diff --git a/backend/apps/agents/core/fault_injection.py b/backend/apps/agents/core/fault_injection.py
index c6a53176..a905f962 100644
--- a/backend/apps/agents/core/fault_injection.py
+++ b/backend/apps/agents/core/fault_injection.py
@@ -25,6 +25,7 @@ KNOWN_FAULTS: Set[str] = {
     "sidecar_wedge",     # a builtin tool never returns (ENG-368 heartbeat ceiling)
     "transport_death",   # the CLI's pipe dies, not the provider (ENG-382 respawn-not-rebuild)
     "empty_finish",      # a turn ends with no answer after tool work (ENG-354, ENG-390)
+    "dead_lane",         # the router has already given up on the credential (ENG-414 preflight)
 }
 
 
diff --git a/backend/apps/agents/manager/run/lane_preflight.py b/backend/apps/agents/manager/run/lane_preflight.py
index ddeb6a6e..e54c6761 100644
--- a/backend/apps/agents/manager/run/lane_preflight.py
+++ b/backend/apps/agents/manager/run/lane_preflight.py
@@ -90,8 +90,19 @@ def connection_is_dead(conn: Dict) -> bool:
     return conn.get("errorCode") in (401, 403)
 
 
+# The shape the router publishes for a credential it has given up on, and the shape a drill injects.
+# It is built to satisfy `connection_is_dead` above, and a test pins that, so a drill can never fire
+# a fault the guard ignores and still report a pass.
+def injected_dead_conn(provider: str) -> Dict:
+    return {"provider": provider, "testStatus": "unavailable", "errorCode": 401}
+
+
 async def dead_connection(provider: str) -> Optional[Dict]:
     """The provider's connection if the router considers it dead, else None. Never raises: a preflight that cannot read health must let the turn proceed, because guessing "dead" would ground a working lane."""
+    from backend.apps.agents.core.fault_injection import armed
+    if armed("dead_lane"):
+        logger.warning("[fault] dead_lane armed: reporting %s as a dead credential", provider)
+        return injected_dead_conn(provider)
     try:
         from backend.apps.nine_router import get_providers
         for conn in await get_providers():
diff --git a/backend/tests/test_lane_preflight_never_declares_death.py b/backend/tests/test_lane_preflight_never_declares_death.py
index cd9c20fe..ed0d551e 100644
--- a/backend/tests/test_lane_preflight_never_declares_death.py
+++ b/backend/tests/test_lane_preflight_never_declares_death.py
@@ -116,3 +116,32 @@ async def test_a_router_that_does_not_come_back_still_stops_the_turn(monkeypatch
 
 async def p_async(value):
     return value
+
+
+# ---------------------------------------------------- the drill seam itself (OSW_FAULT=dead_lane)
+
+def test_the_injected_fault_is_the_shape_the_REAL_classifier_catches():
+    """A drill that fires a fault the guard ignores would report a pass while proving nothing.
+    Same rule the fault_injection module already states for its other faults."""
+    conn = p_pf.injected_dead_conn("claude")
+    assert p_pf.connection_is_dead(conn) is True
+    assert conn["errorCode"] in (401, 403), "connection_is_dead requires auth evidence, not just unavailable"
+
+
+def test_the_fault_is_declared_so_a_typo_cannot_arm_nothing():
+    from backend.apps.agents.core.fault_injection import KNOWN_FAULTS
+    assert "dead_lane" in KNOWN_FAULTS
+
+
+@pytest.mark.asyncio
+async def test_the_fault_is_inert_unless_armed(monkeypatch):
+    monkeypatch.delenv("OSW_FAULT", raising=False)
+    called = {"n": 0}
+
+    async def p_real(provider):
+        called["n"] += 1
+        return None
+
+    monkeypatch.setattr(p_pf, "dead_connection", p_real)
+    await p_pf.preflight_lane("cc/claude-opus-5", p_session())
+    assert called["n"] == 1, "unarmed, the real health read must still happen"