diff --git a/api/routes.py b/api/routes.py
index 6ee22d3b..2aaac2bf 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -5,6 +5,7 @@ Extracted from server.py (Sprint 11) so server.py is a thin shell.
 
 import html as _html
 import copy
+import io
 import json
 import logging
 import os
@@ -49,6 +50,9 @@ _CLIENT_DISCONNECT_ERRORS = (
 # Track job IDs currently being executed so the frontend can poll status.
 _RUNNING_CRON_JOBS: dict[str, float] = {}  # job_id → start_timestamp
 _RUNNING_CRON_LOCK = threading.Lock()
+_MANUAL_COMPRESSION_JOBS: dict[str, dict] = {}
+_MANUAL_COMPRESSION_JOBS_LOCK = threading.Lock()
+_MANUAL_COMPRESSION_JOB_TTL_SECONDS = 10 * 60
 _CRON_OUTPUT_CONTENT_LIMIT = 8000
 _CRON_OUTPUT_HEADER_CONTEXT = 200
 _MESSAGING_RAW_SOURCES = {str(s).strip().lower() for s in MESSAGING_SOURCES}
@@ -3072,6 +3076,10 @@ def handle_get(handler, parsed) -> bool:
             logger.exception("failed to read worktree status for session %s", sid)
             return bad(handler, _sanitize_error(exc), status=500)
 
+    if parsed.path == "/api/session/compress/status":
+        query = parse_qs(parsed.query)
+        return _handle_session_compress_status(handler, query.get("session_id", [""])[0])
+
     if parsed.path == "/api/session":
         import time as _time
         _t0 = _time.monotonic()
@@ -4421,6 +4429,9 @@ def handle_post(handler, parsed) -> bool:
             "parent_session_id": source.session_id,
         })
 
+    if parsed.path == "/api/session/compress/start":
+        return _handle_session_compress_start(handler, body)
+
     if parsed.path == "/api/session/compress":
         return _handle_session_compress(handler, body)
 
@@ -7745,6 +7756,174 @@ def _handle_clarify_respond(handler, body):
     return j(handler, {"ok": True, "response": response})
 
