 }
+
+/* ── /btw ephemeral side-question bubble ── */
+.msg-row-btw {
+  border-left: 3px solid var(--accent-bg-strong);
+  border-radius: 8px;
+  background: var(--accent-bg);
+  margin: 6px 0 6px var(--msg-rail);
+  padding: 8px 12px;
+}
+.msg-row-btw .msg-body { font-size: 14px; line-height: 1.75; }
+.msg-btw-label {
+  font-size: 11px;
+  font-style: italic;
+  color: var(--muted);
+  margin-bottom: 4px;
+  opacity: .8;
+}
+.msg-btw-answer {
+  margin-top: 6px;
+  border-top: 1px solid var(--accent-bg-strong);
+  padding-top: 6px;
+}
+
+/* ── /background badge in composer footer ── */
+.bg-badge {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  min-width: 18px;
+  height: 18px;
+  padding: 0 5px;
+  border-radius: 9px;
+  background: var(--accent-bg-strong);
+  color: var(--accent-text);
+  font-size: 10px;
+  font-weight: 600;
+}

static/ui.js

@@ -386,6 +386,7 @@ function toggleModelDropdown(){
   if(open){closeModelDropdown(); return;}
   if(typeof closeProfileDropdown==='function') closeProfileDropdown();
   if(typeof closeWsDropdown==='function') closeWsDropdown();
+  if(typeof closeReasoningDropdown==='function') closeReasoningDropdown();
   renderModelDropdown();
   dd.classList.add('open');
   _positionModelDropdown();
@@ -407,6 +408,77 @@ window.addEventListener('resize',()=>{
   if(dd&&dd.classList.contains('open')) _positionModelDropdown();
 });
 
+// ── Reasoning effort chip ────────────────────────────────────────────────────
+let _currentReasoningEffort=null;
+
+function _applyReasoningChip(eff){
+  _currentReasoningEffort=eff;
+  const wrap=$('composerReasoningWrap');
+  const label=$('composerReasoningLabel');
+  if(!wrap||!label) return;
+  if(!eff||eff==='none'){wrap.style.display='none';return;}
+  wrap.style.display='';
+  label.textContent='🧠 '+eff;
+  _highlightReasoningOption(eff);
+}
+
+function fetchReasoningChip(){
+  api('/api/reasoning').then(function(st){
+    _applyReasoningChip((st&&st.reasoning_effort)||'');
+  }).catch(function(){_applyReasoningChip('');});
+}
+
+function syncReasoningChip(){
+  if(_currentReasoningEffort===null){fetchReasoningChip();return;}
+  _applyReasoningChip(_currentReasoningEffort);
+}
+
+function _highlightReasoningOption(effort){
+  const dd=$('composerReasoningDropdown');
+  if(!dd) return;
+  dd.querySelectorAll('.reasoning-option').forEach(function(opt){
+    opt.classList.toggle('selected',opt.dataset.effort===effort);
+  });
+}
+
+function toggleReasoningDropdown(){
+  const dd=$('composerReasoningDropdown');
+  const chip=$('composerReasoningChip');
+  if(!dd||!chip) return;
+  const open=dd.classList.contains('open');
+  if(open){closeReasoningDropdown();return;}
+  if(typeof closeProfileDropdown==='function') closeProfileDropdown();
+  if(typeof closeWsDropdown==='function') closeWsDropdown();
+  closeModelDropdown();
+  _highlightReasoningOption(_currentReasoningEffort);
+  dd.classList.add('open');
+  chip.classList.add('active');
+}
+
+function closeReasoningDropdown(){
+  const dd=$('composerReasoningDropdown');
+  const chip=$('composerReasoningChip');
+  if(dd) dd.classList.remove('open');
+  if(chip) chip.classList.remove('active');
+}
+
+document.addEventListener('click',function(e){
+  if(!e.target.closest('#composerReasoningChip')&&!e.target.closest('#composerReasoningDropdown')) closeReasoningDropdown();
+  if(e.target.closest('.reasoning-option')){
+    const opt=e.target.closest('.reasoning-option');
+    const effort=opt&&opt.dataset.effort;
+    if(effort){
+      api('/api/reasoning',{method:'POST',body:JSON.stringify({effort:effort})})
+        .then(function(st){
+          _applyReasoningChip((st&&st.reasoning_effort)||effort);
+          showToast('🧠 Reasoning effort set to '+((st&&st.reasoning_effort)||effort));
+        })
+        .catch(function(){showToast('🧠 Failed to set effort');});
+      closeReasoningDropdown();
+    }
+  }
+});
+
 // ── Scroll pinning ──────────────────────────────────────────────────────────
 // When streaming, auto-scroll only if the user hasn't manually scrolled up.
 // Once the user scrolls back to within 150px of the bottom, re-pin.
@@ -1330,6 +1402,7 @@ function syncTopbar(){
     }
   }
   if(typeof syncModelChip==='function') syncModelChip();
+  if(typeof syncReasoningChip==='function') syncReasoningChip();
   // Show Clear button only when session has messages
   const clearBtn=$('btnClearConv');
   if(clearBtn) clearBtn.style.display=(S.messages&&S.messages.filter(msg=>msg.role!=='tool').length>0)?'':'none';
@@ -1693,6 +1766,7 @@ function renderMessages(){
     const bodyHtml = isUser ? esc(String(content)).replace(/\n/g,'<br>') : renderMd(_stripXmlToolCallsDisplay(String(content)));
     const isEditableUser=isUser&&rawIdx===lastUserRawIdx;
     const editBtn  = isEditableUser ? `<button class="msg-action-btn" title="${t('edit_message')}" onclick="editMessage(this)">${li('pencil',13)}</button>` : '';
+    const undoBtn  = isLastAssistant ? `<button class="msg-action-btn" title="${t('undo_exchange')}" onclick="undoLastExchange()">${li('undo-2',13)}</button>` : '';
     const retryBtn = isLastAssistant ? `<button class="msg-action-btn" title="${t('regenerate')}" onclick="regenerateResponse(this)">${li('rotate-ccw',13)}</button>` : '';
     const copyBtn  = `<button class="msg-copy-btn msg-action-btn" title="${t('copy')}" onclick="copyMsg(this)">${li('copy',13)}</button>`;
     const tsVal=m._ts||m.timestamp;