diff --git a/CHANGELOG.md b/CHANGELOG.md
index 24ba24cb..3cb98f4e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,12 @@
 
 - **Windows pytest-harness compatibility (#3664).** Hardened the test suite to run on Windows: profile-home fallback paths are path-normalized, strict POSIX file-mode (`0o600`) assertions are gated behind `os.name != "nt"` (Linux still asserts them at full strictness), the conftest cleanup handles Windows process-tree/port teardown and the Py3.12+ `shutil.rmtree` `onexc` shim, and tests that require `fork`/`fcntl` carry `@requires_fork` / `@requires_fcntl` markers (which never skip on Linux). Test-only — no runtime or app behavior change, no Linux CI behavior change. (#4254, #4255, #4256, #4257, #4259, #4263, #4266, #4274)
 
+## [v0.51.455] — 2026-06-16 — Release PP (drop stale background-process completions instead of prepending them to a later turn)
+
+### Fixed
+
+- **A background task that finishes long after you've moved on no longer hijacks your next message.** When a `notify_on_complete` background task's completion fired after the user had left the session idle (or it sat queued), the WebUI prepended its `[IMPORTANT: … completed]` notification to whatever the user typed next — so the agent answered the *old* task and already-delivered completion notices reappeared on later turns. The notification drain now age-gates completions: one whose enqueue time is older than a configurable cap (`HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS`, default 6h; set `0` to disable) is silently consumed instead of delivered. Completions without a timestamp (older agent builds) are never dropped, so behavior is unchanged until the agent side stamps `completed_at`. (#4306, #4029)
+
 ## [v0.51.454] — 2026-06-16 — Release PO (complete the Japanese UI translation)
 
 ### Changed
diff --git a/api/streaming.py b/api/streaming.py
index 6dcd8819..44c1b548 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -1536,6 +1536,31 @@ def _bind_turn_session_identity(session_id: str):
         _reset_turn_session_identity(tokens)
 
 
+def _stale_completion_max_age_seconds() -> float:
+    """Max age (seconds) a background-process completion may sit in the queue
+    before the WebUI drain treats it as stale and drops it instead of
+    prepending it to the user's next turn.
+
+    Completions older than this are silently consumed (not requeued) so a
+    notification that finally fires long after the user moved on cannot
+    contaminate an unrelated later turn. See nesquena/hermes-webui#4029.
+
+    Configurable via HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS. A value of
+    0 (or negative) disables age-gating and restores the legacy drain-all
+    behavior. Defaults to 6 hours.
+    """
+    raw = os.environ.get("HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS")
+    if raw is not None:
+        try:
+            return float(raw)
+        except (TypeError, ValueError):
+            logger.warning(
+                "Invalid HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS=%r; using default",
+                raw,
+            )
+    return 6 * 60 * 60  # 6 hours
+
+
 def _format_process_notification(evt: dict) -> str:
     """Format a completed background process notification for agent input."""
     if not isinstance(evt, dict):
@@ -1584,6 +1609,10 @@ def _drain_webui_process_notifications(session_id: str) -> list[str]:
     if completion_queue is None:
         return []
 
+    # Computed once per drain (not per event): reads/validates the env cap a
+    # single time so an invalid value logs at most one warning per drain.
+    stale_completion_max_age = _stale_completion_max_age_seconds()
+
     while True:
         try:
             evt = completion_queue.get_nowait()
@@ -1607,6 +1636,25 @@ def _drain_webui_process_notifications(session_id: str) -> list[str]:
             skipped_events.append(evt)
             continue
 