 
+class _ManualCompressionMemoryHandler:
+    def __init__(self):
+        self.wfile = io.BytesIO()
+        self.status = None
+        self.sent_headers = {}
+
+    def send_response(self, status):
+        self.status = status
+
+    def send_header(self, key, value):
+        self.sent_headers[key] = value
+
+    def end_headers(self):
+        pass
+
+    def payload(self):
+        raw = self.wfile.getvalue().decode("utf-8")
+        return json.loads(raw) if raw else {}
+
+
+def _manual_compression_cleanup_locked(now=None):
+    now = time.time() if now is None else now
+    for sid, job in list(_MANUAL_COMPRESSION_JOBS.items()):
+        if job.get("status") == "running":
+            continue
+        updated_at = float(job.get("updated_at") or job.get("started_at") or now)
+        if now - updated_at > _MANUAL_COMPRESSION_JOB_TTL_SECONDS:
+            _MANUAL_COMPRESSION_JOBS.pop(sid, None)
+
+
+def _manual_compression_status_payload(job):
+    status = job.get("status") or "running"
+    payload = {
+        "ok": status not in {"error", "cancelled"},
+        "status": status,
+        "session_id": job.get("session_id"),
+        "focus_topic": job.get("focus_topic"),
+        "started_at": job.get("started_at"),
+        "updated_at": job.get("updated_at"),
+    }
+    if status == "done":
+        result = job.get("result")
+        if isinstance(result, dict):
+            payload.update(result)
+        payload["status"] = "done"
+        payload["ok"] = True
+    elif status == "error":
+        payload["ok"] = False
+        payload["error"] = job.get("error") or "Compression failed"
+        payload["error_status"] = int(job.get("error_status") or 400)
+    elif status == "cancelled":
+        payload["ok"] = False
+        payload["error"] = job.get("error") or "Compression cancelled"
+        payload["error_status"] = int(job.get("error_status") or 409)
+    return payload
+
+
+def _run_manual_compression_job(sid, body):
+    memory_handler = _ManualCompressionMemoryHandler()
+    try:
+        _handle_session_compress(memory_handler, body)
+        status = int(memory_handler.status or 500)
+        payload = memory_handler.payload()
+        with _MANUAL_COMPRESSION_JOBS_LOCK:
+            job = _MANUAL_COMPRESSION_JOBS.get(sid)
+            if not job:
+                return
+            now = time.time()
+            if status >= 400 or not isinstance(payload, dict) or payload.get("error"):
+                job.update(
+                    {
+                        "status": "error",
+                        "error": str((payload or {}).get("error") or "Compression failed"),
+                        "error_status": status,
+                        "updated_at": now,
+                    }
+                )
+            else:
+                job.update(
+                    {
+                        "status": "done",
+                        "result": payload,
+                        "updated_at": now,
+                    }
+                )
+    except Exception as exc:
+        logger.warning("Manual compression worker failed for session %s: %s", sid, exc)
+        with _MANUAL_COMPRESSION_JOBS_LOCK:
+            job = _MANUAL_COMPRESSION_JOBS.get(sid)
+            if job:
+                job.update(
+                    {
+                        "status": "error",
+                        "error": f"Compression failed: {_sanitize_error(exc)}",
+                        "error_status": 500,
+                        "updated_at": time.time(),
+                    }
+                )
+
+
+def _handle_session_compress_start(handler, body):
+    try:
+        require(body, "session_id")
+    except ValueError as e:
+        return bad(handler, str(e))
+
+    sid = str(body.get("session_id") or "").strip()
+    if not sid:
+        return bad(handler, "session_id is required")
+    try:
+        s = get_session(sid)
+    except KeyError:
+        return bad(handler, "Session not found", 404)
+    if getattr(s, "active_stream_id", None):
+        return bad(handler, "Session is still streaming; wait for the current turn to finish.", 409)
+
+    focus_topic = str(body.get("focus_topic") or body.get("topic") or "").strip()[:500] or None
+    job_body = {"session_id": sid}
+    if focus_topic:
+        job_body["focus_topic"] = focus_topic
+
+    now = time.time()
+    with _MANUAL_COMPRESSION_JOBS_LOCK:
+        _manual_compression_cleanup_locked(now)
+        existing = _MANUAL_COMPRESSION_JOBS.get(sid)
+        if existing:
+            existing_payload = _manual_compression_status_payload(existing)
+            if existing_payload.get("status") == "running":
+                return j(handler, existing_payload)
+            _MANUAL_COMPRESSION_JOBS.pop(sid, None)
+            if existing_payload.get("status") == "done":
+                return j(handler, existing_payload)
+        job = {
+            "session_id": sid,
+            "focus_topic": focus_topic,
+            "status": "running",
+            "started_at": now,
+            "updated_at": now,
+        }
+        _MANUAL_COMPRESSION_JOBS[sid] = job
+
+    worker = threading.Thread(
+        target=_run_manual_compression_job,
+        args=(sid, job_body),
+        name=f"manual-compress-{sid[:8]}",
+        daemon=True,
+    )
+    worker.start()
+
+    with _MANUAL_COMPRESSION_JOBS_LOCK:
+        return j(handler, _manual_compression_status_payload(_MANUAL_COMPRESSION_JOBS.get(sid, job)))
+
+
+def _handle_session_compress_status(handler, sid):
+    sid = str(sid or "").strip()
+    if not sid:
+        return bad(handler, "session_id is required")
+    with _MANUAL_COMPRESSION_JOBS_LOCK:
+        _manual_compression_cleanup_locked()
+        job = _MANUAL_COMPRESSION_JOBS.get(sid)
+        if not job:
+            return j(handler, {"ok": True, "status": "idle", "session_id": sid})
+        payload = _manual_compression_status_payload(job)
+        if payload.get("status") == "done":
+            _MANUAL_COMPRESSION_JOBS.pop(sid, None)
+        return j(handler, payload)
+
+
 def _handle_session_compress(handler, body):
     def _anchor_message_key(m):
         if not isinstance(m, dict):
@@ -7943,6 +8122,12 @@ def _handle_session_compress(handler, body):
         # Lock contract: hold for the in-memory mutation only, never across
         # network I/O.
         original_messages = list(messages)
+        original_stream_state = (
+            getattr(s, "active_stream_id", None),
+            getattr(s, "pending_user_message", None),
+            copy.deepcopy(getattr(s, "pending_attachments", None)),
+            getattr(s, "pending_started_at", None),
+        )
         approx_tokens = _estimate_messages_tokens_rough(original_messages)
 
