diff --git a/tests/test_rlf_env_lease.py b/tests/test_rlf_env_lease.py
index 29571911..58ae9009 100644
--- a/tests/test_rlf_env_lease.py
+++ b/tests/test_rlf_env_lease.py
@@ -416,3 +416,34 @@ def test_finish_trusted_tb2_fallback_byte_unchanged():
                                              task_id="tb2-task", sample_id="s-1", key=HMAC_KEY))
     assert md_timeout[tbench_outcome.OUTCOME_KEY]["verifier"] == tbench_outcome.TIMEOUT_VERIFIER
     assert not any(k.startswith("rlf_") for k in md_timeout)
+
+
+def test_acquire_vm_idle_timeout_follows_ttl(monkeypatch):
+    """Sandbox idle kill must not reclaim a VM mid-generation (S21 gate-5)."""
+    import sys
+    created: dict[str, Any] = {}
+
+    class _Sb:
+        object_id = "sb-x"
+
+        def poll(self):
+            return None
+
+        def terminate(self):
+            pass
+
+    fake_modal = SimpleNamespace(
+        App=SimpleNamespace(lookup=lambda *a, **k: object()),
+        Image=SimpleNamespace(from_registry=lambda *a, **k: object()),
+        Sandbox=SimpleNamespace(create=lambda *a, **k: (created.update(k) or _Sb())),
+    )
+    monkeypatch.setitem(sys.modules, "modal", fake_modal)
+
+    rlf_modal.acquire_vm("t", "ep")
+    assert created["timeout"] == 1800
+    assert created["idle_timeout"] == 1800  # not the old min(ttl, 300)
+
+    created.clear()
+    cfg = rlf_modal.RlfModalVmConfig(ttl_s=600, idle_timeout_s=900)
+    rlf_modal.acquire_vm("t", "ep", config=cfg)
+    assert created["timeout"] == 600 and created["idle_timeout"] == 900
diff --git a/yeto/launcher.py b/yeto/launcher.py
index 00ca4369..ddf73ed7 100644
--- a/yeto/launcher.py
+++ b/yeto/launcher.py
@@ -3491,7 +3491,7 @@ def checkout(
     "OPENENV_RUN_ID",
     "SECRLENV_MAX_TURNS",
 )
-HARNESS_PASSTHROUGH_ENV_PREFIXES = ("YETO_HARNESS_TB2_",)
+HARNESS_PASSTHROUGH_ENV_PREFIXES = ("YETO_HARNESS_TB2_", "YETO_RLF_")
 # tb2_provider's Modal Sandbox backend runs inside the island (rollout
 # workers create the task sandboxes), so the island python needs the Modal
 # client.  The ports image does not ship it; install it in setup without
@@ -3500,7 +3500,8 @@ def checkout(
 MODAL_SANDBOX_PROVIDER = "yeto.rl.harness.codex.tb2_provider:modal_provider"
 # rl-agentic-reward-env: same TB2 provider, sandboxes from prebaked images.
 PREBAKED_MODAL_SANDBOX_PROVIDER = "yeto.cloud.modal_reward_env:modal_provider"
-MODAL_SANDBOX_PROVIDERS = frozenset({MODAL_SANDBOX_PROVIDER, PREBAKED_MODAL_SANDBOX_PROVIDER})
+RLF_MODAL_SANDBOX_PROVIDER = "yeto.cloud.modal_reward_env:rlf_modal_provider"
+MODAL_SANDBOX_PROVIDERS = frozenset({MODAL_SANDBOX_PROVIDER, PREBAKED_MODAL_SANDBOX_PROVIDER, RLF_MODAL_SANDBOX_PROVIDER})
 SANDBOX_MODAL_TOKEN_ENVS = ("YETO_SANDBOX_MODAL_TOKEN_ID", "YETO_SANDBOX_MODAL_TOKEN_SECRET")
 
 
diff --git a/yeto/rl/rlf/modal.py b/yeto/rl/rlf/modal.py
index 84a35f8f..07124bb3 100644
--- a/yeto/rl/rlf/modal.py
+++ b/yeto/rl/rlf/modal.py
@@ -55,6 +55,11 @@ class RlfModalVmConfig:
     cpu: float = 2.0
     memory_mb: int = 4096
     ttl_s: int = 1800
+    # Idle kill must not fire below ttl_s: agent turns can spend minutes in model
+    # generation with no exec running; a shorter idle timeout reclaims the VM
+    # mid-episode (Modal NotFoundError) and the fail-closed preflight then drops
+    # the whole group. Default: follow ttl_s (hard cap still bounds leaks).
+    idle_timeout_s: int | None = None
     image: str = 'debian:bookworm-slim'
     runtime: str = 'vm'
     image_run_commands: tuple = ()
@@ -106,8 +111,9 @@ def acquire_vm(task_id: str, episode_id: str, *, config: RlfModalVmConfig | None
         image = modal.Image.from_registry(cfg.image)
         if cfg.image_run_commands:
             image = image.run_commands(*cfg.image_run_commands)
+    idle_timeout_s = cfg.idle_timeout_s if cfg.idle_timeout_s is not None else cfg.ttl_s
     sb = modal.Sandbox.create('sleep', 'infinity', app=app, image=image, runtime='vm', cpu=cfg.cpu,
-                              memory=cfg.memory_mb, timeout=cfg.ttl_s, idle_timeout=min(cfg.ttl_s, 300),
+                              memory=cfg.memory_mb, timeout=cfg.ttl_s, idle_timeout=idle_timeout_s,
                               tags={'yeto-purpose': 'rlf-environment', 'yeto-task': task_id[:63],
                                     'yeto-episode': episode_id[:63]})
     return RlfVmLease(sb, task_id=task_id, episode_id=episode_id, trajectory_id=trajectory_id)