diff --git a/src/forge_loop/runner/tick_checks.py b/src/forge_loop/runner/tick_checks.py
index af8796b..37b4229 100644
--- a/src/forge_loop/runner/tick_checks.py
+++ b/src/forge_loop/runner/tick_checks.py
@@ -535,6 +535,38 @@ def _inflight_worktrees(cfg: Config) -> set[str]:
         return set()
 
 
+_SYSTEM_ROOTS = {"/", "/tmp", "/var/tmp", "/home"}
+
+
+def _clamp_overbroad_root(cfg: Config, root, tick: int):
+    """#451 — never honor a worktree_root that is a SYSTEM directory.
+
+    A live run configured ``worktree_root: /tmp``; the sweep then treated
+    every /tmp worktree as loop-owned and reaped two OPERATOR worktrees
+    (one seconds old, uncommitted work destroyed). Roots resolving to /,
+    /tmp, /var/tmp, /home or $HOME are clamped to the loop's own per-repo
+    namespace (``worktree_base(repo)``) and a typed event is emitted so
+    the operator sees the config smell."""
+    import os
+    from pathlib import Path as _P
+
+    from forge_loop.worker_worktree import worktree_base
+
+    resolved = str(_P(root).resolve())
+    home = os.path.expanduser("~")
+    if resolved in _SYSTEM_ROOTS or resolved == home:
+        clamped = worktree_base(cfg.repo)
+        append_event(
+            cfg.events_file,
+            "worktree_root_clamped",
+            tick=tick,
+            configured=str(root),
+            clamped_to=str(clamped),
+        )
+        return clamped
+    return root
+
+
 def run_worktree_sweep(
     cfg: Config,
     tick: int,
@@ -558,6 +590,7 @@ def run_worktree_sweep(
     root = getattr(cfg, "worktree_root", None)
     if not root:
         return None
+    root = _clamp_overbroad_root(cfg, root, tick)
     wts = worktrees if worktrees is not None else _list_worktrees(cfg.repo)
     live = (
         live_paths
diff --git a/tests/test_worktree_sweep_agent.py b/tests/test_worktree_sweep_agent.py
index 584fc24..32ec724 100644
--- a/tests/test_worktree_sweep_agent.py
+++ b/tests/test_worktree_sweep_agent.py
@@ -204,3 +204,35 @@ def test_e2e_reaps_intact_stale_agent_worktree(tmp_path: Path) -> None:
     assert not wt.exists()  # orphan gone
     assert (repo / ".git").exists()  # checkout intact
     assert (repo / "f.txt").read_text() == "x\n"
+
+
+def test_run_worktree_sweep_clamps_system_temp_root(tmp_path: Path) -> None:
+    """#451 — `worktree_root: /tmp` (a system temp dir) must NOT make every
+    /tmp worktree reapable. A live run reaped two OPERATOR worktrees
+    seconds after creation because the config declared all of /tmp as the
+    loop's namespace. Over-broad roots get clamped to the loop's own
+    per-repo base (worktree_base) and a clamp event is emitted."""
+    cfg = Config(
+        repo=tmp_path,
+        github_repo="o/r",
+        labels=Labels(),
+        briefs=Briefs(),
+        worktree_root=Path("/tmp"),  # the footgun config
+        maintenance_every_n_ticks=5,
+    )
+    removed: list[str] = []
+    report = tc.run_worktree_sweep(
+        cfg,
+        tick=5,
+        worktrees=[
+            "/tmp/wt-operator",            # operator worktree → MUST survive
+            "/tmp/clone-x",                # operator clone-ish dir → MUST survive
+        ],
+        live_paths=set(),
+        remove=lambda p: removed.append(p) or True,
+    )
+    assert removed == []
+    assert report is None or report.reaped == []
+    evts = [e for e in _read_events(cfg) if e["kind"] == "worktree_root_clamped"]
+    assert len(evts) == 1
+    assert "/tmp" in evts[0]["configured"]