         agent = _run_agent.AIAgent(
@@ -7974,6 +8159,14 @@ def _handle_session_compress(handler, body):
         with _cfg._get_session_agent_lock(sid):
             # Re-read messages to detect concurrent edits during the LLM call.
             # If the history changed, the compression result is stale — abort.
+            current_stream_state = (
+                getattr(s, "active_stream_id", None),
+                getattr(s, "pending_user_message", None),
+                copy.deepcopy(getattr(s, "pending_attachments", None)),
+                getattr(s, "pending_started_at", None),
+            )
+            if current_stream_state != original_stream_state:
+                return bad(handler, "Session stream state changed during compression; please retry.", 409)
             if _sanitize_messages_for_api(s.messages) != original_messages:
                 return bad(handler, "Session was modified during compression; please retry.", 409)
 
diff --git a/static/commands.js b/static/commands.js
index bd2f146f..c09cbf54 100644
--- a/static/commands.js
+++ b/static/commands.js
@@ -382,6 +382,131 @@ async function cmdNew(){
   showToast(t('new_session'));
 }
 
+function _manualCompressionVisibleMessages(){
+  return (S.messages||[]).filter(m=>{
+    if(!m||!m.role||m.role==='tool') return false;
+    if(m.role==='assistant'){
+      const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
+      const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
+      if(hasTc||hasTu|| (typeof _messageHasReasoningPayload==='function' && _messageHasReasoningPayload(m))) return true;
+    }
+    return typeof msgContent==='function' ? !!msgContent(m) || !!m.attachments?.length : !!m.content || !!m.attachments?.length;
+  });
+}
+
+function _manualCompressionSleep(ms){
+  return new Promise(resolve=>setTimeout(resolve, ms));
+}
+
+async function _pollManualCompressionResult(sid){
+  let delay=700;
+  while(true){
+    const data=await api(`/api/session/compress/status?session_id=${encodeURIComponent(sid)}`);
+    if(data&&data.status==='done') return data;
+    if(data&&data.status==='error'){
+      const err=new Error(data.error||'Compression failed');
+      err.status=data.error_status||400;
+      throw err;
+    }
+    if(data&&data.status==='idle') throw new Error('Compression job is no longer available');
+    await _manualCompressionSleep(delay);
+    delay=Math.min(2000, delay+300);
+  }
+}
+
+async function _applyManualCompressionResult(data, focusTopic, visibleCount, commandText){
+  if(data&&data.session){
+    const currentSid=S.session&&S.session.session_id;
+    if(data.session.session_id&&data.session.session_id!==currentSid){
+      await loadSession(data.session.session_id);
+    }else{
+      S.session=data.session;
+      S.messages=data.session.messages||[];
+      S.toolCalls=data.session.tool_calls||[];
+      clearLiveToolCards();
+      localStorage.setItem('hermes-webui-session',S.session.session_id);
+      if(typeof _setActiveSessionUrl==='function') _setActiveSessionUrl(S.session.session_id);
+      syncTopbar();
+      renderMessages();
+      await renderSessionList();
+      updateQueueBadge(S.session.session_id);
+    }
+  }
+  const summary=data&&data.summary;
+  if(typeof setCompressionUi==='function'&&S.session){
+    const referenceMsg=(S.messages||[]).find(m=>typeof _isContextCompactionMessage==='function'&&_isContextCompactionMessage(m));
+    const messageRef=referenceMsg?msgContent(referenceMsg)||String(referenceMsg.content||''):'';
+    const summaryRef=summary&&typeof summary.reference_message==='string' ? String(summary.reference_message||'').trim() : '';
+    // Prefer the persisted compaction handoff when it already exists in session state.
+    // The short summary fallback is only for environments where that message is unavailable.
+    const referenceText=messageRef || summaryRef;
