diff --git a/AGENTS.md b/AGENTS.md
index 0e9f6f34..b103761e 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -89,6 +89,12 @@ Follow that checklist's safety rules:
   by publishing finalizing under STREAMS_LOCK before the last pending-steer
   drain; earlier accepted guidance is drained, later guidance is rejected.
   Test both drain/Steer orderings, including compression-rotated identities.
+- Inactive-session recovery is separate from live Steer. Resolve durable
+  compression lineage in the session's profile database, read-only, even when
+  the WebUI sidecar has no snapshot flag. Never reopen a sealed parent. Reject
+  stale chat POSTs before workspace/model/pending-state mutation; the browser
+  loads the continuation and preserves the draft without automatic replay.
+  Explicit closures and unknown terminal reasons do not authorize a redirect.
 - For Docker build changes in `docker_init.bash`, mirror directory exclusions
   in both the `rsync` and `cp -a` paths — `/opt/hermes` may contain subdirectories
   with restricted permissions (e.g. `.playwright/`).
diff --git a/api/compression_continuation.py b/api/compression_continuation.py
new file mode 100644
index 00000000..d4233116
--- /dev/null
+++ b/api/compression_continuation.py
@@ -0,0 +1,60 @@
+"""Read-only routing hints for externally compressed WebUI sessions.
+
+SQLite owns compression lineage; sidecar snapshot flags may predate a
+Desktop/CLI rotation. Never reopen a sealed parent or mutate Agent state here.
+"""
+import logging
+from pathlib import Path
+
+logger = logging.getLogger(__name__)
+
+
+def durable_compression_continuation(session):
+    """Return (sealed, resumable tip), without making a recovery write.
+
+    A known sealed parent without a safe tip stays sealed (no sidecar fallback).
+    Older Agent installations without this read API retain legacy behavior.
+    """
+    from api.profiles import _PROFILE_ID_RE, _resolve_profile_home_for_name
+
+    sid = str(getattr(session, "session_id", "") or "")
+    profile = str(getattr(session, "profile", None) or "default")
+    if not sid or (profile != "default" and not _PROFILE_ID_RE.fullmatch(profile)):
+        return False, None
+    db = None
+    sealed = False
+    try:
+        from hermes_state import SessionDB
+
+        path = Path(_resolve_profile_home_for_name(profile)) / "state.db"
+        if not path.is_file():
+            return False, None
+        db = SessionDB(path, read_only=True)
+        parent = db.get_session(sid)
+        if not parent or parent.get("end_reason") != "compression":
+            return False, None
+        sealed = True
+        tip = db.get_compression_tip(sid)
+        if not tip or tip == sid:
+            return True, None
+        child = db.get_session(tip)
+        if not child or child.get("source") not in {"webui", "tui", "cli"}:
+            return True, None
+        for row in (parent, child):
+            if row.get("profile_name") not in (None, "", profile):
+                return True, None
+        # Only the observed automatic idle closure is resumable here. Explicit
+        # resets/closures and unknown future reasons must not become redirects.
+        if child.get("ended_at") is not None and child.get("end_reason") != "idle_timeout":
+            return True, None
+        if db.get_compression_tip(sid) != tip:
+            return True, None
+        return True, tip
+    except (ImportError, AttributeError, TypeError):
+        return sealed, None
+    except Exception:
+        logger.debug("Could not resolve durable compression continuation", exc_info=True)
+        return sealed, None
+    finally:
+        if db is not None:
+            db.close()
diff --git a/api/routes.py b/api/routes.py
index 0e159784..7bfa4347 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -10488,6 +10488,11 @@ def _pre_compression_continuation_session_id(session) -> str | None:
     exists either in memory or on disk. Follow bounded snapshot-to-snapshot hops
     so repeated compression still lands on the latest visible continuation.
