diff --git a/tests/dfx/reliability/test_reliability_qwen3_omni.py b/tests/dfx/reliability/test_reliability_qwen3_omni.py
index 428323638..32c660954 100644
--- a/tests/dfx/reliability/test_reliability_qwen3_omni.py
+++ b/tests/dfx/reliability/test_reliability_qwen3_omni.py
@@ -315,11 +315,32 @@ def _assert_post_fault_health_terminal(host: str, port: int, *, scenario: str) -
     pytest.fail(f"[{scenario} health] no terminal post-fault health observed: {last_observation}")
 
 
+def _assert_server_alive_after_fault(host: str, port: int, *, scenario: str) -> None:
+    """#4285: a single-stage engine death must NOT take down the API server.
+
+    The process must stay up and keep answering /health (503 is acceptable —
+    errored but listening). A connection-refused means the server process
+    exited, which is the bug this fix closes.
+    """
+    deadline = time.monotonic() + 15.0
+    responded = False
+    while time.monotonic() < deadline:
+        try:
+            status, _ = get_health_raw(host, port, timeout_sec=5)
+            responded = True
+            assert status in (200, 503), f"[{scenario} alive] unexpected /health status={status}"
+        except Exception as exc:  # noqa: BLE001
+            if _looks_like_server_unreachable(exc):
+                pytest.fail(f"[{scenario} alive] server exited after single-stage OOM (#4285): {exc!r}")
+            raise
+        time.sleep(0.5)
+    assert responded, f"[{scenario} alive] /health never responded after fault"
+
+
 QWEN_PARAMS = create_reliability_omni_server_params(RELIABILITY_SCENARIOS, DEPLOY_CONFIGS_DIR)
 
 
 @pytest.mark.slow
-@pytest.mark.skip(reason="issue#4285")
 @hardware_test(res={"cuda": "H100"}, num_cards=2)
 @pytest.mark.parametrize("omni_server_function", QWEN_PARAMS, indirect=True)
 def test_reliability_fault_gpu_oom_error_contract_consistent_chat_speech(
@@ -386,9 +407,11 @@ def test_reliability_fault_gpu_oom_error_contract_consistent_chat_speech(
     assert "code" in chat_error, f"chat error lacks code field: {chat_error!r}"
     assert "code" in speech_error, f"speech error lacks code field: {speech_error!r}"
 
+    # #4285: a single dead stage must not take the whole API server down.
+    _assert_server_alive_after_fault(host, port, scenario="oom_contract")
+
 
 @pytest.mark.slow
-@pytest.mark.skip(reason="issue#4285")
 @hardware_test(res={"cuda": "H100"}, num_cards=2)
 @pytest.mark.parametrize("omni_server_function", QWEN_PARAMS, indirect=True)
 def test_reliability_fault_gpu_oom_chat_large_payload_failure(omni_server_function, openai_client_function) -> None:
@@ -431,7 +454,6 @@ def test_reliability_fault_gpu_oom_chat_large_payload_failure(omni_server_functi
 
 
 @pytest.mark.slow
-@pytest.mark.skip(reason="issue#4285")
 @hardware_test(res={"cuda": "H100"}, num_cards=2)
 @pytest.mark.parametrize("omni_server_function", QWEN_PARAMS, indirect=True)
 def test_reliability_fault_gpu_oom_concurrent_pressure_failure(omni_server_function, openai_client_function) -> None:
@@ -774,7 +796,11 @@ def test_reliability_fault_process_kill_tree_with_load_fast_fail_and_cleanup(
 
 
 @pytest.mark.slow
-@pytest.mark.skip(reason="issue#4285")
+@pytest.mark.skip(
+    reason="needs request-scoped OOM recovery: under transient pressure the stage "
+    "EngineCore still dies and is evicted (not respawned), so the engine does not "
+    "recover after the hog stops. Out of scope for the #4285 fault-isolation fix."
+)
 @hardware_test(res={"cuda": "H100"}, num_cards=2)
 @pytest.mark.parametrize("omni_server_function", QWEN_PARAMS, indirect=True)
 def test_reliability_fault_gpu_oom_state_converges_after_fault_removed(
diff --git a/tests/engine/test_cfg_companion_lifecycle.py b/tests/engine/test_cfg_companion_lifecycle.py
index 5cf2fd072..6b907c3d2 100644
--- a/tests/engine/test_cfg_companion_lifecycle.py
+++ b/tests/engine/test_cfg_companion_lifecycle.py
@@ -83,6 +83,11 @@ class _FakePool:
         self.stage_client = SimpleNamespace(requires_multimodal_data=False, custom_process_input_func=None)
         self.aborted: list[list[str]] = []
 
+    def live_replica_ids(self) -> list[int]:
+        # Single always-live replica: these tests cover CFG bundling, not
+        # replica death, and the forward path fast-fails on an empty pool.
+        return [0]
+
     async def abort_requests(self, request_ids):
         self.aborted.append(list(request_ids))
 
diff --git a/tests/engine/test_orchestrator.py b/tests/engine/test_orchestrator.py
index b6c4f8509..3d63943a7 100644
--- a/tests/engine/test_orchestrator.py
+++ b/tests/engine/test_orchestrator.py
@@ -74,8 +74,6 @@ async def test_engine_dead_broadcasts_fatal_to_rpc_waiters(monkeypatch: pytest.M
         rpc_async_queue=rpc_queue,
         stage_pools=[],
     )
-    orchestrator._fatal_error = "stage engine died"
-    orchestrator._fatal_error_stage_id = 2
 
     async def wait_for_requests() -> None:
         await asyncio.Event().wait()
@@ -91,8 +89,7 @@ async def test_engine_dead_broadcasts_fatal_to_rpc_waiters(monkeypatch: pytest.M
     fatal = rpc_queue.get_nowait()
     assert isinstance(fatal, ErrorMessage)
     assert fatal.fatal is True
-    assert fatal.error == "stage engine died"
-    assert fatal.stage_id == 2
+    assert "stage engine died" in fatal.error
 
 
 @dataclass
diff --git a/tests/engine/test_orchestrator_error_handling.py b/tests/engine/test_orchestrator_error_handling.py
index d30b1eee4..780c5a17b 100644
--- a/tests/engine/test_orchestrator_error_handling.py
+++ b/tests/engine/test_orchestrator_error_handling.py
@@ -3,7 +3,7 @@
 """Tests for error propagation paths within the Orchestrator.
 
 Covers:
-- EngineDeadError from an LLM stage poll → fatal error broadcast + shutdown
+- EngineDeadError from an LLM stage poll → fatal error broadcast + replica eviction
 - Diffusion stage error output (OmniRequestOutput.from_error) → routed correctly
 """
 
@@ -14,17 +14,30 @@ import queue
 import time
 from types import SimpleNamespace
 
+import janus
 import pytest
 from vllm.v1.engine.exceptions import EngineDeadError
 
-from vllm_omni.engine.messages import EngineQueueMessage, ErrorMessage, ShutdownRequestMessage
+from vllm_omni.engine.messages import (
+    AddCompanionRequestMessage,
+    EngineQueueMessage,
+    ErrorMessage,
+    ShutdownRequestMessage,
+    StageSubmissionMessage,
+)
+from vllm_omni.engine.orchestrator import Orchestrator, OrchestratorRequestState
+from vllm_omni.engine.stage_pool import StageUnavailableError
 from vllm_omni.inputs.data import OmniDiffusionSamplingParams
 from vllm_omni.outputs import OmniRequestOutput
 
 from .test_orchestrator import (
+    FakeOutputProcessor,
     FakeStageClient,
     OrchestratorFixture,
     _build_harness,
+    _build_request_output,
+    _build_stage_pools,
+    _engine_core_outputs,
     _enqueue_add_request,
     _wait_for,
 )
@@ -50,6 +63,22 @@ async def _get_any_output_message(fixture: OrchestratorFixture, *, timeout: floa
             await asyncio.sleep(0.01)
 
 
+async def _wait_for_error_message(
+    fixture: OrchestratorFixture,
+    *,
+    request_id: str | None = None,
+    max_messages: int = 50,
+) -> ErrorMessage:
+    """Drain output messages until the ErrorMessage arrives, skipping the
+    non-error outputs (e.g. stage metrics, forwarded stage results) that a
+    driven pipeline can interleave ahead of the failure."""
+    for _ in range(max_messages):
+        msg = await _get_any_output_message(fixture)
+        if isinstance(msg, ErrorMessage) and (request_id is None or msg.request_id == request_id):
+            return msg
+    raise AssertionError(f"No ErrorMessage for req={request_id} within {max_messages} messages")
+
+
 @pytest.fixture
 def orchestrator_factory():
     fixtures: list[OrchestratorFixture] = []
@@ -72,32 +101,14 @@ def orchestrator_factory():
 # ───────── EngineDeadError from LLM stage poll ─────────
 
 
-class FakeDeadLLMStageClient(FakeStageClient):
-    """LLM stage client that raises EngineDeadError once a request is in-flight.
-
-    Raising on the first poll races with ``add_request``: under suite load the
-    orchestrator can die and drain before the test enqueues anything, so no
-    fatal ErrorMessage is ever published. Keep returning empty outputs until
-    ``add_request_async`` has been called, then fail on the next poll.
-    """
-
-    async def get_output_async(self):
-        if not self.add_request_calls:
-            return SimpleNamespace(
-                outputs=[],
-                scheduler_stats=None,
-                finished_requests=None,
-            )
-        raise EngineDeadError("Stage-0 engine core is dead")
-
-
 @pytest.mark.asyncio
-async def test_engine_dead_error_broadcasts_fatal_and_shuts_down(orchestrator_factory) -> None:
-    """When a stage raises EngineDeadError during poll, the orchestrator must:
+async def test_engine_dead_error_evicts_replica_and_keeps_running(orchestrator_factory) -> None:
+    """When a stage replica raises EngineDeadError during poll, the orchestrator must:
     1. Enqueue a fatal error message for each affected request
-    2. Shut itself down (thread exits)
+    2. Evict the dead replica and keep running so the API server stays up
+       (per-replica fault isolation, #4285)
     """
-    stage0 = FakeDeadLLMStageClient(stage_type="llm", final_output=True)
+    stage0 = _DieOnDemandLLMStageClient(stage_type="llm", final_output=True)
     orchestrator_fixture = orchestrator_factory([stage0])
     request = SimpleNamespace(request_id="req-dead", prompt_token_ids=[1, 2])
 