+    const effectiveFocus=(data&&data.focus_topic)||focusTopic||'';
+    setCompressionUi({
+      sessionId:S.session.session_id,
+      phase:'done',
+      focusTopic:effectiveFocus,
+      commandText:effectiveFocus?`/compress ${effectiveFocus}`:(commandText||'/compress'),
+      beforeCount:visibleCount,
+      summary:summary||null,
+      referenceText,
+      anchorVisibleIdx: data?.session?.compression_anchor_visible_idx,
+      anchorMessageKey: data?.session?.compression_anchor_message_key||null,
+    });
+  }
+  if(typeof setComposerStatus==='function') setComposerStatus('');
+  renderMessages();
+  if(typeof _setCompressionSessionLock==='function') _setCompressionSessionLock(null);
+}
+
+async function resumeManualCompressionForSession(sid){
+  if(!sid) return;
+  try{
+    const status=await api(`/api/session/compress/status?session_id=${encodeURIComponent(sid)}`);
+    if(!status||status.status!=='running') return;
+    const visibleMessages=_manualCompressionVisibleMessages();
+    const visibleCount=visibleMessages.length;
+    const anchorMessageKey=_compressionAnchorMessageKey(visibleMessages[visibleMessages.length-1]||null);
+    if(typeof setBusy==='function') setBusy(true);
+    if(typeof setComposerStatus==='function') setComposerStatus(t('compressing'));
+    if(typeof setCompressionUi==='function'){
+      setCompressionUi({
+        sessionId:sid,
+        phase:'running',
+        focusTopic:status.focus_topic||'',
+        commandText:status.focus_topic?`/compress ${status.focus_topic}`:'/compress',
+        beforeCount:visibleCount,
+        anchorVisibleIdx:Math.max(0, visibleCount-1),
+        anchorMessageKey,
+      });
+    }
+    renderMessages();
+    const done=await _pollManualCompressionResult(sid);
+    if(!S.session||S.session.session_id!==sid) return;
+    await _applyManualCompressionResult(done, status.focus_topic||'', visibleCount, status.focus_topic?`/compress ${status.focus_topic}`:'/compress');
+  }catch(e){
+    if(S.session&&S.session.session_id===sid&&typeof setCompressionUi==='function'){
+      const visibleMessages=_manualCompressionVisibleMessages();
+      setCompressionUi({
+        sessionId:sid,
+        phase:'error',
+        focusTopic:'',
+        commandText:'/compress',
+        beforeCount:visibleMessages.length,
+        errorText:`Compression failed: ${e.message}`,
+        anchorVisibleIdx:Math.max(0, visibleMessages.length-1),
+        anchorMessageKey:null,
+      });
+      renderMessages();
+    }
+  }finally{
+    if(S.session&&S.session.session_id===sid){
+      if(typeof _setCompressionSessionLock==='function') _setCompressionSessionLock(null);
+      if(typeof setBusy==='function') setBusy(false);
+      if(typeof setComposerStatus==='function') setComposerStatus('');
+    }
+  }
+}
+
 async function _runManualCompression(focusTopic){
   if(!S.session){showToast(t('no_active_session'));return;}
   let visibleCount=0;
@@ -410,15 +535,7 @@ async function _runManualCompression(focusTopic){
     if(typeof setBusy==='function') setBusy(true);
     const body={session_id:sid};
     if(focusTopic) body.focus_topic=focusTopic;
-    const visibleMessages=(S.messages||[]).filter(m=>{
-      if(!m||!m.role||m.role==='tool') return false;
-      if(m.role==='assistant'){
-        const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
-        const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
-        if(hasTc||hasTu|| (typeof _messageHasReasoningPayload==='function' && _messageHasReasoningPayload(m))) return true;
-      }
-      return typeof msgContent==='function' ? !!msgContent(m) || !!m.attachments?.length : !!m.content || !!m.attachments?.length;
-    });
+    const visibleMessages=_manualCompressionVisibleMessages();
     visibleCount=visibleMessages.length;
     const anchorVisibleIdx=Math.max(0, visibleCount - 1);
