diff --git a/tests/tui_gateway/test_delegation_session_lifecycle.py b/tests/tui_gateway/test_delegation_session_lifecycle.py
new file mode 100644
index 00000000000..57a96469e23
--- /dev/null
+++ b/tests/tui_gateway/test_delegation_session_lifecycle.py
@@ -0,0 +1,158 @@
+"""Fail-closed ownership + session-scoped delegation lifecycle (#55578).
+
+Covers the two hardening rules layered on top of the origin-routing salvage:
+
+1. ``_session_owns_notification_event`` — positive-proof ownership. An
+   async-delegation completion may only be injected into a session that
+   PROVABLY commissioned it (origin UI id, or session-key/lineage match).
+   Orphans are never adopted by a foreign chat.
+
+2. ``interrupt_for_session`` — a session's in-flight async delegations end
+   with the session. ``_finalize_session`` interrupts delegations owned by
+   the closing session (by origin UI id always; by durable key only when the
+   TUI owns the lifecycle).
+"""
+
+import threading
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+import tools.async_delegation as ad
+from tui_gateway.server import (
+    _finalize_session,
+    _session_owns_notification_event,
+)
+
+
+@pytest.fixture(autouse=True)
+def _reset_async_delegation():
+    ad._reset_for_tests()
+    yield
+    ad._reset_for_tests()
+
+
+class TestSessionOwnsNotificationEvent:
+    def _session(self, key="sess_key_1"):
+        return {"session_key": key, "_finalized": False}
+
+    def test_origin_ui_match_owns(self):
+        evt = {"type": "async_delegation", "origin_ui_session_id": "tab1", "session_key": "other"}
+        assert _session_owns_notification_event("tab1", self._session(), evt) is True
+
+    def test_session_key_match_owns(self):
+        evt = {"type": "async_delegation", "origin_ui_session_id": "", "session_key": "sess_key_1"}
+        assert _session_owns_notification_event("tabX", self._session("sess_key_1"), evt) is True
+
+    def test_orphan_is_not_owned(self):
+        """No origin match, no key match, owner gone → NOT ours (fail closed)."""
+        evt = {"type": "async_delegation", "origin_ui_session_id": "dead_tab", "session_key": "gone_key"}
+        assert _session_owns_notification_event("tab1", self._session(), evt) is False
+
+    def test_empty_key_and_origin_not_owned(self):
+        """A delegation event with no return address at all is never adopted."""
+        evt = {"type": "async_delegation", "origin_ui_session_id": "", "session_key": ""}
+        assert _session_owns_notification_event("tab1", self._session(), evt) is False
+
+    def test_finalized_session_owns_nothing(self):
+        evt = {"type": "async_delegation", "origin_ui_session_id": "tab1", "session_key": "sess_key_1"}
+        sess = self._session()
+        sess["_finalized"] = True
+        assert _session_owns_notification_event("tab1", sess, evt) is False
+
+    def test_compression_chain_resolution_owns(self):
+        evt = {"type": "async_delegation", "origin_ui_session_id": "", "session_key": "parent_key"}
+        db = MagicMock()
+        db.resolve_resume_session_id.return_value = "child_key"
+        with patch("tui_gateway.server._get_db", return_value=db):
+            assert _session_owns_notification_event("tabX", self._session("child_key"), evt) is True
+
+
+class TestInterruptForSession:
+    def _seed_record(self, delegation_id, session_key="", origin_ui_session_id="", status="running"):
+        fn = MagicMock()
+        with ad._records_lock:
+            ad._records[delegation_id] = {
+                "delegation_id": delegation_id,
+                "status": status,
+                "session_key": session_key,
+                "origin_ui_session_id": origin_ui_session_id,
+                "interrupt_fn": fn,
+            }
+        return fn
+
+    def test_interrupts_only_matching_session(self):
+        mine = self._seed_record("d1", session_key="sess_A")
+        other = self._seed_record("d2", session_key="sess_B")
+        n = ad.interrupt_for_session(session_key="sess_A")
+        assert n == 1
+        mine.assert_called_once()
+        other.assert_not_called()
+
+    def test_matches_by_origin_ui_session_id(self):
+        mine = self._seed_record("d1", origin_ui_session_id="tab1")
+        other = self._seed_record("d2", origin_ui_session_id="tab2")
+        n = ad.interrupt_for_session(origin_ui_session_id="tab1")
+        assert n == 1
+        mine.assert_called_once()
+        other.assert_not_called()
+
+    def test_no_selector_is_noop(self):
+        fn = self._seed_record("d1", session_key="sess_A")
+        assert ad.interrupt_for_session() == 0
+        fn.assert_not_called()
+
+    def test_completed_records_untouched(self):
+        fn = self._seed_record("d1", session_key="sess_A", status="completed")
+        assert ad.interrupt_for_session(session_key="sess_A") == 0
+        fn.assert_not_called()
+
+
+class TestFinalizeInterruptsOwnDelegations:
+    def _make_session(self, session_key="sess_A", sid="tab1"):
+        agent = MagicMock()
+        agent.session_id = session_key
+        agent._session_messages = None
+        agent.model = "m"
+        agent.platform = "tui"
+        return {
+            "agent": agent,
+            "history": [{"role": "user", "content": "x"}],
+            "history_lock": threading.Lock(),
+            "session_key": session_key,
+            "_finalized": False,
+            "_sid": sid,
+        }
+
+    @patch("tui_gateway.server._get_db")
+    def test_finalize_interrupts_sessions_delegations(self, mock_get_db):
+        mock_db = MagicMock()
+        mock_db.get_session.return_value = {"source": "tui"}
+        mock_get_db.return_value = mock_db
+
+        with patch("tools.async_delegation.interrupt_for_session") as mock_int:
+            _finalize_session(self._make_session(), end_reason="tui_close")
+
+        mock_int.assert_called_once()
+        kwargs = mock_int.call_args.kwargs
+        assert kwargs["session_key"] == "sess_A"
+        assert kwargs["origin_ui_session_id"] == "tab1"
+
+    @patch("tui_gateway.server._get_db")
+    def test_viewer_of_gateway_session_only_interrupts_by_origin(self, mock_get_db):
+        """Closing a TUI viewer tab on a live gateway session must not kill
+        the gateway's own background work — key-based interrupt is skipped,
+        origin-id interrupt (this tab's own dispatches) still applies."""
+        mock_db = MagicMock()
+        mock_db.get_session.return_value = {"source": "telegram"}
+        mock_get_db.return_value = mock_db
+
+        with patch("tools.async_delegation.interrupt_for_session") as mock_int:
+            _finalize_session(
+                self._make_session(session_key="agent:main:telegram:dm:123", sid="tab9"),
+                end_reason="ws_orphan_reap",
+            )
+
+        kwargs = mock_int.call_args.kwargs
+        assert kwargs["session_key"] == ""
+        assert kwargs["origin_ui_session_id"] == "tab9"
diff --git a/tools/async_delegation.py b/tools/async_delegation.py
index 710ab5c1d3b..cc22a7f6e9a 100644
--- a/tools/async_delegation.py
+++ b/tools/async_delegation.py
@@ -525,6 +525,54 @@ def interrupt_all(reason: str = "shutdown") -> int:
     return count
 