@@ -110,6 +121,12 @@ async def test_engine_dead_error_broadcasts_fatal_and_shuts_down(orchestrator_fa
             sampling_params_list=[_sampling_params()],
             final_stage_id=0,
         )
+        # Let the request dispatch before the replica dies; a replica dead on
+        # its very first poll can be evicted before dispatch, which routes the
+        # failure through the add-request guard instead (that ordering is
+        # covered by test_add_request_to_dead_stage_fails_request_not_server).
+        await _wait_for(lambda: len(stage0.add_request_calls) == 1)
+        stage0.die = True
 
         # Collect the fatal error message.
         msg = await _get_any_output_message(orchestrator_fixture)
@@ -118,13 +135,17 @@ async def test_engine_dead_error_broadcasts_fatal_and_shuts_down(orchestrator_fa
         assert msg.type == "error"
         assert msg.fatal is True
         assert msg.request_id == "req-dead"
-        assert "Stage-0 engine core is dead" in msg.error
+        assert "Stage-0 replica-0 is dead" in msg.error
 
-        # The orchestrator thread should exit after the fatal error.
-        orchestrator_fixture.thread.join(timeout=5)
-        assert not orchestrator_fixture.thread.is_alive()
+        # The dead replica is evicted; the orchestrator keeps running.
+        pool = orchestrator_fixture.orchestrator.stage_pools[0]
+        deadline = time.monotonic() + 5.0
+        while pool.live_replica_ids() and time.monotonic() < deadline:
+            await asyncio.sleep(0.02)
+        assert pool.live_replica_ids() == []
+        assert orchestrator_fixture.thread.is_alive()
 
-        # Request state should be cleaned up.
+        # Affected request state should be cleaned up.
         assert "req-dead" not in orchestrator_fixture.orchestrator.request_states
     finally:
         if orchestrator_fixture.thread.is_alive():
@@ -132,6 +153,596 @@ async def test_engine_dead_error_broadcasts_fatal_and_shuts_down(orchestrator_fa
             orchestrator_fixture.thread.join(timeout=5)
 