     const anchorMessageKey=_compressionAnchorMessageKey(visibleMessages[visibleMessages.length-1]||null);
@@ -436,48 +553,14 @@ async function _runManualCompression(focusTopic){
     }
     if(typeof setComposerStatus==='function') setComposerStatus(t('compressing'));
     renderMessages();
-    const data=await api('/api/session/compress',{method:'POST',body:JSON.stringify(body)});
-    if(data&&data.session){
-      const currentSid=S.session&&S.session.session_id;
-      if(data.session.session_id&&data.session.session_id!==currentSid){
-        await loadSession(data.session.session_id);
-      }else{
-        S.session=data.session;
-        S.messages=data.session.messages||[];
-        S.toolCalls=data.session.tool_calls||[];
-        clearLiveToolCards();
-        localStorage.setItem('hermes-webui-session',S.session.session_id);
-        if(typeof _setActiveSessionUrl==='function') _setActiveSessionUrl(S.session.session_id);
-        syncTopbar();
-        renderMessages();
-        await renderSessionList();
-        updateQueueBadge(S.session.session_id);
-      }
+    const started=await api('/api/session/compress/start',{method:'POST',body:JSON.stringify(body)});
+    if(started&&started.status==='error'){
+      const err=new Error(started.error||'Compression failed');
+      err.status=started.error_status||400;
+      throw err;
     }
-    const summary=data&&data.summary;
-    if(typeof setCompressionUi==='function'&&S.session){
-      const referenceMsg=(S.messages||[]).find(m=>typeof _isContextCompactionMessage==='function'&&_isContextCompactionMessage(m));
-      const messageRef=referenceMsg?msgContent(referenceMsg)||String(referenceMsg.content||''):'';
-      const summaryRef=summary&&typeof summary.reference_message==='string' ? String(summary.reference_message||'').trim() : '';
-      // Prefer the persisted compaction handoff when it already exists in session state.
-      // The short summary fallback is only for environments where that message is unavailable.
-      const referenceText=messageRef || summaryRef;
-      const effectiveFocus=(data&&data.focus_topic)||focusTopic||'';
-      setCompressionUi({
-        sessionId:S.session.session_id,
-        phase:'done',
-        focusTopic:effectiveFocus,
-        commandText:effectiveFocus?`/compress ${effectiveFocus}`:'/compress',
-        beforeCount:visibleCount,
-        summary:summary||null,
-        referenceText,
-        anchorVisibleIdx: data?.session?.compression_anchor_visible_idx,
-        anchorMessageKey: data?.session?.compression_anchor_message_key||null,
-      });
-    }
-    if(typeof setComposerStatus==='function') setComposerStatus('');
-    renderMessages();
-    if(typeof _setCompressionSessionLock==='function') _setCompressionSessionLock(null);
+    const data=(started&&started.status==='done')?started:await _pollManualCompressionResult(sid);
+    await _applyManualCompressionResult(data, focusTopic, visibleCount, commandText);
   }catch(e){
     if(typeof setCompressionUi==='function'){
       const currentSid=S.session&&S.session.session_id;
diff --git a/static/sessions.js b/static/sessions.js
index f9451a91..17ee262c 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -653,6 +653,7 @@ async function loadSession(sid){
       setComposerStatus('');
       updateQueueBadge(sid);
       syncTopbar();renderMessages();
+      if(typeof resumeManualCompressionForSession==='function') resumeManualCompressionForSession(sid);
       // Kick off loadDir first (issues network requests), then highlight code.
       // The fetch is dispatched before the CPU-bound Prism pass begins.
       const _dirP=loadDir('.');
diff --git a/tests/test_sprint46.py b/tests/test_sprint46.py
index 35145c95..e502ea9b 100644
--- a/tests/test_sprint46.py
+++ b/tests/test_sprint46.py
@@ -6,6 +6,8 @@ import contextlib
 import io
 import json
 import sys
+import threading
+import time
 import types
 