     """
+    from api.compression_continuation import durable_compression_continuation
+
+    sealed, tip = durable_compression_continuation(session)
+    if sealed:
+        return tip
     if not getattr(session, "pre_compression_snapshot", False):
         return None
     sid = _safe_first(getattr(session, "session_id", None))
@@ -24355,6 +24360,16 @@ def _handle_chat_start(handler, body, diag=None):
                 s.profile = requested_profile
             else:
                 return bad(handler, "Session not found", 404)
+        # Resolve durable rotations before any workspace/model/pending mutation.
+        # GET navigation adopts the tip; POST never silently replays a user turn.
+        from api.compression_continuation import durable_compression_continuation
+        sealed, continuation = durable_compression_continuation(s)
+        if sealed:
+            return j(handler, {
+                "error": "This session was compressed. Open its continuation before sending.",
+                "code": "session_rotated",
+                "continuation_session_id": continuation,
+            }, status=409)
         regeneration = None
         if body.get("regenerate") is True:
             if any(key in body for key in ("message", "attachments", "keep_count", "prompt", "prompt_index")):
diff --git a/docs/rfcs/webui-run-state-consistency-contract.md b/docs/rfcs/webui-run-state-consistency-contract.md
index ca185de6..9b6d02ec 100644
--- a/docs/rfcs/webui-run-state-consistency-contract.md
+++ b/docs/rfcs/webui-run-state-consistency-contract.md
@@ -30,6 +30,25 @@ This RFC defines a consistency contract for those layers. It complements the
 larger run adapter direction in #1925 by documenting what must remain coherent
 while WebUI still has multiple overlapping state stores.
 
+## Inactive compression continuation recovery
+
+The Agent profile's SQLite compression lineage owns the canonical continuation,
+including when Desktop/CLI compressed a session without updating WebUI's
+`pre_compression_snapshot` sidecar flag. `GET /api/session` may expose the
+existing `continuation_session_id` hint from that read-only lineage. Automatic
+`idle_timeout` closure does not hide the continuation; explicit/unknown terminal
+reasons, foreign-profile rows and delegated/tool children do not authorize it.
+This read does not reopen sessions or copy ancestor display history into context.
+
+A stale `POST /api/chat/start` returns HTTP 409 with `code=session_rotated`
+and the continuation hint before workspace, model, pending-turn or worker
+mutation. The browser loads the continuation through normal session access
+checks and restores the rejected text and attachments as a draft. The user
+sends again explicitly; there is no automatic POST replay or migration of the
+parent's workspace binding. Clients without this handling must reload the
+session before retrying. Server wakeups, regeneration semantics and Gateway
+routing are not silently retargeted by this recovery path.
+
 ## Goals
 
 - Define the state layers involved in active and recovered WebUI turns.
diff --git a/static/messages.js b/static/messages.js
index d1dbac12..86aa3b29 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -1296,6 +1296,31 @@ function applySessionTitleUpdate(sid, titleText, options={}){
 // BEFORE slash rewrites (/moa, bundles) mutate the payload and BEFORE
 // uploadPendingFiles() drains S.pendingFiles — so we restore what the user
 // actually typed, not the transformed send payload.
+async function _recoverCompressedSend(error,sid,draftText,filesSnapshot,clearPromise){
+  let payload;
+  try{ payload=JSON.parse(error&&error.body||'{}'); }catch(_){ return false; }
+  const target=payload&&payload.continuation_session_id;
+  if(!error||error.status!==409||!payload||payload.code!=='session_rotated'||typeof target!=='string'||!target||target===sid) return false;
+  // A failed POST has not admitted a turn. Never resend automatically: the
+  // continuation may already be busy, and attachments must remain a draft.
+  if(!S.session||S.session.session_id!==sid) return false;
+  delete INFLIGHT[sid];
+  if(typeof clearInflightState==='function') clearInflightState(sid);
+  if(typeof clearOptimisticSessionStreaming==='function') clearOptimisticSessionStreaming(sid);
+  stopApprovalPolling();stopClarifyPolling();removeThinking();setBusy(false);
+  try{
+    await loadSession(target);
+    if(!S.session||S.session.session_id===sid) return false;
+    // loadSession can lose its navigation race to another tab selection. Never
+    // place the rejected message into that unrelated session's composer.