 
+class _DieOnDemandLLMStageClient(FakeStageClient):
+    """LLM stage replica that raises EngineDeadError on poll once ``die`` is set."""
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.die = False
+
+    async def get_output_async(self):
+        if self.die:
+            raise EngineDeadError(f"Stage-{self.stage_id} replica-{self.replica_id} is dead")
+        return await super().get_output_async()
+
+
+class _DieOnDemandDiffusionStageClient(FakeStageClient):
+    """Diffusion stage replica that raises EngineDeadError on poll once ``die`` is set."""
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.die = False
+
+    def get_diffusion_output_nowait(self):
+        if self.die:
+            raise EngineDeadError(f"Stage-{self.stage_id} diffusion replica-{self.replica_id} is dead")
+        return super().get_diffusion_output_nowait()
+
+
+class _UnavailableDiffusionStageClient(FakeStageClient):
+    """Diffusion replica that stays live (so the pool keeps stage_type) but whose
+    dispatch fails with StageUnavailableError, as a downstream prewarm would when
+    its stage has no capacity."""
+
+    async def add_request_async(self, *args, **kwargs) -> None:
+        raise StageUnavailableError(f"stage {self.stage_id} has no live replica")
+
+
+@pytest.mark.asyncio
+async def test_engine_dead_error_fails_only_dead_replica_requests(orchestrator_factory) -> None:
+    """Multi-replica stage: one replica dying must fail only the requests bound
+    to it. Requests bound to a surviving replica keep running, and the stage
+    (still having a live replica) is not torn down (per-replica fault isolation,
+    #4285).
+    """
+    r0 = _DieOnDemandLLMStageClient(stage_type="llm", final_output=True)
+    r1 = FakeStageClient(stage_type="llm", final_output=True)
+    stage_pools = _build_stage_pools([[r0, r1]])
+    orchestrator_fixture = orchestrator_factory([], stage_pools=stage_pools)
+    pool = orchestrator_fixture.orchestrator.stage_pools[0]
+
+    try:
+        # req-0 → replica 0 (round-robin starts at 0)
+        await _enqueue_add_request(
+            orchestrator_fixture,
+            request_id="req-0",
+            prompt=SimpleNamespace(request_id="req-0", prompt_token_ids=[1, 2]),
+            original_prompt={"prompt": "a"},
+            sampling_params_list=[_sampling_params()],
+            final_stage_id=0,
+        )
+        await _wait_for(lambda: len(r0.add_request_calls) == 1)
+
+        # req-1 → replica 1
+        await _enqueue_add_request(
+            orchestrator_fixture,
+            request_id="req-1",
+            prompt=SimpleNamespace(request_id="req-1", prompt_token_ids=[3, 4]),
+            original_prompt={"prompt": "b"},
+            sampling_params_list=[_sampling_params()],
+            final_stage_id=0,
+        )
+        await _wait_for(lambda: len(r1.add_request_calls) == 1)
+        assert pool.get_bound_replica_id("req-0") == 0
+        assert pool.get_bound_replica_id("req-1") == 1
+
+        # Kill replica 0 on its next poll.
+        r0.die = True
+
+        msg = await _get_any_output_message(orchestrator_fixture)
+        assert isinstance(msg, ErrorMessage)
+        assert msg.fatal is True
+        assert msg.request_id == "req-0"
+
+        # Replica 0 evicted, replica 1 still serving; req-1 untouched.
+        await _wait_for(lambda: pool.live_replica_ids() == [1])
+        assert orchestrator_fixture.thread.is_alive()
+        assert "req-0" not in orchestrator_fixture.orchestrator.request_states
+        assert "req-1" in orchestrator_fixture.orchestrator.request_states
+    finally:
+        if orchestrator_fixture.thread.is_alive():
+            orchestrator_fixture.request_sync_q.put_nowait(ShutdownRequestMessage())
+            orchestrator_fixture.thread.join(timeout=5)
+
+
+@pytest.mark.asyncio
+async def test_forward_to_dead_downstream_stage_fails_request_not_server(orchestrator_factory) -> None:
+    """A request mid-pipeline whose downstream stage has lost all replicas must
+    fail that request, not tear down the orchestrator. Forwarding to a stage with
+    no live replica is handled gracefully (per-replica fault isolation, #4285).
+    """
+    stage0 = FakeStageClient(stage_type="llm", final_output=False, next_inputs=[{"prompt_token_ids": [7, 8]}])
+    stage1 = _DieOnDemandLLMStageClient(stage_type="llm", final_output=True)
+    proc0 = FakeOutputProcessor(request_outputs=[_build_request_output("req-x", token_ids=[3], finished=True)])
+    proc1 = FakeOutputProcessor()
+    stage_pools = _build_stage_pools([[stage0], [stage1]], output_processors=[proc0, proc1])
+    orchestrator_fixture = orchestrator_factory([], stage_pools=stage_pools)
+    pool1 = orchestrator_fixture.orchestrator.stage_pools[1]
+
+    try:
+        await _enqueue_add_request(
+            orchestrator_fixture,
+            request_id="req-x",
+            prompt=SimpleNamespace(request_id="req-x", prompt_token_ids=[1, 2]),
+            original_prompt={"prompt": "hello"},
+            sampling_params_list=[_sampling_params(), _sampling_params()],
+            final_stage_id=1,
+        )
+        await _wait_for(lambda: len(stage0.add_request_calls) == 1)
+
+        # Kill stage-1's only replica before stage-0 output is forwarded.
+        stage1.die = True
+        await _wait_for(lambda: pool1.live_replica_ids() == [])
+
+        # Completing stage 0 triggers a forward to the now-empty stage 1.
+        stage0.push_engine_core_outputs(_engine_core_outputs("s0-raw", 1.0))
+
+        # Completing stage 0 also emits its normal outputs; skip past them to
+        # the downstream-forward failure.
+        error_msg = await _wait_for_error_message(orchestrator_fixture, request_id="req-x")
+        assert error_msg.fatal is True
+        assert error_msg.stage_id == 1
+
+        assert orchestrator_fixture.thread.is_alive()
+        assert "req-x" not in orchestrator_fixture.orchestrator.request_states
+    finally:
+        if orchestrator_fixture.thread.is_alive():
+            orchestrator_fixture.request_sync_q.put_nowait(ShutdownRequestMessage())
+            orchestrator_fixture.thread.join(timeout=5)
+
+
+@pytest.mark.asyncio
+async def test_add_request_to_dead_stage_fails_request_not_server(orchestrator_factory) -> None:
+    """A new request entering a stage that already lost all replicas fails that
+    request instead of raising out of the request handler and tearing the server
+    down. The guard runs before request state is registered, so nothing leaks.
+    """
+    r0 = _DieOnDemandLLMStageClient(stage_type="llm", final_output=True)
+    stage_pools = _build_stage_pools([[r0]])
+    orchestrator_fixture = orchestrator_factory([], stage_pools=stage_pools)
+    pool = orchestrator_fixture.orchestrator.stage_pools[0]
+
+    try:
+        # Send a first request so the replica is polled and dies → evicted.
+        await _enqueue_add_request(
+            orchestrator_fixture,
+            request_id="req-0",
+            prompt=SimpleNamespace(request_id="req-0", prompt_token_ids=[1, 2]),
+            original_prompt={"prompt": "a"},
+            sampling_params_list=[_sampling_params()],
+            final_stage_id=0,
+        )
+        r0.die = True
+        await _wait_for(lambda: pool.live_replica_ids() == [])
+
+        # A new request now enters a stage with no live replica.
+        await _enqueue_add_request(
+            orchestrator_fixture,
+            request_id="req-after",
+            prompt=SimpleNamespace(request_id="req-after", prompt_token_ids=[3, 4]),
+            original_prompt={"prompt": "b"},
+            sampling_params_list=[_sampling_params()],
+            final_stage_id=0,
+        )
+
+        error_msg = await _wait_for_error_message(orchestrator_fixture, request_id="req-after")
+        assert error_msg.fatal is True
+        assert error_msg.stage_id == 0
+
+        assert orchestrator_fixture.thread.is_alive()
+        # Guard runs before registration, so the failed request never enters state.
+        assert "req-after" not in orchestrator_fixture.orchestrator.request_states
+    finally:
+        if orchestrator_fixture.thread.is_alive():
+            orchestrator_fixture.request_sync_q.put_nowait(ShutdownRequestMessage())
+            orchestrator_fixture.thread.join(timeout=5)
+
+
+@pytest.mark.asyncio
+async def test_diffusion_replica_death_on_poll_keeps_server(orchestrator_factory) -> None:
+    """A diffusion stage replica raising EngineDeadError during poll must be
+    evicted while the orchestrator keeps serving. The per-replica catch has to
+    cover the diffusion polling path, not just the LLM one (#4285).
+    """
+    stage0 = _DieOnDemandDiffusionStageClient(stage_type="diffusion", final_output=True, final_output_type="image")
+    stage_pools = _build_stage_pools([[stage0]])
+    orchestrator_fixture = orchestrator_factory([], stage_pools=stage_pools)
+    pool = orchestrator_fixture.orchestrator.stage_pools[0]
+
+    try:
+        await _enqueue_add_request(
+            orchestrator_fixture,
+            request_id="req-dit",
+            prompt={"prompt": "a cat"},
+            original_prompt={"prompt": "a cat"},
+            sampling_params_list=[OmniDiffusionSamplingParams()],
+            final_stage_id=0,
+        )
+        await _wait_for(lambda: len(stage0.add_request_calls) == 1)
+
+        stage0.die = True
+
+        error_msg = await _wait_for_error_message(orchestrator_fixture, request_id="req-dit")
+        assert error_msg.fatal is True
+        assert error_msg.stage_id == 0
+
+        await _wait_for(lambda: pool.live_replica_ids() == [])
+        assert orchestrator_fixture.thread.is_alive()
+        assert "req-dit" not in orchestrator_fixture.orchestrator.request_states
+    finally:
+        if orchestrator_fixture.thread.is_alive():
+            orchestrator_fixture.request_sync_q.put_nowait(ShutdownRequestMessage())
+            orchestrator_fixture.thread.join(timeout=5)
+
+
+@pytest.mark.asyncio
+async def test_async_chunk_prewarm_failure_reports_failing_downstream_stage(orchestrator_factory) -> None:
+    """When an async-chunk prewarm dispatch to a downstream stage fails, the
+    failure must be attributed to that stage, not stage 0 (the prewarm's own
+    dispatch stage). Regression for the guard using stage_id=0.
+    """
+    stage0 = FakeStageClient(stage_type="llm", final_output=False)
+    stage1 = FakeStageClient(stage_type="diffusion", final_output=False, final_output_type="image")
+    stage2 = _UnavailableDiffusionStageClient(stage_type="diffusion", final_output=True, final_output_type="image")
+    stage_pools = _build_stage_pools([[stage0], [stage1], [stage2]])
+    orchestrator_fixture = orchestrator_factory([], stage_pools=stage_pools, async_chunk=True)
+
+    try:
+        await _enqueue_add_request(
+            orchestrator_fixture,
+            request_id="req-x",
+            prompt=SimpleNamespace(request_id="req-x", prompt_token_ids=[1, 2, 3]),
+            original_prompt={"prompt": "a cat"},
+            sampling_params_list=[_sampling_params(), OmniDiffusionSamplingParams(), OmniDiffusionSamplingParams()],
+            final_stage_id=2,
+        )
+
+        error_msg = await _wait_for_error_message(orchestrator_fixture, request_id="req-x")
+        assert error_msg.fatal is True
+        assert error_msg.stage_id == 2
+        assert "Stage-2" in error_msg.error
+
+        # Stage 1 prewarm ran before the stage-2 failure; server stays up.
+        assert len(stage1.add_request_calls) == 1
+        assert orchestrator_fixture.thread.is_alive()
+    finally:
+        if orchestrator_fixture.thread.is_alive():
+            orchestrator_fixture.request_sync_q.put_nowait(ShutdownRequestMessage())
+            orchestrator_fixture.thread.join(timeout=5)
+
+
+# ───────── Direct unit tests for the fault-isolation helpers ─────────
+
+
+def _build_bare_orchestrator(stage_pools) -> tuple[Orchestrator, tuple[janus.Queue, ...]]:
+    """Construct an Orchestrator on the current event loop without running it.
+
+    Lets tests await the fault-isolation helpers directly instead of driving
+    them through the orchestration loop.
+    """
+    queues = (janus.Queue(), janus.Queue(), janus.Queue())
+    orchestrator = Orchestrator(
+        request_async_queue=queues[0].async_q,
+        output_async_queue=queues[1].async_q,
+        rpc_async_queue=queues[2].async_q,
+        stage_pools=stage_pools,
+    )
+    return orchestrator, queues
+
+
+def _register_request(orchestrator: Orchestrator, request_id: str, *, submitted_stage: int | None = 0) -> None:
+    state = OrchestratorRequestState(request_id=request_id)
+    if submitted_stage is not None:
+        state.stage_submit_ts[submitted_stage] = time.time()
+    orchestrator.request_states[request_id] = state
+
+
+@pytest.mark.asyncio
+async def test_handle_dead_replica_fails_only_bound_requests() -> None:
+    """Direct helper test: with a surviving replica, only requests bound to the
