diff --git a/api/routes.py b/api/routes.py
index 85a1fd38548..842ad3efb8a 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -22178,6 +22178,12 @@ def start_session_turn(
     PR #2279 next-turn drain delivers the wakeup when the active turn ends.
     """
     msg = str(message or "").strip()
+    if _is_silent_control_message(msg):
+        return {
+            "status": "suppressed",
+            "reason": "silent_control_message",
+            "_status": 200,
+        }
     if not msg:
         return {"error": "message is required", "_status": 400}
     stale_response = _agent_runtime_barrier_response(runner_local_owned=True)
@@ -22674,6 +22680,18 @@ def _legacy_goal_update(session_id: str, _action: str, text: str) -> dict:
     return j(handler, payload)
 
 
+def _is_silent_control_message(message) -> bool:
+    """Return True only for the scheduler's exact suppression sentinel.
+
+    ``[SILENT]`` is control-plane output, never conversation content. If a wake
+    relay POSTs it and 8701 restarts while the turn is pending, recovery
+    materializes it as a visible ``_recovered`` user message. Suppress it before
+    session lookup or pending-state mutation. Matching stays exact and
+    case-sensitive so ordinary user text is unaffected.
+    """
+    return str(message or "").strip() == "[SILENT]"
+
+
 def _handle_chat_start(handler, body, diag=None):
     try:
         diag.stage("validate_session_id") if diag else None
@@ -22681,6 +22699,12 @@ def _handle_chat_start(handler, body, diag=None):
             require(body, "session_id")
         except ValueError as e:
             return bad(handler, str(e))
+        if _is_silent_control_message(body.get("message")):
+            return j(
+                handler,
+                {"status": "suppressed", "reason": "silent_control_message"},
+                status=200,
+            )
         # Reject a stale local Agent runtime before materialising, claiming, or
         # mutating any session state. Gateway-backed turns run in the gateway's
         # process and do not depend on this WebUI process's imported checkout.
diff --git a/tests/test_silent_control_suppression.py b/tests/test_silent_control_suppression.py
new file mode 100644
index 00000000000..b5a2c535c46
--- /dev/null
+++ b/tests/test_silent_control_suppression.py
@@ -0,0 +1,95 @@
+"""Reserved ``[SILENT]`` must never become a WebUI conversation turn.
+
+Cron agents use the exact final response ``[SILENT]`` as a delivery-suppression
+sentinel. If an external wake relay accidentally POSTs that sentinel to
+``/api/chat/start`` and 8701 restarts while the turn is pending, session repair
+materializes it as ``{"role": "user", "_recovered": True}``. That creates a
+visible user turn and can repeat on every restart.
+
+Both server-side entry points therefore treat the exact normalized sentinel as
+a successful no-op *before* session lookup or pending-state mutation.
+"""
+from __future__ import annotations
+
+import io
+import json
+
+from api import routes
+
+
+class _JSONHandler:
+    headers = {}
+
+    def __init__(self):
+        self.status = None
+        self.wfile = io.BytesIO()
+        self.headers_sent = {}
+
+    def send_response(self, status):
+        self.status = status
+
+    def send_header(self, key, value):
+        self.headers_sent[key] = value
+
+    def end_headers(self):
+        pass
+
+
+def _payload(handler):
+    raw = handler.wfile.getvalue().decode("utf-8")
+    return json.loads(raw) if raw else {}
+
+
+def test_http_chat_start_suppresses_silent_before_session_lookup(monkeypatch):
+    looked_up = []
+
+    def _unexpected_lookup(*args, **kwargs):
+        looked_up.append((args, kwargs))
+        raise AssertionError("[SILENT] must be suppressed before session lookup")
+
+    monkeypatch.setattr(routes, "_get_or_materialize_session", _unexpected_lookup)
+    handler = _JSONHandler()
+
+    routes._handle_chat_start(
+        handler,
+        {"session_id": "does-not-need-to-exist", "message": "  [SILENT]\n"},
+    )
+
+    assert handler.status == 200
+    assert _payload(handler) == {
+        "status": "suppressed",
+        "reason": "silent_control_message",
+    }
+    assert looked_up == []
+
+
+def test_server_side_start_suppresses_silent_before_session_lookup(monkeypatch):
+    looked_up = []
+
+    def _unexpected_lookup(*args, **kwargs):
+        looked_up.append((args, kwargs))
+        raise AssertionError("[SILENT] must be suppressed before session lookup")
+
+    monkeypatch.setattr(routes, "get_session", _unexpected_lookup)
+
+    result = routes.start_session_turn(
+        "does-not-need-to-exist",
+        "\t[SILENT] ",
+        source="process_wakeup",
+    )
+
+    assert result == {
+        "status": "suppressed",
+        "reason": "silent_control_message",
+        "_status": 200,
+    }
+    assert looked_up == []
+
+
+def test_silent_suppression_is_exact_and_case_sensitive():
+    assert routes._is_silent_control_message("[SILENT]") is True
+    assert routes._is_silent_control_message("  [SILENT]\n") is True
+    assert routes._is_silent_control_message("[silent]") is False
+    assert routes._is_silent_control_message("prefix [SILENT]") is False
+    assert routes._is_silent_control_message("") is False
+    assert routes._is_silent_control_message(None) is False