 
+def interrupt_for_session(
+    session_key: str = "",
+    origin_ui_session_id: str = "",
+    reason: str = "session_end",
+) -> int:
+    """Signal running async delegations owned by ONE session to stop.
+
+    A delegation's lifecycle is bound to the session that spawned it: when
+    that session ends, its in-flight background subagents must end with it —
+    a completed orphan would otherwise sit on the shared completion queue
+    with no live owner, either leaking into another chat or burning tokens
+    with no one listening (#55578).
+
+    Matches on ``origin_ui_session_id`` (the live UI session that
+    commissioned the work) and/or the durable ``session_key``; either
+    matching field claims the record. Returns how many were interrupted.
+    """
+    if not session_key and not origin_ui_session_id:
+        return 0
+    count = 0
+    with _records_lock:
+        targets = [
+            r for r in _records.values()
+            if r.get("status") == "running"
+            and (
+                (origin_ui_session_id and str(r.get("origin_ui_session_id") or "") == origin_ui_session_id)
+                or (session_key and str(r.get("session_key") or "") == session_key)
+            )
+        ]
+    for r in targets:
+        fn = r.get("interrupt_fn")
+        if callable(fn):
+            try:
+                fn()
+                count += 1
+            except Exception as exc:
+                logger.debug(
+                    "interrupt_for_session: %s interrupt failed: %s",
+                    r.get("delegation_id"), exc,
+                )
+    if count:
+        logger.info(
+            "Interrupted %d async delegation(s) for ending session (%s)",
+            count, reason,
+        )
+    return count
+
+
 def _reset_for_tests() -> None:
     """Test-only: clear all state and tear down the executor."""
     global _executor, _executor_max_workers
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index 29c7dd42d20..80fa9cea95e 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -625,6 +625,7 @@ def _finalize_session(session: dict | None, end_reason: str = "tui_close") -> No
     # Use session_id (from agent.session_id) not session_key — after compression,
     # session_key may be stale (the ended parent) while session_id is the live
     # continuation. Fix for #20001.
+    _tui_owns_lifecycle = True
     if session_id:
         try:
             db = _get_db()
@@ -638,11 +639,40 @@ def _finalize_session(session: dict | None, end_reason: str = "tui_close") -> No
                 # repeats on every inbound message.  (#60609)
                 row = db.get_session(session_id)
                 source = (row or {}).get("source", "")
-                if not _is_gateway_owned_source(source):
+                _tui_owns_lifecycle = not _is_gateway_owned_source(source)
+                if _tui_owns_lifecycle:
                     db.end_session(session_id, end_reason)
         except Exception:
             pass
 