+    dead replica fail; the survivor's requests and bindings are untouched.
+    """
+    r0 = FakeStageClient(stage_type="llm", final_output=True)
+    r1 = FakeStageClient(stage_type="llm", final_output=True)
+    orchestrator, queues = _build_bare_orchestrator(_build_stage_pools([[r0, r1]]))
+    try:
+        pool = orchestrator.stage_pools[0]
+        _register_request(orchestrator, "req-0")
+        _register_request(orchestrator, "req-1")
+        assert pool.select_replica_id("req-0") == 0
+        assert pool.select_replica_id("req-1") == 1
+
+        await orchestrator._handle_dead_replica(0, 0, EngineDeadError("replica-0 dead"))
+
+        assert pool.live_replica_ids() == [1]
+        assert r0.shutdown_calls == 1
+        msg = queues[1].async_q.get_nowait()
+        assert isinstance(msg, ErrorMessage)
+        assert msg.fatal is True
+        assert msg.request_id == "req-0"
+        assert msg.stage_id == 0
+        assert "replica-0 dead" in msg.error
+        assert queues[1].async_q.empty()
+        assert "req-0" not in orchestrator.request_states
+        assert "req-1" in orchestrator.request_states
+        assert pool.get_bound_replica_id("req-0") is None
+        assert pool.get_bound_replica_id("req-1") == 1
+    finally:
+        for q in queues:
+            q.close()
+
+
+@pytest.mark.asyncio
+async def test_handle_dead_replica_last_replica_fails_all_stage_requests() -> None:
+    """Direct helper test: when the dead replica was the stage's last one, every
+    request routed through the stage fails — bound or not — while requests that
+    never entered the stage survive.
+    """
+    orchestrator, queues = _build_bare_orchestrator(_build_stage_pools([[FakeStageClient(stage_type="llm")]]))
+    try:
+        pool = orchestrator.stage_pools[0]
+        _register_request(orchestrator, "req-bound")
+        _register_request(orchestrator, "req-unbound")
+        _register_request(orchestrator, "req-elsewhere", submitted_stage=None)
+        assert pool.select_replica_id("req-bound") == 0
+
+        await orchestrator._handle_dead_replica(0, 0, EngineDeadError("last replica dead"))
+
+        assert pool.live_replica_ids() == []
+        failed = {queues[1].async_q.get_nowait().request_id for _ in range(2)}
+        assert failed == {"req-bound", "req-unbound"}
+        assert queues[1].async_q.empty()
+        assert set(orchestrator.request_states) == {"req-elsewhere"}
+    finally:
+        for q in queues:
+            q.close()
+
+
+@pytest.mark.asyncio
+async def test_fail_request_dead_stage_cleans_registered_request() -> None:
+    """Direct helper test: a registered request failed against a dead stage gets
+    a fatal ErrorMessage, its state and bindings are released, and its
+    in-flight work on live replicas is aborted.
+    """
+    r0 = FakeStageClient(stage_type="llm")
+    orchestrator, queues = _build_bare_orchestrator(_build_stage_pools([[r0]]))
+    try:
+        pool = orchestrator.stage_pools[0]
+        _register_request(orchestrator, "req-a")
+        assert pool.select_replica_id("req-a") == 0
+
+        await orchestrator._fail_request_dead_stage("req-a", 0)
+
+        msg = queues[1].async_q.get_nowait()
+        assert isinstance(msg, ErrorMessage)
+        assert msg.error == "Stage-0 has no live replica"
+        assert msg.fatal is True
+        assert msg.request_id == "req-a"
+        assert msg.stage_id == 0
+        assert "req-a" not in orchestrator.request_states
+        assert pool.get_bound_replica_id("req-a") is None
+        # abort=True: the failed request's work on the still-live replica is
+        # stopped, not left to run to completion.
+        assert r0.abort_calls == [["req-a"]]
+    finally:
+        for q in queues:
+            q.close()
+
+
+@pytest.mark.asyncio
+async def test_streaming_update_racing_eviction_fails_request_not_server() -> None:
+    """A streaming_update can find its request state still present while the
+    stage has already lost all replicas (_handle_dead_replica awaits between
+    eviction and cleanup). Submitting then raises from replica selection; the
+    handler must fail that request instead of letting the raise escape
+    _request_handler and shut the server down (#4285).
+    """
+    orchestrator, queues = _build_bare_orchestrator(_build_stage_pools([[FakeStageClient(stage_type="llm")]]))
+    try:
+        pool = orchestrator.stage_pools[0]
+        _register_request(orchestrator, "req-s")
+        orchestrator.request_states["req-s"].sampling_params_list = [_sampling_params()]
+        assert pool.select_replica_id("req-s") == 0
+        pool.evict_replica(0)
+
+        msg = StageSubmissionMessage(
+            type="streaming_update",
+            request_id="req-s",
+            prompt=SimpleNamespace(request_id="req-s", prompt_token_ids=[1]),
+            original_prompt={"prompt": "hi"},
+            output_prompt_text=None,
+            sampling_params_list=[],
+            final_stage_id=0,
+            preprocess_ms=0.0,
+            request_timestamp=time.time(),
+            enqueue_ts=time.time(),
+        )
+        await orchestrator._handle_streaming_update(msg)
+
+        err = queues[1].async_q.get_nowait()
+        assert isinstance(err, ErrorMessage)
+        assert err.fatal is True
+        assert err.request_id == "req-s"
+        assert "req-s" not in orchestrator.request_states
+    finally:
+        for q in queues:
+            q.close()
+
+
+@pytest.mark.asyncio
+async def test_streaming_update_unrelated_error_is_not_masked() -> None:
+    """The dispatch guard converts only StageUnavailableError/EngineDeadError;
+    a genuine bug surfacing as a plain RuntimeError must propagate (fail
+    fast), not be misreported as a dead-stage request failure.
+    """
+
+    class _BuggyClient(FakeStageClient):
+        async def add_request_async(self, *args, **kwargs):
+            raise RuntimeError("boom: unrelated bug")
+
+    orchestrator, queues = _build_bare_orchestrator(
+        _build_stage_pools([[_BuggyClient(stage_type="llm", final_output=True)]])
+    )
+    try:
+        _register_request(orchestrator, "req-b")
+        orchestrator.request_states["req-b"].sampling_params_list = [_sampling_params()]
+
+        msg = StageSubmissionMessage(
+            type="streaming_update",
+            request_id="req-b",
+            prompt=SimpleNamespace(request_id="req-b", prompt_token_ids=[1]),
+            original_prompt={"prompt": "hi"},
+            output_prompt_text=None,
+            sampling_params_list=[],
+            final_stage_id=0,
+            preprocess_ms=0.0,
+            request_timestamp=time.time(),
+            enqueue_ts=time.time(),
+        )
+        with pytest.raises(RuntimeError, match="unrelated bug"):
+            await orchestrator._handle_streaming_update(msg)
+    finally:
+        for q in queues:
+            q.close()
+
+
+@pytest.mark.asyncio
+async def test_add_companion_racing_eviction_fails_parent_not_server() -> None:
+    """A CFG companion submission hitting a fully dead stage must fail the
+    parent request (whose cleanup also releases the companion), not raise out
+    of _request_handler (#4285).
+    """
+    orchestrator, queues = _build_bare_orchestrator(_build_stage_pools([[FakeStageClient(stage_type="llm")]]))
+    try:
+        pool = orchestrator.stage_pools[0]
+        _register_request(orchestrator, "req-p")
+        orchestrator.request_states["req-p"].sampling_params_list = [_sampling_params()]
+        pool.evict_replica(0)
+
+        msg = AddCompanionRequestMessage(
+            companion_id="req-p-cfg",
+            parent_id="req-p",
+            role="negative",
+            prompt=SimpleNamespace(request_id="req-p-cfg", prompt_token_ids=[2]),
+            companion_prompt_text=None,
+            sampling_params_list=[_sampling_params()],
+        )
+        await orchestrator._handle_add_companion(msg)
+
+        err = queues[1].async_q.get_nowait()
+        assert isinstance(err, ErrorMessage)
+        assert err.fatal is True
+        assert err.request_id == "req-p"
+        assert "req-p" not in orchestrator.request_states
+        assert "req-p-cfg" not in orchestrator.request_states
+    finally:
+        for q in queues:
+            q.close()
+
+
+@pytest.mark.asyncio
+async def test_fail_request_dead_stage_is_noop_safe_for_unregistered_request() -> None:
+    """Direct helper test: the stage-0 dispatch guard calls the helper before the
+    request is registered; the cleanup must be a safe no-op and still emit the
+    fatal ErrorMessage.
+    """
+    orchestrator, queues = _build_bare_orchestrator(_build_stage_pools([[FakeStageClient(stage_type="llm")]]))
+    try:
+        await orchestrator._fail_request_dead_stage("req-unknown", 0)
+
+        msg = queues[1].async_q.get_nowait()
+        assert isinstance(msg, ErrorMessage)
+        assert msg.error == "Stage-0 has no live replica"
+        assert msg.fatal is True
+        assert msg.request_id == "req-unknown"
+        assert orchestrator.request_states == {}
+    finally:
+        for q in queues:
+            q.close()
+
+
+@pytest.mark.asyncio
+async def test_dispatch_death_evicts_so_subsequent_requests_reroute_to_survivor() -> None:
+    """A replica dying mid-submit (EngineDeadError from dispatch, before the poll
+    loop evicts it) is evicted by the guard so subsequent requests reroute to the
+    survivor instead of round-robining back onto the dead slot until the poll
+    catches up. The survivor and its request are untouched (#4285).
+    """
+    r0 = FakeStageClient(stage_type="llm")
+    r1 = FakeStageClient(stage_type="llm")
+    orchestrator, queues = _build_bare_orchestrator(_build_stage_pools([[r0, r1]]))
+    try:
+        pool = orchestrator.stage_pools[0]
+        _register_request(orchestrator, "req-dead")
+        _register_request(orchestrator, "req-live")
+        assert pool.select_replica_id("req-dead") == 0
+        assert pool.select_replica_id("req-live") == 1
+
+        async def _die() -> None:
+            raise EngineDeadError("stage-0 replica-0 died on submit")
+
+        failed = await orchestrator._dispatch_or_fail_request(
+            _die, req_id="req-dead", stage_id=0, operation="add_request"
+        )
+        assert failed is False
+
+        # The dead replica is evicted; only the request on it fails, the survivor
+        # and its request are untouched.
+        assert pool.live_replica_ids() == [1]
+        assert r0.shutdown_calls == 1
+        assert r1.shutdown_calls == 0
+        msg = queues[1].async_q.get_nowait()
+        assert isinstance(msg, ErrorMessage) and msg.fatal is True and msg.request_id == "req-dead"
+        assert queues[1].async_q.empty()
+        assert "req-dead" not in orchestrator.request_states
+        assert "req-live" in orchestrator.request_states and pool.get_bound_replica_id("req-live") == 1
+
+        # The point of eviction: a burst of subsequent requests all route to the
+        # survivor and never land back on the evicted replica.
+        for rid in ("next-1", "next-2", "next-3"):
+            assert pool.select_replica_id(rid) == 1
+    finally:
+        for q in queues:
+            q.close()
+
+
+@pytest.mark.asyncio
+async def test_dispatch_death_evicts_so_subsequent_requests_fast_fail() -> None:
+    """When the replica dying on submit was the stage's only one, the guard evicts
+    it (leaving the stage empty). A subsequent request then fast-fails at routing —
+    the ``_handle_add_request`` entry guard turns ``StageUnavailableError`` into a
+    fatal 5xx immediately, rather than dispatching onto the dead replica (#4285).
+    """
+    r0 = FakeStageClient(stage_type="llm")
+    orchestrator, queues = _build_bare_orchestrator(_build_stage_pools([[r0]]))
+    try:
+        pool = orchestrator.stage_pools[0]
+        _register_request(orchestrator, "req-a")
+        assert pool.select_replica_id("req-a") == 0
+
+        async def _die() -> None:
+            raise EngineDeadError("stage-0 replica-0 died on submit")
+
+        failed = await orchestrator._dispatch_or_fail_request(_die, req_id="req-a", stage_id=0, operation="add_request")
+        assert failed is False
+
+        assert pool.live_replica_ids() == []
+        assert r0.shutdown_calls == 1
+        msg = queues[1].async_q.get_nowait()
+        assert isinstance(msg, ErrorMessage) and msg.request_id == "req-a"
+        assert queues[1].async_q.empty()
+        assert "req-a" not in orchestrator.request_states
+
+        # A subsequent request cannot be routed at all: the stage has no live
+        # replica, so selection raises fast instead of dispatching onto the dead
+        # slot. _handle_add_request converts this to a fatal 5xx at its entry guard.
+        with pytest.raises(StageUnavailableError):
+            pool.select_replica_id("req-next")
+    finally:
+        for q in queues:
+            q.close()
+
+
 # ───────── Diffusion stage error output routing ─────────
 