+    _restoreComposerDraftAfterFailedSend(draftText,filesSnapshot,target,clearPromise);
+    if(S.session.session_id!==target) return true;
+    setComposerStatus('Session resumed. Your message is preserved; send it when ready.');
+    showToast('Session resumed after compression. Your draft is preserved.',4000);
+    return true;
+  }catch(_){ return false; }
+}
+
 function _restoreComposerDraftAfterFailedSend(draftText, filesSnapshot, sid, clearPromise){
   const restore=String(draftText||'');
   const files=Array.isArray(filesSnapshot)?filesSnapshot.filter(Boolean):[];
@@ -1850,6 +1875,7 @@ async function send(){
       if(typeof renderSessionList==='function') void renderSessionList();
       return;
     }
+    if(await _recoverCompressedSend(e,activeSid,_failedSendDraftText,_failedSendFilesSnapshot,_composerDraftClearPromise)) return;
     const conflictActiveStream=/session already has an active stream/i.test(errMsg);
     if(conflictActiveStream){
       delete INFLIGHT[activeSid];
diff --git a/tests/test_compressed_idle_resume.py b/tests/test_compressed_idle_resume.py
new file mode 100644
index 00000000..7644460a
--- /dev/null
+++ b/tests/test_compressed_idle_resume.py
@@ -0,0 +1,130 @@
+"""A stale WebUI sidecar must not hide a durable compression continuation."""
+import io
+import json
+import subprocess
+from pathlib import Path
+from types import SimpleNamespace
+
+import pytest
+
+from api import routes, profiles
+
+
+@pytest.fixture
+def lineage(tmp_path, monkeypatch):
+    SessionDB = pytest.importorskip("hermes_state").SessionDB
+    db = SessionDB(tmp_path / "state.db")
+    db.create_session("sealedparent", source="webui")
+    db.create_session("idlechild", source="tui", parent_session_id="sealedparent")
+    db.append_message("idlechild", "user", "retained task")
+    db.end_session("sealedparent", "compression")
+    db.end_session("idlechild", "idle_timeout")
+    monkeypatch.setattr(profiles, "_resolve_profile_home_for_name", lambda name: str(tmp_path))
+    session = SimpleNamespace(session_id="sealedparent", profile="default", pre_compression_snapshot=False)
+    yield db, session
+    db.close()
+
+
+def test_reload_resolves_sqlite_rotation_without_sidecar_flag(lineage):
+    db, session = lineage
+    before = db.get_session("idlechild")["ended_at"]
+    assert routes._pre_compression_continuation_session_id(session) == "idlechild"
+    assert db.get_session("idlechild")["ended_at"] == before
+    assert db.get_session("sealedparent")["end_reason"] == "compression"
+
+
+class Handler:
+    headers = {}
+    def __init__(self):
+        self.wfile = io.BytesIO()
+    def send_response(self, status):
+        self.status = status
+    def send_header(self, *args):
+        pass
+    def end_headers(self):
+        pass
+
+
+def test_stale_post_rejected_before_workspace_or_worker_mutation(lineage, monkeypatch):
+    _, session = lineage
+    monkeypatch.setattr(routes, "_agent_runtime_barrier_response", lambda **kw: None)
+    monkeypatch.setattr(routes, "_get_or_materialize_session", lambda *a, **kw: session)
+    monkeypatch.setattr(routes, "_get_active_profile_name", lambda: "default")
+    monkeypatch.setattr(routes, "_session_visible_to_active_profile", lambda *a: True)
+    monkeypatch.setattr(routes, "_resolve_chat_workspace_with_recovery", lambda *a: pytest.fail("stale turn must not reach workspace mutation"))
+    h = Handler()
+    routes._handle_chat_start(h, {"session_id": session.session_id, "message": "conclusion?"})
+    assert h.status == 409
+    payload = json.loads(h.wfile.getvalue())
+    assert payload["code"] == "session_rotated"
+    assert payload["continuation_session_id"] == "idlechild"
+
+