+    # A session's in-flight async delegations end WITH the session (#55578):
+    # once nobody owns the return address, a still-running background subagent
+    # can only burn tokens and park an orphaned completion on the shared
+    # queue. Always interrupt delegations commissioned by THIS live UI session
+    # (its sid); additionally interrupt by durable session_key, but only when
+    # the TUI owns the lifecycle — closing a viewer tab on a live gateway
+    # session must not kill the gateway's own background work.
+    try:
+        from tools.async_delegation import interrupt_for_session
+
+        _own_sid = str(session.get("_sid") or "")
+        if not _own_sid:
+            try:
+                with _sessions_lock:
+                    for _cand_sid, _cand in _sessions.items():
+                        if _cand is session:
+                            _own_sid = _cand_sid
+                            break
+            except Exception:
+                _own_sid = ""
+        interrupt_for_session(
+            session_key=str(session_key or "") if _tui_owns_lifecycle else "",
+            origin_ui_session_id=_own_sid,
+            reason=end_reason,
+        )
+    except Exception:
+        pass
+
     # Close the slash-worker subprocess as part of finalize itself, not just
     # in the callers. Defense-in-depth: every session-end path goes through
     # _finalize_session (it's the single ``_finalized``-guarded chokepoint), so
@@ -712,6 +742,10 @@ def _close_session_by_id(sid: str, *, end_reason: str = "tui_close") -> bool:
         session = _sessions.pop(sid, None)
     if session is None:
         return False
+    # The session is already out of _sessions here, so downstream teardown
+    # (e.g. _finalize_session's per-session async-delegation interrupt) can't
+    # recover its live id by scanning the dict — stamp it on the record.
+    session["_sid"] = sid
     _teardown_session(session, end_reason=end_reason)
     return True
 
@@ -8550,6 +8584,40 @@ def _notification_event_belongs_elsewhere(sid: str, session: dict, evt: dict) ->
     )
 
 
+def _session_owns_notification_event(sid: str, session: dict, evt: dict) -> bool:
+    """True iff *this* session PROVABLY owns ``evt``.
+
+    Positive ownership — the mirror of ``_notification_event_belongs_elsewhere``
+    minus its orphan-adoption fallback. An event owns-matches when its
+    ``origin_ui_session_id`` is this live session, or its ``session_key``
+    (raw or resolved through the compression chain) matches this session's
+    key/lineage. Used as a fail-closed gate for async-delegation payloads:
+    "not provably elsewhere" is NOT good enough to inject a conversation
+    payload into this chat (#55578).
+    """
+    if session.get("_finalized"):
+        return False
+    if str(evt.get("origin_ui_session_id") or "") == str(sid or ""):
+        return True
+    evt_key = str(evt.get("session_key") or "")
+    if not evt_key:
+        return False
+    current_keys = {
+        str(session.get("session_key") or ""),
+        _session_lookup_key(session, fallback=sid),
+    }
+    if evt_key in current_keys:
+        return True
+    try:
+        db = _get_db()
+        resolved_key = (
+            db.resolve_resume_session_id(evt_key) if db is not None else evt_key
+        ) or evt_key
+    except Exception:
+        resolved_key = evt_key
+    return resolved_key in current_keys
+
+
 def _notification_event_dedup_key(evt: dict) -> tuple:
     """Return the UI-emission identity for a process notification event.
 
@@ -8620,6 +8688,32 @@ def _notification_poller_loop(
             time.sleep(0.1)
             continue
 
+        # Fail closed for async-delegation results (#55578): these carry a
+        # conversation payload, and injecting one into any chat other than the
+        # one that commissioned it is a hard cross-session leak. The
+        # belongs-elsewhere check above already re-queued events owned by
+        # another LIVE session; what reaches here is either ours or an
+        # orphan whose owner is gone. Orphaned delegation payloads are
+        # DROPPED, not adopted — the subagent's summary is already persisted
+        # in the delegation records/output store, so nothing is lost, whereas
+        # a wrong-chat injection is unrecoverable. Non-delegation events
+        # (background process completions etc.) keep the historical
+        # adopt-orphans behavior.
+        if evt.get("type") == "async_delegation" and not _session_owns_notification_event(
+            sid, session, evt
+        ):
+            logger.warning(
+                "async-delegation completion %s has no live owner "
+                "(origin=%r key=%r); dropping from injection instead of "
+                "delivering to session %s (#55578 fail-closed; result "
+                "remains in the delegation records)",
+                evt.get("delegation_id", "?"),
+                str(evt.get("origin_ui_session_id") or ""),
+                str(evt.get("session_key") or ""),
+                sid,
+            )
+            continue
+
         _evt_sid = evt.get("session_id", "")
         if evt.get("type") == "completion" and process_registry.is_completion_consumed(_evt_sid):
             continue
@@ -8676,6 +8770,15 @@ def _notification_poller_loop(
         if _notification_event_belongs_elsewhere(sid, session, evt):
             deferred.append(evt)
             continue
+        # Same fail-closed rule as the live loop: an orphaned async-delegation
+        # payload is never adopted by a foreign session — defer it (a later
+        # resume of the owner's lineage can still claim it) rather than
+        # injecting another chat's conversation here (#55578).
+        if evt.get("type") == "async_delegation" and not _session_owns_notification_event(
+            sid, session, evt
+        ):
+            deferred.append(evt)
+            continue
         _evt_sid = evt.get("session_id", "")
         if evt.get("type") == "completion" and process_registry.is_completion_consumed(_evt_sid):
             continue