 
diff --git a/tests/entrypoints/test_omni_entrypoints.py b/tests/entrypoints/test_omni_entrypoints.py
index 5d9b85938..d7f424d6e 100644
--- a/tests/entrypoints/test_omni_entrypoints.py
+++ b/tests/entrypoints/test_omni_entrypoints.py
@@ -104,6 +104,9 @@ class FakeAsyncOmniEngine:
         ]
         self.supported_tasks = ("generate",)
         self.stage_clients = [SimpleNamespace(is_comprehension=False) for _ in range(self.num_stages)]
+        # One replica per stage, sharing the stage_clients objects so a test that
+        # marks stage_clients[i] dead is reflected in the pool's liveness.
+        self.stage_pools = [_FakeStagePool([client], stage_id=i) for i, client in enumerate(self.stage_clients)]
         self.stage_vllm_configs = [None for _ in range(self.num_stages)]
         self.output_processors = [SimpleNamespace(tokenizer=None) for _ in range(self.num_stages)]
         self.input_processor = None
@@ -969,11 +972,17 @@ async def test_async_omni_propagates_non_400_client_error_status(
     assert str(exc_info.value) == error_text
 
 
+class _FakeStagePool:
+    def __init__(self, clients: list, stage_id: int = 0):
+        self.clients = clients
+        self.stage_id = stage_id
+
+
 def test_async_omni_errored_property_alive():
     omni = object.__new__(AsyncOmni)
     omni.engine = SimpleNamespace(
         is_alive=lambda: True,
-        stage_clients=[SimpleNamespace(is_comprehension=False)],
+        stage_pools=[_FakeStagePool(clients=[SimpleNamespace()])],
     )
 
     assert omni.errored is False
@@ -983,21 +992,47 @@ def test_async_omni_errored_property_dead_engine():
     omni = object.__new__(AsyncOmni)
     omni.engine = SimpleNamespace(
         is_alive=lambda: False,
-        stage_clients=[SimpleNamespace(is_comprehension=False)],
+        stage_pools=[_FakeStagePool(clients=[SimpleNamespace()])],
     )
 
     assert omni.errored is True
 
 
-def test_async_omni_errored_property_dead_stage():
+def test_async_omni_errored_false_when_stage_dead():
+    # errored is process-fatal only: a dead stage must not trip the serving
+    # precheck, or requests that never touch that stage would be rejected too.
+    # Stage liveness surfaces via check_health / per-request dispatch failures.
     omni = object.__new__(AsyncOmni)
-    dead_stage = SimpleNamespace(is_comprehension=False, _engine_dead=True)
     omni.engine = SimpleNamespace(
         is_alive=lambda: True,
-        stage_clients=[dead_stage],
+        stage_pools=[_FakeStagePool(clients=[SimpleNamespace(_engine_dead=True)])],
     )
 
-    assert omni.errored is True
+    assert omni.errored is False
+
+
+def test_async_omni_errored_false_when_a_replica_survives():
+    # Per-replica fault isolation (#4285): one replica dies, another lives →
+    # the stage is still serving, so the engine is not errored.
+    omni = object.__new__(AsyncOmni)
+    omni.engine = SimpleNamespace(
+        is_alive=lambda: True,
+        stage_pools=[_FakeStagePool(clients=[SimpleNamespace(_engine_dead=True), SimpleNamespace()])],
+    )
+
+    assert omni.errored is False
+
+
+def test_async_omni_errored_false_when_evicted_replica_leaves_stage_empty():
+    # Even a fully evicted stage (all slots None/dead) is not process-fatal;
+    # only orchestrator death makes errored True.
+    omni = object.__new__(AsyncOmni)
+    omni.engine = SimpleNamespace(
+        is_alive=lambda: True,
+        stage_pools=[_FakeStagePool(clients=[None, SimpleNamespace(_engine_dead=True)])],
+    )
+
+    assert omni.errored is False
 
 
 def _enqueue_stage_error(
@@ -1067,20 +1102,22 @@ async def test_async_omni_propagates_engine_generate_error(monkeypatch: pytest.M
 
 def test_check_health_passes_when_all_healthy():
     base = _make_base()
-    healthy_stage = MagicMock()
-    healthy_stage.check_health = MagicMock()
     base.engine.is_alive.return_value = True
-    base.engine.stage_clients = [healthy_stage]
+    healthy_stage = SimpleNamespace(check_health=lambda: None)
+    base.engine.stage_pools = [_FakeStagePool([healthy_stage])]
     base.check_health()  # should not raise
 
 
 def test_check_health_raises_when_stage_dead():
     base = _make_base()
-    dead_stage = MagicMock()
-    dead_stage.check_health = MagicMock(side_effect=EngineDeadError("Stage-1 dead"))
     base.engine.is_alive.return_value = True
-    base.engine.stage_clients = [dead_stage]
-    with pytest.raises(EngineDeadError, match="Stage-1 dead"):
+
+    def _raise_dead() -> None:
+        raise EngineDeadError("Stage-1 engine core is dead")
+
+    dead_stage = SimpleNamespace(check_health=_raise_dead)
+    base.engine.stage_pools = [_FakeStagePool([dead_stage], stage_id=1)]
+    with pytest.raises(EngineDeadError, match="Stage-1"):
         base.check_health()
 
 
@@ -1098,22 +1135,25 @@ def test_check_health_raises_when_orchestrator_dead():
 def test_omni_base_errored_false_when_alive():
     base = _make_base()
     base.engine.is_alive.return_value = True
-    base.engine.stage_clients = [SimpleNamespace()]
+    base.engine.stage_pools = [_FakeStagePool([SimpleNamespace()])]
     assert base.errored is False
 
 
-def test_omni_base_is_running_false_when_stage_engine_dead():
+def test_omni_base_is_running_true_when_stage_engine_dead():
+    # is_running tracks the orchestrator only; a dead stage must not make
+    # `errored and not is_running` true, which would let terminate_if_errored
+    # kill the server on a partial failure.
     base = _make_base()
     base.engine.is_alive.return_value = True
-    base.engine.stage_clients = [SimpleNamespace(_engine_dead=True)]
-    assert base.is_running is False
+    base.engine.stage_pools = [_FakeStagePool([SimpleNamespace(_engine_dead=True)])]
+    assert base.is_running is True
 
 
-def test_omni_base_is_running_false_when_stage_resources_engine_dead():
+def test_omni_base_is_running_true_when_stage_resources_engine_dead():
     base = _make_base()
     base.engine.is_alive.return_value = True
-    base.engine.stage_clients = [SimpleNamespace(resources=SimpleNamespace(engine_dead=True))]
-    assert base.is_running is False
+    base.engine.stage_pools = [_FakeStagePool([SimpleNamespace(resources=SimpleNamespace(engine_dead=True))])]
+    assert base.is_running is True
 
 
 def test_omni_base_errored_true_when_orchestrator_dead():
@@ -1123,20 +1163,22 @@ def test_omni_base_errored_true_when_orchestrator_dead():
     assert base.errored is True
 
 
-def test_omni_base_errored_true_when_stage_engine_dead():
+def test_omni_base_errored_false_when_stage_engine_dead():
+    # Process-fatal only: the serving precheck reads errored before routing,
+    # so a dead stage here would reject requests that never touch it.
     base = _make_base()
     base.engine.is_alive.return_value = True
     dead_stage = SimpleNamespace(_engine_dead=True)
-    base.engine.stage_clients = [dead_stage]
-    assert base.errored is True
+    base.engine.stage_pools = [_FakeStagePool([dead_stage])]
+    assert base.errored is False
 
 
-def test_omni_base_errored_true_when_stage_resources_engine_dead():
+def test_omni_base_errored_false_when_stage_resources_engine_dead():
     base = _make_base()
     base.engine.is_alive.return_value = True
     dead_stage = SimpleNamespace(resources=SimpleNamespace(engine_dead=True))
-    base.engine.stage_clients = [dead_stage]
-    assert base.errored is True
+    base.engine.stage_pools = [_FakeStagePool([dead_stage])]
+    assert base.errored is False
 
 
 # ───────── Omni (sync) EngineDeadError / EngineGenerateError ─────────
@@ -1202,13 +1244,14 @@ def test_omni_errored_property_dead_engine(monkeypatch: pytest.MonkeyPatch):
 
 
 def test_omni_errored_property_dead_stage(monkeypatch: pytest.MonkeyPatch):
-    """Omni.errored returns True when a stage client is marked dead."""
+    """Omni.errored stays False when only a stage client dies (process-fatal
+    semantics): the request-level failure and check_health carry the signal."""
     engine = FakeAsyncOmniEngine(stage_metadata=THREE_STAGE_META)
     _patch_engine(monkeypatch, engine)
 
     app = Omni("dummy-model")
     try:
         engine.stage_clients[0]._engine_dead = True
-        assert app.errored is True
+        assert app.errored is False
     finally:
         app.shutdown()
diff --git a/tests/model_executor/models/voxtral_tts/test_cuda_graph_acoustic_transformer.py b/tests/model_executor/models/voxtral_tts/test_cuda_graph_acoustic_transformer.py
index cf40f9ad4..8ff67b5ff 100644
--- a/tests/model_executor/models/voxtral_tts/test_cuda_graph_acoustic_transformer.py
+++ b/tests/model_executor/models/voxtral_tts/test_cuda_graph_acoustic_transformer.py
@@ -83,11 +83,16 @@ class SyntheticAcousticTransformer(nn.Module):
         # time_embedding: (B, 1) -> (B, dim)
         self.time_embedding = nn.Linear(1, HIDDEN_DIM, bias=False)
 
-        # _predict_velocity: takes x_t (B, C), llm_output (B, D), t_emb (B, D) -> (B, C)
+        # Identities: callers pass pre-projected inputs; the synthetic transformer folds
+        # everything into _velocity_proj.
+        self.llm_projection = nn.Identity()
+        self.time_projection = nn.Identity()
+
+        # _predict_velocity: takes x_t (B, C), llm_proj (B, D), t_proj (B, D) -> (B, C)
         self._velocity_proj = nn.Linear(N_ACOUSTIC_CODEBOOK + HIDDEN_DIM + HIDDEN_DIM, N_ACOUSTIC_CODEBOOK, bias=False)
 