 from api.models import Session
@@ -59,39 +61,9 @@ class _FakeAgent:
         _FakeAgent.last_instance = self
 
 
-def _make_session(messages=None):
-    SESSION_DIR.mkdir(parents=True, exist_ok=True)
-    messages = messages or [
-        {"role": "user", "content": "one"},
-        {"role": "assistant", "content": "two"},
-        {"role": "user", "content": "three"},
-        {"role": "assistant", "content": "four"},
-    ]
-    s = Session(
-        session_id="compress_test_001",
-        title="Untitled",
-        workspace="/tmp/hermes-webui-test",
-        model="openai/gpt-5.4-mini",
-        messages=messages,
-    )
-    s.save(touch_updated_at=False)
-    return s.session_id
-
-
-def test_session_compress_requires_session_id(cleanup_test_sessions):
-    handler = _FakeHandler()
-    _handle_session_compress(handler, {})
-    assert handler.status == 400
-    assert handler.payload()["error"] == "Missing required field(s): session_id"
-
-
-def test_session_compress_roundtrip(monkeypatch, cleanup_test_sessions):
-    created = cleanup_test_sessions
-    sid = _make_session()
-    created.append(sid)
-
+def _install_fake_compression_runtime(monkeypatch, agent_cls):
     fake_run_agent = types.ModuleType("run_agent")
-    fake_run_agent.AIAgent = _FakeAgent
+    fake_run_agent.AIAgent = agent_cls
     monkeypatch.setitem(sys.modules, "run_agent", fake_run_agent)
 
     import api.config as _cfg
@@ -128,6 +100,40 @@ def test_session_compress_roundtrip(monkeypatch, cleanup_test_sessions):
         },
     )
 
+
+def _make_session(messages=None):
+    SESSION_DIR.mkdir(parents=True, exist_ok=True)
+    messages = messages or [
+        {"role": "user", "content": "one"},
+        {"role": "assistant", "content": "two"},
+        {"role": "user", "content": "three"},
+        {"role": "assistant", "content": "four"},
+    ]
+    s = Session(
+        session_id=f"compress_test_{time.time_ns()}",
+        title="Untitled",
+        workspace="/tmp/hermes-webui-test",
+        model="openai/gpt-5.4-mini",
+        messages=messages,
+    )
+    s.save(touch_updated_at=False)
+    return s.session_id
+
+
+def test_session_compress_requires_session_id(cleanup_test_sessions):
+    handler = _FakeHandler()
+    _handle_session_compress(handler, {})
+    assert handler.status == 400
+    assert handler.payload()["error"] == "Missing required field(s): session_id"
+
+
+def test_session_compress_roundtrip(monkeypatch, cleanup_test_sessions):
+    created = cleanup_test_sessions
+    sid = _make_session()
+    created.append(sid)
+
+    _install_fake_compression_runtime(monkeypatch, _FakeAgent)
+
     handler = _FakeHandler()
     _handle_session_compress(handler, {"session_id": sid, "focus_topic": "database schema"})
 
@@ -153,6 +159,253 @@ def test_session_compress_roundtrip(monkeypatch, cleanup_test_sessions):
     assert _FakeAgent.last_instance.context_compressor.calls[0]["focus_topic"] == "database schema"
 