+def test_idle_tip_accepts_normal_persistence_without_reopening_parent(lineage):
+    db, session = lineage
+    tip = routes._pre_compression_continuation_session_id(session)
+    db.append_message(tip, "user", "resume task")
+    assert db.get_session("sealedparent")["end_reason"] == "compression"
+    assert db.get_messages(tip)[-1]["content"] == "resume task"
+
+
+@pytest.mark.parametrize("reason", ["reset", "user_closed", "compression"])
+def test_unknown_or_sealed_terminal_tip_is_not_redirected(lineage, reason):
+    db, session = lineage
+    db._conn.execute("UPDATE sessions SET end_reason=? WHERE id='idlechild'", (reason,))
+    db._conn.commit()
+    assert routes._pre_compression_continuation_session_id(session) is None
+
+
+def test_newer_branch_does_not_replace_canonical_continuation(lineage):
+    db, session = lineage
+    db.create_session("branchchild", source="webui", parent_session_id="sealedparent")
+    db._conn.execute("UPDATE sessions SET message_count=20, model_config=? WHERE id='branchchild'", (json.dumps({'_branched_from': 'sealedparent'}),))
+    db._conn.commit()
+    assert routes._pre_compression_continuation_session_id(session) == "idlechild"
+
+
+def test_foreign_profile_and_subagent_are_not_redirected(lineage):
+    db, session = lineage
+    db._conn.execute("UPDATE sessions SET profile_name='foreign' WHERE id='idlechild'")
+    db._conn.commit()
+    assert routes._pre_compression_continuation_session_id(session) is None
+    db._conn.execute("UPDATE sessions SET profile_name=NULL, source='subagent' WHERE id='idlechild'")
+    db._conn.commit()
+    assert routes._pre_compression_continuation_session_id(session) is None
+
+
+def test_noncompressed_parent_and_absent_database_keep_legacy_behavior(lineage, monkeypatch, tmp_path):
+    db, session = lineage
+    db._conn.execute("UPDATE sessions SET end_reason='idle_timeout' WHERE id='sealedparent'")
+    db._conn.commit()
+    assert routes._pre_compression_continuation_session_id(session) is None
+    monkeypatch.setattr(profiles, "_resolve_profile_home_for_name", lambda name: str(tmp_path / 'missing'))
+    assert routes._pre_compression_continuation_session_id(session) is None
+    assert not (tmp_path / 'missing').exists()
+
+
+def test_browser_rotation_restores_draft_without_reposting():
+    source = (Path(__file__).parents[1] / 'static/messages.js').read_text()
+    helper = source[source.index('async function _recoverCompressedSend('):source.index('function _restoreComposerDraftAfterFailedSend(')]
+    script = r'''
+const assert = require('node:assert/strict');
+let S={session:{session_id:'old'}}, INFLIGHT={old:{}};
+const calls=[];
+const stopApprovalPolling=()=>{},stopClarifyPolling=()=>{},removeThinking=()=>{},setBusy=()=>{},setComposerStatus=()=>{},showToast=()=>{};
+const loadSession=async sid=>{calls.push(['load',sid]);S.session={session_id:sid}};
+const _restoreComposerDraftAfterFailedSend=(...args)=>calls.push(['restore',...args]);
+const api=()=>{throw Error('must not repost')};
+(async()=>{
+const err={status:409,body:JSON.stringify({code:'session_rotated',continuation_session_id:'new'})};
+const files=[{name:'drawing.png'}], promise=Promise.resolve();
+assert.equal(await _recoverCompressedSend(err,'old','conclusion?',files,promise),true);
+assert.deepEqual(calls[0],['load','new']);
+assert.deepEqual(calls[1],['restore','conclusion?',files,'new',promise]);
+assert.equal(INFLIGHT.old,undefined);
+assert.equal(await _recoverCompressedSend({status:500},'new','x',[],promise),false);
+assert.equal(await _recoverCompressedSend(err,'old','x',[],promise),false);
+assert.equal(calls.length,2);
+})().catch(e=>{console.error(e);process.exit(1)});
+'''
+    subprocess.run(['node', '-e', helper + script], check=True, timeout=15)