-    def _predict_velocity(self, x_t: torch.Tensor, llm_output: torch.Tensor, t_emb: torch.Tensor) -> torch.Tensor:
-        combined = torch.cat([x_t, llm_output, t_emb], dim=-1)
+    def _predict_velocity(self, x_t: torch.Tensor, llm_proj: torch.Tensor, t_proj: torch.Tensor) -> torch.Tensor:
+        combined = torch.cat([x_t, llm_proj, t_proj], dim=-1)
         return self._velocity_proj(combined)
 
 
@@ -129,14 +134,18 @@ class SyntheticModel(nn.Module):
         hidden_zero = torch.zeros_like(hidden_states)
         timesteps = torch.linspace(0, 1, 16, device=hidden_states.device, dtype=hidden_states.dtype)
 
+        # llm_projection is constant across steps so hoist it (mirrors decode_one_frame).
+        llm_batched = torch.cat([hidden_states, hidden_zero], dim=0)
+        llm_proj_batched = at.llm_projection(llm_batched)
+
         for i in range(len(timesteps) - 1):
             t = timesteps[i]
             dt = timesteps[i + 1] - timesteps[i]
             t_emb = at.time_embedding(t.view(-1, 1).repeat(B, 1)).to(hidden_states.dtype)
+            t_proj = at.time_projection(t_emb)
             x_batched = torch.cat([x, x], dim=0)
-            llm_batched = torch.cat([hidden_states, hidden_zero], dim=0)
-            t_emb_batched = t_emb.repeat(2, 1)
-            v_all = at._predict_velocity(x_t=x_batched, llm_output=llm_batched, t_emb=t_emb_batched)
+            t_proj_batched = t_proj.repeat(2, 1)
+            v_all = at._predict_velocity(x_t=x_batched, llm_proj=llm_proj_batched, t_proj=t_proj_batched)
             v_t, uncond_v_t = v_all[:B], v_all[B:]
             cfg_alpha = 1.2
             v_t = cfg_alpha * v_t + (1 - cfg_alpha) * uncond_v_t
diff --git a/vllm_omni/engine/orchestrator.py b/vllm_omni/engine/orchestrator.py
index 0b4f17aae..c539e9559 100644
--- a/vllm_omni/engine/orchestrator.py
+++ b/vllm_omni/engine/orchestrator.py
@@ -50,7 +50,7 @@ from vllm_omni.engine.messages import (
 )
 from vllm_omni.engine.orchestrator_monitor import create_orch_monitor, replica_key
 from vllm_omni.engine.serialization import serialize_additional_information
-from vllm_omni.engine.stage_pool import StagePool
+from vllm_omni.engine.stage_pool import StagePool, StageUnavailableError
 from vllm_omni.metrics.prometheus import OmniRequestCounter
 from vllm_omni.metrics.stat_logger import OmniPrometheusStatLogger
 from vllm_omni.outputs import OmniRequestOutput
@@ -417,8 +417,6 @@ class Orchestrator:
 
         self._shutdown_event = asyncio.Event()
         self._stages_shutdown = False
-        self._fatal_error: str | None = None
-        self._fatal_error_stage_id: int | None = None
 
         # Distributed membership (optional, injected by DistStageRuntime)
         self._membership = membership_controller
@@ -532,19 +530,17 @@ class Orchestrator:
         except asyncio.CancelledError:
             raise
         except EngineDeadError as e:
-            # EngineDeadError from _orchestration_loop means the diffusion
-            # engine died.  All pending requests were already notified and
-            # _shutdown_event was already set by the loop's handler.
-            # During teardown this is expected; the finally block handles
-            # proper cleanup.  Do not re-raise.
-            logger.info("[Orchestrator] Engine dead during shutdown: %s", e)
-            if self._fatal_error is None:
-                self._fatal_error = str(e) or "Stage engine died"
+            # The orchestration loop isolates per-replica EngineDeadError
+            # (evict + continue), so this only fires if one escapes outside the
+            # poll handler (e.g. from output post-processing). Treat it as a
+            # teardown signal and let the finally block clean up; do not re-raise.
+            logger.info("[Orchestrator] Engine dead, shutting down: %s", e)
+            # Unblock any collective_rpc callers waiting on rpc_async_queue
+            # instead of leaving them to hit their own timeout.
             await self.rpc_async_queue.put(
                 ErrorMessage(
-                    error=self._fatal_error or str(e),
+                    error=str(e) or "Stage engine died",
                     fatal=True,
-                    stage_id=self._fatal_error_stage_id,
                 )
             )
         except Exception:
@@ -562,9 +558,6 @@ class Orchestrator:
             if self.duplex_control_plane is not None:
                 await self.duplex_control_plane.shutdown()
 
-            if self._fatal_error is not None:
-                await self._drain_pending_requests_on_fatal()
-
             if self._membership is not None:
                 await self._membership.drain_tasks(timeout=10.0)
                 self._membership.shutdown()
@@ -651,6 +644,13 @@ class Orchestrator:
         final_stage_id = msg.final_stage_id
         final_output_stage_ids = set(msg.final_output_stage_ids or [final_stage_id])
 
+        if not self.stage_pools[stage_id].live_replica_ids():
+            # Stage 0 lost all replicas between the HTTP-layer errored check and
+            # dispatch. Runs before request state / running counter registration,
+            # so the helper's cleanup is a no-op here.
+            await self._fail_request_dead_stage(request_id, stage_id)
+            return
+
         logger.debug(
             "[Orchestrator] _handle_add_request: stage=%s req=%s "
             "prompt_type=%s original_prompt_type=%s final_stage=%s "
@@ -682,12 +682,18 @@ class Orchestrator:
         preprocess_ms = msg.preprocess_ms
         if preprocess_ms > 0:
             req_state.pipeline_timings["preprocess_ms"] = preprocess_ms
-        await self.stage_pools[stage_id].submit_initial(
-            request_id,
-            req_state,
-            prompt,
-            prompt_text=msg.output_prompt_text,
-        )
+        if not await self._dispatch_or_fail_request(
+            lambda: self.stage_pools[stage_id].submit_initial(
+                request_id,
+                req_state,
+                prompt,
+                prompt_text=msg.output_prompt_text,
+            ),
+            req_id=request_id,
+            stage_id=stage_id,
+            operation="add_request",
+        ):
+            return
 
         if self.async_chunk and stage_id == 0 and final_stage_id > 0:
             await self._prewarm_async_chunk_stages(request_id, prompt, req_state)
@@ -717,12 +723,18 @@ class Orchestrator:
 
         req_state.streaming.enabled = True
         req_state.stage_submit_ts[stage_id] = _time.time()
-        await self.stage_pools[stage_id].submit_update(
-            request_id,
-            req_state,
-            request,
-            prompt_text=msg.output_prompt_text,
-        )
+        if not await self._dispatch_or_fail_request(
+            lambda: self.stage_pools[stage_id].submit_update(
+                request_id,
+                req_state,
+                request,
+                prompt_text=msg.output_prompt_text,
+            ),
+            req_id=request_id,
+            stage_id=stage_id,
+            operation="streaming_update",
+        ):
+            return
 
         if self.async_chunk and stage_id == 0 and final_stage_id > 0:
             await self._prewarm_async_chunk_stages(request_id, request, req_state)
@@ -757,20 +769,29 @@ class Orchestrator:
         )
         self.request_states[companion_id] = companion_state
         companion_state.stage_submit_ts[0] = _time.time()
-        companion_replica_id = await self.stage_pools[0].submit_initial(
-            companion_id,
-            companion_state,
-            companion_prompt,
-            prompt_text=msg.companion_prompt_text,
-            affinity_request_id=parent_id,
-        )
+        # A dead-stage failure is attributed to the parent request; its cleanup
+        # also releases this companion.
+        if not await self._dispatch_or_fail_request(
+            lambda: self.stage_pools[0].submit_initial(
+                companion_id,
+                companion_state,
+                companion_prompt,
+                prompt_text=msg.companion_prompt_text,
+                affinity_request_id=parent_id,
+            ),
+            req_id=parent_id,
+            stage_id=0,
+            operation=f"companion {companion_id}",
+            dispatch_req_id=companion_id,
+        ):
+            return
 
         logger.info(
             "[Orchestrator] CFG companion submitted: %s (role=%s, parent=%s, stage-0 replica-%s)",
             companion_id,
             role,
             parent_id,
-            companion_replica_id,
+            self.stage_pools[0].get_bound_replica_id(companion_id),
         )
 
     async def _handle_abort(self, msg: AbortRequestMessage) -> None:
@@ -903,16 +924,17 @@ class Orchestrator:
                     if self._shutdown_event.is_set():
                         return
 
-                    if pool.stage_type == "diffusion":
-                        output = pool.poll_diffusion_output(replica_id)
-                        if output is None:
-                            continue
+                    # Shared catch so a dead replica on either poll path is
+                    # evicted rather than tearing down every stage (#4285).
+                    try:
+                        if pool.stage_type == "diffusion":
+                            diffusion_output = pool.poll_diffusion_output(replica_id)
+                            if diffusion_output is None:
+                                continue
 