 
+def test_session_compress_start_is_async_and_reuses_running_job(monkeypatch, cleanup_test_sessions):
+    import api.routes as routes
+
+    assert hasattr(routes, "_handle_session_compress_start")
+    assert hasattr(routes, "_handle_session_compress_status")
+
+    class BlockingCompressor:
+        entered = threading.Event()
+        release = threading.Event()
+        calls = []
+
+        def compress(self, messages, current_tokens=None, focus_topic=None):
+            self.calls.append({"messages": list(messages), "focus_topic": focus_topic})
+            self.entered.set()
+            assert self.release.wait(timeout=5), "test timed out waiting to release compression"
+            return [messages[0], messages[-1]]
+
+    class BlockingAgent:
+        instances = []
+
+        def __init__(self, **kwargs):
+            self.context_compressor = BlockingCompressor()
+            self.instances.append(self)
+
+    created = cleanup_test_sessions
+    sid = _make_session()
+    created.append(sid)
+    _install_fake_compression_runtime(monkeypatch, BlockingAgent)
+    try:
+        first = _FakeHandler()
+        routes._handle_session_compress_start(first, {"session_id": sid, "focus_topic": "slow"})
+        assert first.status == 200
+        first_payload = first.payload()
+        assert first_payload["ok"] is True
+        assert first_payload["status"] == "running"
+        assert first_payload["session_id"] == sid
+        assert first_payload["focus_topic"] == "slow"
+        assert BlockingCompressor.entered.wait(timeout=2)
+
+        second = _FakeHandler()
+        routes._handle_session_compress_start(second, {"session_id": sid, "focus_topic": "slow"})
+        assert second.status == 200
+        second_payload = second.payload()
+        assert second_payload["status"] == "running"
+        assert len(BlockingAgent.instances) == 1
+
+        running = _FakeHandler()
+        routes._handle_session_compress_status(running, sid)
+        assert running.status == 200
+        assert running.payload()["status"] == "running"
+    finally:
+        BlockingCompressor.release.set()
+
+    deadline = time.time() + 5
+    done_payload = None
+    while time.time() < deadline:
+        done = _FakeHandler()
+        routes._handle_session_compress_status(done, sid)
+        payload = done.payload()
+        if payload["status"] == "done":
+            done_payload = payload
+            break
+        time.sleep(0.02)
+    assert done_payload is not None
+    assert done_payload["summary"]["headline"] == "Compressed: 4 → 2 messages"
+    assert done_payload["session"]["messages"] == [
+        {"role": "user", "content": "one"},
+        {"role": "assistant", "content": "four"},
+    ]
+
+
+def test_session_compress_status_reports_worker_error_without_raw_paths(monkeypatch, cleanup_test_sessions):
+    import api.routes as routes
+
+    assert hasattr(routes, "_handle_session_compress_start")
+    assert hasattr(routes, "_handle_session_compress_status")
+
+    class FailingCompressor:
+        entered = threading.Event()
+
+        def compress(self, messages, current_tokens=None, focus_topic=None):
+            self.entered.set()
+            raise RuntimeError("provider log at /Users/alice/.hermes/secrets/token.txt failed")
+
+    class FailingAgent:
+        def __init__(self, **kwargs):
+            self.context_compressor = FailingCompressor()
+
+    created = cleanup_test_sessions
+    sid = _make_session()
+    created.append(sid)
+    _install_fake_compression_runtime(monkeypatch, FailingAgent)
+
+    start = _FakeHandler()
+    routes._handle_session_compress_start(start, {"session_id": sid})
+    assert start.status == 200
+    assert FailingCompressor.entered.wait(timeout=2)
+
+    deadline = time.time() + 5
+    error_payload = None
+    while time.time() < deadline:
+        status = _FakeHandler()
+        routes._handle_session_compress_status(status, sid)
+        payload = status.payload()
+        if payload["status"] == "error":
+            error_payload = payload
+            break
+        time.sleep(0.02)
+    assert error_payload is not None
+    assert error_payload["ok"] is False
+    assert error_payload["error_status"] == 400
+    assert "<path>" in error_payload["error"]
+    assert "/Users/alice" not in error_payload["error"]
+
+
+def test_session_compress_start_retries_after_terminal_error(monkeypatch, cleanup_test_sessions):
+    import api.routes as routes
+
+    class BlockingCompressor:
+        entered = threading.Event()
+        release = threading.Event()
+
+        def compress(self, messages, current_tokens=None, focus_topic=None):
+            self.entered.set()
+            assert self.release.wait(timeout=5), "test timed out waiting to release compression"
+            return [messages[0], messages[-1]]
+
+    class BlockingAgent:
+        def __init__(self, **kwargs):
+            self.context_compressor = BlockingCompressor()
+
+    created = cleanup_test_sessions
+    sid = _make_session()
+    created.append(sid)
+    _install_fake_compression_runtime(monkeypatch, BlockingAgent)