+        # Age-gate stale completions: a completion that fires long after the
+        # user moved on must not be prepended to an unrelated later turn
+        # (nesquena/hermes-webui#4029). Drop (consume, do not requeue) any
+        # completion whose enqueue time is older than the configured cap.
+        # Events without a 'completed_at' (older agent builds) are never
+        # dropped here, preserving backward-compatible behavior.
+        if stale_completion_max_age > 0 and isinstance(evt, dict):
+            completed_at = evt.get('completed_at')
+            if isinstance(completed_at, (int, float)) and completed_at > 0:
+                age = time.time() - completed_at
+                if age > stale_completion_max_age:
+                    logger.info(
+                        "Dropping stale background-process completion for "
+                        "session %s (age %.0fs > cap %.0fs)",
+                        evt_sid, age, stale_completion_max_age,
+                    )
+                    _mark_process_completion_consumed(process_registry, evt_sid)
+                    continue
+
         notification = _format_process_notification(evt)
         if notification:
             notifications.append(notification)
diff --git a/tests/test_issue4029_stale_completion_age_gate.py b/tests/test_issue4029_stale_completion_age_gate.py
new file mode 100644
index 00000000..62f13a1c
--- /dev/null
+++ b/tests/test_issue4029_stale_completion_age_gate.py
@@ -0,0 +1,125 @@
+"""Behavioral tests for issue #4029 stale-completion age gate.
+
+These exercise the real `_drain_webui_process_notifications` against a fake
+process_registry, proving that:
+  - a completion older than the cap is dropped (consumed, not requeued),
+  - a fresh completion is still delivered,
+  - events without `completed_at` are never dropped (backward compat),
+  - the env override (incl. disable via 0) is honored.
+"""
+import importlib
+import queue
+import sys
+import threading
+import time
+import types
+
+import pytest
+
+
+def _install_fake_registry(monkeypatch, events):
+    """Build a fake `tools.process_registry` module whose `process_registry`
+    exposes the surface `_drain_webui_process_notifications` touches."""
+    q = queue.Queue()
+    for e in events:
+        q.put(e)
+
+    class _FakeProc:
+        def __init__(self, session_key):
+            self.session_key = session_key
+
+    class _FakeRegistry:
+        def __init__(self):
+            self.completion_queue = q
+            self._lock = threading.RLock()
+            self._completion_consumed = set()
+            # map session_id -> session_key so ownership check passes
+            self._owner = {}
+
+        def is_completion_consumed(self, sid):
+            return sid in self._completion_consumed
+
+        def get(self, sid):
+            key = self._owner.get(sid)
+            return _FakeProc(key) if key is not None else None
+
+    reg = _FakeRegistry()
+    mod = types.ModuleType("tools.process_registry")
+    mod.process_registry = reg
+    # ensure parent package exists
+    if "tools" not in sys.modules:
+        pkg = types.ModuleType("tools")
+        pkg.__path__ = []
+        monkeypatch.setitem(sys.modules, "tools", pkg)
+    monkeypatch.setitem(sys.modules, "tools.process_registry", mod)
+    return reg
+
+
+def _make_event(sid, completed_at, session_key="websess-1"):
+    return {
+        "type": "completion",
+        "session_id": sid,
+        "session_key": session_key,
+        "command": f"cmd-{sid}",
+        "exit_code": 0,
+        "output": f"out-{sid}",
+        **({"completed_at": completed_at} if completed_at is not None else {}),
+    }
+
+
+@pytest.fixture
+def streaming():
+    return importlib.import_module("api.streaming")
+
+
+def test_stale_completion_is_dropped_and_fresh_is_delivered(streaming, monkeypatch):
+    now = time.time()
+    fresh = _make_event("fresh", now - 60)          # 1 min old -> keep
+    stale = _make_event("stale", now - 7 * 3600)    # 7 h old -> drop (cap 6h)
+    reg = _install_fake_registry(monkeypatch, [stale, fresh])
+    reg._owner["fresh"] = "websess-1"
+    reg._owner["stale"] = "websess-1"
+    # default cap = 6h
+    monkeypatch.delenv("HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS", raising=False)
+
+    out = streaming._drain_webui_process_notifications("websess-1")
+
+    joined = "\n".join(out)
+    assert "fresh" in joined, "fresh completion should be delivered"
+    assert "stale" not in joined, "stale completion must be dropped"
+    # stale was consumed (won't come back), fresh consumed after delivery
+    assert "stale" in reg._completion_consumed
+    assert "fresh" in reg._completion_consumed
+    # nothing belonging to this session is left requeued
+    assert reg.completion_queue.empty()
+
+
+def test_event_without_completed_at_is_never_dropped(streaming, monkeypatch):
+    legacy = _make_event("legacy", None)  # no completed_at -> backward compat keep
+    reg = _install_fake_registry(monkeypatch, [legacy])
+    reg._owner["legacy"] = "websess-1"
+    monkeypatch.delenv("HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS", raising=False)
+
+    out = streaming._drain_webui_process_notifications("websess-1")
+
+    assert any("legacy" in n for n in out), "legacy event (no timestamp) must be delivered"
+
+
+def test_env_zero_disables_age_gate(streaming, monkeypatch):
+    now = time.time()
+    ancient = _make_event("ancient", now - 10 * 24 * 3600)  # 10 days old
+    reg = _install_fake_registry(monkeypatch, [ancient])
+    reg._owner["ancient"] = "websess-1"
+    monkeypatch.setenv("HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS", "0")
+
+    out = streaming._drain_webui_process_notifications("websess-1")
+
+    assert any("ancient" in n for n in out), "age gate disabled -> even ancient delivered"
+
+
+def test_helper_reads_env_override(streaming, monkeypatch):
+    monkeypatch.setenv("HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS", "120")
+    assert streaming._stale_completion_max_age_seconds() == 120.0
+    monkeypatch.setenv("HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS", "not-a-number")
+    # invalid -> falls back to default 6h
+    assert streaming._stale_completion_max_age_seconds() == 6 * 60 * 60
diff --git a/tests/test_notify_on_complete_webui.py b/tests/test_notify_on_complete_webui.py
index 8b8e88b6..b74f45fd 100644
--- a/tests/test_notify_on_complete_webui.py
+++ b/tests/test_notify_on_complete_webui.py
@@ -28,3 +28,20 @@ def test_webui_sets_gateway_session_platform_for_background_watchers():
     assert "os.environ['HERMES_SESSION_PLATFORM'] = 'webui'" in src
     assert "old_session_platform = os.environ.get('HERMES_SESSION_PLATFORM')" in src
     assert "os.environ.pop('HERMES_SESSION_PLATFORM', None)" in src
+
+
+def test_webui_age_gates_stale_background_completion_events():
+    """Issue #4029: drain must drop completions older than the configured cap
+    so stale notifications can't be prepended to an unrelated later turn."""
+    src = Path("api/streaming.py").read_text(encoding="utf-8")
+
+    # The age-gate helper + its env override exist.
+    assert "def _stale_completion_max_age_seconds()" in src
+    assert "HERMES_WEBUI_STALE_COMPLETION_MAX_AGE_SECONDS" in src
+    # The drain reads completed_at and drops over-age events without requeueing.
+    assert "completed_at = evt.get('completed_at')" in src
+    assert "age = time.time() - completed_at" in src
+    assert "if age > stale_completion_max_age:" in src
+    # Over-age events are consumed (marked), not requeued, so they vanish.
+    assert "_mark_process_completion_consumed(process_registry, evt_sid)" in src
+