-                        pool.record_output_timestamps([output])
-                        await self._handle_processed_outputs(stage_id, replica_id, [output])
-                        idle = False
-                    else:
-                        try:
+                            pool.record_output_timestamps([diffusion_output])
+                            processed = [diffusion_output]
+                        else:
                             raw_outputs = await pool.poll_llm_raw_output(replica_id, timeout_s=0.001)
                             if raw_outputs is None:
                                 continue
@@ -944,7 +966,7 @@ class Orchestrator:
                             iteration_stats = (
                                 IterationStats() if (self._stat_logger is not None and raw_outputs.outputs) else None
                             )
-                            raw_output = await pool.process_llm_raw_outputs(
+                            processed = await pool.process_llm_raw_outputs(
                                 replica_id,
                                 raw_outputs,
                                 iteration_stats=iteration_stats,
@@ -957,80 +979,23 @@ class Orchestrator:
                                     iteration_stats,
                                     engine_idx=self._stage_replica_to_engine_idx[(stage_id, replica_id)],
                                 )
-                        except asyncio.CancelledError:
-                            raise
-                        except EngineDeadError as e:
-                            logger.error(
-                                "[Orchestrator] Stage-%s replica-%s is dead: %s",
-                                stage_id,
-                                replica_id,
-                                e,
-                            )
-                            affected_request_ids = pool.mark_replica_unavailable(replica_id)
-                            closed_sessions = (
-                                self.duplex_control_plane.close_sessions_for_request_ids(
-                                    affected_request_ids,
-                                    abort=False,
-                                )
-                                if self.duplex_control_plane is not None
-                                else {}
-                            )
-                            for session_id, stale_request_ids in closed_sessions.items():
-                                affected_request_ids.extend(stale_request_ids)
-                                logger.warning(
-                                    "[Orchestrator] closed duplex session %s after stage-%s replica-%s died; "
-                                    "stale_request_ids=%s",
-                                    session_id,
-                                    stage_id,
-                                    replica_id,
-                                    stale_request_ids,
-                                )
-                            affected_request_ids = list(dict.fromkeys(affected_request_ids))
-                            if pool.available_replica_ids():
-                                for req_id in affected_request_ids:
-                                    await self.output_async_queue.put(
-                                        ErrorMessage(
-                                            error=str(e),
-                                            fatal=False,
-                                            request_id=req_id,
-                                            stage_id=stage_id,
-                                        )
-                                    )
-                                await self._cleanup_request_ids(
-                                    affected_request_ids,
-                                    close_duplex_sessions=True,
-                                )
-                                continue
+                    except asyncio.CancelledError:
+                        raise
+                    except EngineDeadError as e:
+                        await self._handle_dead_replica(stage_id, replica_id, e)
+                        continue
+                    except Exception:
+                        if self._shutdown_event.is_set():
+                            return
+                        logger.exception(
+                            "[Orchestrator] Stage-%s replica-%s processing failed",
+                            stage_id,
+                            replica_id,
+                        )
+                        raise
 
-                            self._fatal_error = str(e)
-                            self._fatal_error_stage_id = stage_id
-                            for req_id in affected_request_ids:
-                                await self.output_async_queue.put(
-                                    ErrorMessage(
-                                        error=str(e),
-                                        fatal=True,
-                                        request_id=req_id,
-                                        stage_id=stage_id,
-                                    )
-                                )
-                            await self._cleanup_request_ids(
-                                affected_request_ids,
-                                close_duplex_sessions=True,
-                            )
-                            self._shutdown_event.set()
-                            raise
-                        except Exception:
-                            if self._shutdown_event.is_set():
-                                return
-                            logger.exception(
-                                "[Orchestrator] Stage-%s replica-%s processing failed",
-                                stage_id,
-                                replica_id,
-                            )
-                            raise
-
-                        await self._handle_processed_outputs(stage_id, replica_id, raw_output)
-                        idle = False
+                    await self._handle_processed_outputs(stage_id, replica_id, processed)
+                    idle = False
 
             self._orch_monitor.note_loop(idle=idle)
             if idle:
@@ -1091,6 +1056,143 @@ class Orchestrator:
             close_duplex_sessions=True,
         )
 
+    async def _handle_dead_replica(self, stage_id: int, replica_id: int, error: EngineDeadError) -> None:
+        """Evict a dead stage replica and fail the requests stranded on it (#4285).
+
+        A dead replica must not tear down the whole server: evict it and keep the
+        orchestrator loop alive so remaining live replicas / stages keep serving.
+        If the stage still has a live replica, fail only the requests bound to the
+        dead replica; requests bound to a surviving replica (or not yet bound)
+        keep running and can be (re)dispatched. If the stage has no live replica
+        left, every request routed through it must fail since none can be served.
+        """
+        pool = self.stage_pools[stage_id]
+        logger.error(
+            "[Orchestrator] Stage-%s replica-%s is dead; evicting it and failing its in-flight requests: %s",
+            stage_id,
+            replica_id,
+            error,
+        )
+        pool.evict_replica(replica_id)
+        stage_has_live = bool(pool.live_replica_ids())
+        failed_ids: list[str] = []
+        for req_id, req_state in list(self.request_states.items()):
+            if stage_id not in req_state.stage_submit_ts:
+                continue
+            bound = pool.get_bound_replica_id(req_id)
+            if bound == replica_id or not stage_has_live:
+                await self.output_async_queue.put(
+                    ErrorMessage(
+                        error=str(error),
+                        fatal=True,
+                        request_id=req_id,
+                        stage_id=stage_id,
+                    )
+                )
+                failed_ids.append(req_id)
+        # Use the shared cleanup path so the running counter, PD/CFG state, and
+        # bindings across every stage pool are released (not just this pool).
+        # abort=True stops the failed requests' in-flight work on surviving
+        # stages (abort skips dead/evicted bindings), matching the
+        # distributed-membership eviction path.
+        for req_id in failed_ids:
+            await self._cleanup_request_ids(
+                [req_id, *self._cfg_tracker.cleanup_parent(req_id)],
+                abort=True,
+            )
+
+    async def _fail_request_dead_stage(self, req_id: str, stage_id: int) -> None:
+        """Fail one request whose target stage has no live replica (#4285).
+
+        Used at dispatch sites so a fully dead stage fails just that request
+        instead of raising out of the request/orchestration task and tearing
+        down the server. The cleanup releases the running counter, PD/CFG state,
+        and bindings across every stage pool; it is a no-op for a request that
+        was never registered.
+        """
+        logger.error(
+            "[Orchestrator] req=%s: stage-%d has no live replica; failing the request",
+            req_id,
+            stage_id,
+        )
+        await self.output_async_queue.put(
+            ErrorMessage(
+                error=f"Stage-{stage_id} has no live replica",
+                fatal=True,
+                request_id=req_id,
+                stage_id=stage_id,
+            )
+        )
+        await self._cleanup_request_ids(
+            [req_id, *self._cfg_tracker.cleanup_parent(req_id)],
+            abort=True,
+        )
+
+    async def _dispatch_or_fail_request(
+        self,
+        dispatch: Callable[[], Awaitable[Any]],
+        *,
+        req_id: str,
+        stage_id: int,
+        operation: str,
+        dispatch_req_id: str | None = None,
+    ) -> bool:
+        """Run a dispatch action, converting stage unavailability into a
+        single-request failure.
+
+        ``dispatch`` is a thunk rather than a bare coroutine so that replica
+        selection and other argument setup run inside this guard: a synchronous
+        StageUnavailableError raised while building the dispatch is caught here,
+        not left to propagate out of the caller.
+
+        Replica eviction can interleave with any dispatch, surfacing as
+        StageUnavailableError (no live replica / evicted slot) or
+        EngineDeadError (replica died mid-submit before the poll loop evicted
+        it). Both mean "this request cannot be placed", not "the server is
+        broken": fail the request and keep serving (#4285). Unrelated errors
+        propagate. Returns False when the request was failed.
+
+        ``req_id`` is the request the failure is attributed to; ``dispatch_req_id``
+        is the id actually being dispatched when it differs (e.g. a CFG companion
+        submitted under its parent's attribution) and is used to locate the
+        replica that died.
+        """
+        try:
+            await dispatch()
+            return True
+        except StageUnavailableError as e:
+            # No specific replica to evict: the stage already has no live
+            # replica or the chosen slot was evicted. Fail just this request.
+            logger.error(
+                "[Orchestrator] %s dispatch for req=%s hit unavailable stage-%s: %s",
+                operation,
+                req_id,
+                stage_id,
+                e,
+            )
+            await self._fail_request_dead_stage(req_id, stage_id)
+            return False
+        except EngineDeadError as e:
+            # A live replica died mid-submit before the poll loop evicted it.
+            # Evict it now (and fail every request bound to it) so a burst of
+            # new requests stops round-robining onto the dead slot until the
+            # poll catches up. Capture the replica before failing this request,
+            # since the failure releases its binding.
+            pool = self.stage_pools[stage_id]
+            dead_replica = pool.get_bound_replica_id(dispatch_req_id or req_id)
+            logger.error(
+                "[Orchestrator] %s dispatch for req=%s hit dead stage-%s replica-%s: %s",
+                operation,
+                req_id,
+                stage_id,
+                dead_replica,
+                e,
+            )
+            await self._fail_request_dead_stage(req_id, stage_id)
+            if dead_replica is not None and dead_replica in pool.live_replica_ids():
+                await self._handle_dead_replica(stage_id, dead_replica, e)
+            return False
+
     # ---- Shared helpers ----
 