+
+    with routes._MANUAL_COMPRESSION_JOBS_LOCK:
+        routes._MANUAL_COMPRESSION_JOBS[sid] = {
+            "session_id": sid,
+            "focus_topic": None,
+            "status": "error",
+            "error": "previous failure",
+            "error_status": 400,
+            "started_at": time.time(),
+            "updated_at": time.time(),
+        }
+
+    try:
+        retry = _FakeHandler()
+        routes._handle_session_compress_start(retry, {"session_id": sid})
+        assert retry.status == 200
+        retry_payload = retry.payload()
+        assert retry_payload["status"] == "running"
+        assert retry_payload["ok"] is True
+        assert BlockingCompressor.entered.wait(timeout=2)
+    finally:
+        BlockingCompressor.release.set()
+
+
+def test_session_compress_async_reports_stale_session_guard(monkeypatch, cleanup_test_sessions):
+    import api.routes as routes
+
+    created = cleanup_test_sessions
+    sid = _make_session()
+    created.append(sid)
+
+    class MutatingCompressor:
+        entered = threading.Event()
+
+        def compress(self, messages, current_tokens=None, focus_topic=None):
+            live = get_session(sid)
+            live.messages.append({"role": "user", "content": "concurrent edit"})
+            self.entered.set()
+            return [messages[0], messages[-1]]
+
+    class MutatingAgent:
+        def __init__(self, **kwargs):
+            self.context_compressor = MutatingCompressor()
+
+    _install_fake_compression_runtime(monkeypatch, MutatingAgent)
+
+    start = _FakeHandler()
+    routes._handle_session_compress_start(start, {"session_id": sid})
+    assert start.status == 200
+    assert MutatingCompressor.entered.wait(timeout=2)
+
+    deadline = time.time() + 5
+    error_payload = None
+    while time.time() < deadline:
+        status = _FakeHandler()
+        routes._handle_session_compress_status(status, sid)
+        payload = status.payload()
+        if payload["status"] == "error":
+            error_payload = payload
+            break
+        time.sleep(0.02)
+    assert error_payload is not None
+    assert error_payload["ok"] is False
+    assert error_payload["error_status"] == 409
+    assert "modified during compression" in error_payload["error"]
+    assert get_session(sid).messages[-1]["content"] == "concurrent edit"
+
+
+def test_session_compress_async_reports_stream_state_guard(monkeypatch, cleanup_test_sessions):
+    import api.routes as routes
+
+    created = cleanup_test_sessions
+    sid = _make_session()
+    created.append(sid)
+
+    class StreamMutatingCompressor:
+        entered = threading.Event()
+
+        def compress(self, messages, current_tokens=None, focus_topic=None):
+            live = get_session(sid)
+            live.active_stream_id = "stream-concurrent"
+            self.entered.set()
+            return [messages[0], messages[-1]]
+
+    class StreamMutatingAgent:
+        def __init__(self, **kwargs):
+            self.context_compressor = StreamMutatingCompressor()
+
+    _install_fake_compression_runtime(monkeypatch, StreamMutatingAgent)
+
+    start = _FakeHandler()
+    routes._handle_session_compress_start(start, {"session_id": sid})
+    assert start.status == 200
+    assert StreamMutatingCompressor.entered.wait(timeout=2)
+
+    deadline = time.time() + 5
+    error_payload = None
+    while time.time() < deadline:
+        status = _FakeHandler()
+        routes._handle_session_compress_status(status, sid)
+        payload = status.payload()
+        if payload["status"] == "error":
+            error_payload = payload
+            break
+        time.sleep(0.02)
+    assert error_payload is not None
+    assert error_payload["ok"] is False
+    assert error_payload["error_status"] == 409
+    assert "stream state changed" in error_payload["error"]
+    assert get_session(sid).active_stream_id == "stream-concurrent"
+
+
 def test_static_commands_js_registers_compress_alias(cleanup_test_sessions):
     from pathlib import Path
 
@@ -160,7 +413,10 @@ def test_static_commands_js_registers_compress_alias(cleanup_test_sessions):
         src = f.read()
     assert "name:'compress'" in src
     assert "name:'compact'" in src
-    assert "/api/session/compress" in src
+    assert "/api/session/compress/start" in src
+    assert "/api/session/compress/status" in src
+    assert "await api('/api/session/compress'," not in src
+    assert "beforeCount:visibleCount" in src
     assert "cmdCompress" in src
     assert "cmdCompact" in src
 
@@ -173,3 +429,12 @@ def test_static_commands_js_prefers_persisted_reference_message(cleanup_test_ses
 
     assert "const messageRef=referenceMsg?msgContent(referenceMsg)||String(referenceMsg.content||''):'';" in src
     assert "const referenceText=messageRef || summaryRef;" in src
+
+
+def test_static_session_load_resumes_manual_compression_polling(cleanup_test_sessions):
+    from pathlib import Path
+
+    with open(Path(__file__).resolve().parents[1] / "static" / "sessions.js", encoding="utf-8") as f:
+        src = f.read()
+
+    assert "resumeManualCompressionForSession" in src