     async def _cleanup_request_ids(
@@ -1735,10 +1837,42 @@ class Orchestrator:
         src_replica_id: int | None = None,
         is_streaming_session: bool = False,
         is_final_update: bool = False,
+    ) -> None:
+        """Forward output to the next stage; a dead stage fails only this request."""
+        await self._dispatch_or_fail_request(
+            lambda: self._forward_to_next_stage_unguarded(
+                req_id,
+                src_stage_id,
+                output,
+                req_state,
+                src_replica_id=src_replica_id,
+                is_streaming_session=is_streaming_session,
+                is_final_update=is_final_update,
+            ),
+            req_id=req_id,
+            stage_id=src_stage_id + 1,
+            operation="inter-stage forward",
+        )
+
+    async def _forward_to_next_stage_unguarded(
+        self,
+        req_id: str,
+        src_stage_id: int,
+        output: Any,
+        req_state: OrchestratorRequestState,
+        *,
+        src_replica_id: int | None = None,
+        is_streaming_session: bool = False,
+        is_final_update: bool = False,
     ) -> None:
         """Forward output from the current logical stage to the next one."""
         next_logical = src_stage_id + 1
         next_pool = self.stage_pools[next_logical]
+        if not next_pool.live_replica_ids():
+            # The downstream stage lost all of its replicas before this in-flight
+            # request could be forwarded.
+            await self._fail_request_dead_stage(req_id, next_logical)
+            return
         next_client = next_pool.stage_client
         params = req_state.sampling_params_list[next_logical]
         source_outputs = [output]
@@ -2101,16 +2235,22 @@ class Orchestrator:
             _t_submit_start = _time.perf_counter()
 
             if next_pool.stage_type == "diffusion":
-                replica_id = await next_pool.submit_initial(
-                    request_id,
-                    req_state,
-                    req_state.prompt,
-                    submit_kwargs={
-                        "kv_sender_info": self._build_kv_sender_info(
-                            list(getattr(next_pool.stage_client, "engine_input_source", None) or [next_stage_id - 1]),
-                            request_id=request_id,
-                        )
-                    },
+                submit_kwargs = {
+                    "kv_sender_info": self._build_kv_sender_info(
+                        list(getattr(next_pool.stage_client, "engine_input_source", None) or [next_stage_id - 1]),
+                        request_id=request_id,
+                    )
+                }
+                submitted = await self._dispatch_or_fail_request(
+                    lambda: next_pool.submit_initial(
+                        request_id,
+                        req_state,
+                        req_state.prompt,
+                        submit_kwargs=submit_kwargs,
+                    ),
+                    req_id=request_id,
+                    stage_id=next_stage_id,
+                    operation="async-chunk prewarm",
                 )
             else:
                 import copy
@@ -2140,16 +2280,30 @@ class Orchestrator:
                     resumable=downstream_resumable,
                 )
                 request.external_req_id = request.request_id
-                replica_id = await next_pool.submit_initial(
-                    request_id,
-                    req_state,
-                    request,
-                    prompt_text=None,
+                submitted = await self._dispatch_or_fail_request(
+                    lambda: next_pool.submit_initial(
+                        request_id,
+                        req_state,
+                        request,
+                        prompt_text=None,
+                    ),
+                    req_id=request_id,
+                    stage_id=next_stage_id,
+                    operation="async-chunk prewarm",
                 )
+
+            # Attribute the failure to the stage that failed, not stage 0; the
+            # request is already cleaned up, so stop prewarming.
+            if not submitted:
+                return
+
+            # The guard swallows submit_initial's return value; the pool binding
+            # it recorded carries the same replica.
+            bound_replica_id = next_pool.get_bound_replica_id(request_id)
             self._record_duplex_stage_submission(
                 next_stage_id,
                 request_id,
-                replica_id,
+                bound_replica_id if bound_replica_id is not None else 0,
                 req_state,
             )
 
@@ -2201,53 +2355,6 @@ class Orchestrator:
 
     # ---- Shutdown / lifecycle ----
 
-    async def _drain_pending_requests_on_fatal(self) -> None:
-        """Drain the request queue and broadcast fatal errors for any
-        pending add_request messages that were never processed.
-
-        Called from the ``run()`` finally block when a fatal error
-        (e.g. ``EngineDeadError``) caused the orchestrator to shut down
-        before the request handler could process all queued messages.
-        Also broadcasts for any already-tracked requests still in
-        ``request_states`` that were not yet notified.
-        """
-        assert self._fatal_error is not None
-
-        notified: set[str] = set()
-
-        # 1) Drain pending messages from the request queue.
-        while True:
-            try:
-                msg = self.request_async_queue.get_nowait()
-            except Exception:
-                break
-            if msg.type == "add_request":
-                req_id = msg.request_id
-                await self.output_async_queue.put(
-                    ErrorMessage(
-                        error=self._fatal_error,
-                        fatal=True,
-                        request_id=req_id,
-                        stage_id=self._fatal_error_stage_id,
-                    )
-                )
-                notified.add(req_id)
-
-        # 2) Broadcast for any tracked requests not already notified
-        #    (e.g. request was registered but the EngineDeadError handler
-        #    missed it because it wasn't submitted to the dead stage yet).
-        for req_id in list(self.request_states):
-            if req_id not in notified:
-                await self.output_async_queue.put(
-                    ErrorMessage(
-                        error=self._fatal_error,
-                        fatal=True,
-                        request_id=req_id,
-                        stage_id=self._fatal_error_stage_id,
-                    )
-                )
-            self.request_states.pop(req_id, None)
-
     def _shutdown_stages(self) -> None:
         """Shutdown all stage pools."""
         if self._stages_shutdown:
diff --git a/vllm_omni/engine/stage_pool.py b/vllm_omni/engine/stage_pool.py
index 354032a7d..7d1bfc60a 100644
--- a/vllm_omni/engine/stage_pool.py
+++ b/vllm_omni/engine/stage_pool.py
@@ -44,6 +44,19 @@ if TYPE_CHECKING:
 logger = init_logger(__name__)
 
 
+class StageUnavailableError(RuntimeError):
+    """Raised when dispatch cannot reach a live replica of a stage.
+
+    Deliberately a ``RuntimeError`` subclass (not ``EngineDeadError``): no
+    engine died at the raise site — the stage's replica set is empty or the
+    chosen slot was evicted, which is a routing/capacity condition and, in
+    distributed mode, recoverable. Dispatch guards catch exactly this type so
+    unrelated ``RuntimeError``s keep failing fast, and existing
+    ``EngineDeadError`` handlers (teardown-on-dead, poll-path eviction) are
+    not silently enrolled.
+    """
+
+
 @dataclass
 class _ReplicaMetrics:
     """Per-replica metrics accumulators owned by a stage pool."""
@@ -329,7 +342,9 @@ class StagePool:
 
             now = _time.monotonic()
             if now >= deadline:
-                raise RuntimeError(f"no UP replica for stage {self.stage_id} after {self.DISPATCH_WAIT_TIMEOUT_S:.1f}s")
+                raise StageUnavailableError(
+                    f"no UP replica for stage {self.stage_id} after {self.DISPATCH_WAIT_TIMEOUT_S:.1f}s"
+                )
             await asyncio.sleep(min(self.DISPATCH_RETRY_INTERVAL_S, deadline - now))
 
     def preselect_replica_id(
@@ -555,7 +570,7 @@ class StagePool:
             # Prefer replicas that are both live (client up) and available.
             live = [r for r in self.live_replica_ids() if self.is_replica_available(r)]
             if not live:
-                raise RuntimeError(f"stage {self.stage_id} has no live replicas")
+                raise StageUnavailableError(f"stage {self.stage_id} has no live replicas")
             if len(live) == 1:
                 chosen = live[0]
             else:
@@ -570,13 +585,13 @@ class StagePool:
     def _llm_client(self, replica_id: int) -> StagePoolLLMClient:
         client = self.clients[replica_id]
         if client is None:
-            raise RuntimeError(f"stage {self.stage_id} replica {replica_id} is not attached")
+            raise StageUnavailableError(f"stage {self.stage_id} replica {replica_id} is not attached")
         return cast(StagePoolLLMClient, client)
 
     def _diffusion_client(self, replica_id: int) -> StagePoolDiffusionClient:
         client = self.clients[replica_id]
         if client is None:
-            raise RuntimeError(f"stage {self.stage_id} replica {replica_id} is not attached")
+            raise StageUnavailableError(f"stage {self.stage_id} replica {replica_id} is not attached")
         return cast(StagePoolDiffusionClient, client)
 
     # ---- Metrics ----
@@ -961,7 +976,7 @@ class StagePool:
         )
         client = self.clients[replica_id]
         if client is None:
-            raise RuntimeError(f"stage {self.stage_id} replica {replica_id} is not attached")
+            raise StageUnavailableError(f"stage {self.stage_id} replica {replica_id} is not attached")
         try:
             self.output_processor.add_request(
                 request=request,
@@ -1010,7 +1025,7 @@ class StagePool:
 
         client = self.clients[replica_id]
         if client is None:
-            raise RuntimeError(f"stage {self.stage_id} replica {replica_id} is not attached")
+            raise StageUnavailableError(f"stage {self.stage_id} replica {replica_id} is not attached")
 
         if self.stage_type == "diffusion":
             if isinstance(request, list):
@@ -1246,3 +1261,21 @@ class StagePool:
                 replica_id,
                 e,
             )
+
+    def evict_replica(self, replica_id: int) -> None:
+        """Shut down a replica and remove it from the live set.
+
+        After eviction ``live_replica_ids`` / ``live_num_replicas`` no longer
+        include this slot, so the orchestrator stops polling and dispatching to
+        it (per-replica fault isolation). The slot is left as a ``None`` hole,
+        consistent with ``num_replicas``.
+        """
+        # replica_id should always come from live_replica_ids() (a valid, live
+        # index). Fail loud on an out-of-range id rather than letting a negative
+        # index wrap or IndexError on the next line; an ``assert`` would be
+        # stripped under ``python -O``, silently leaving a dead replica in the
+        # live set and breaking isolation.
+        if not 0 <= replica_id < len(self.clients):
+            raise ValueError(f"evict_replica: replica_id {replica_id} out of range (num_replicas={len(self.clients)})")
+        self.shutdown_replica(replica_id)
+        self.clients[replica_id] = None
diff --git a/vllm_omni/entrypoints/async_omni.py b/vllm_omni/entrypoints/async_omni.py
index f0af77699..f6899deaa 100644
--- a/vllm_omni/entrypoints/async_omni.py
+++ b/vllm_omni/entrypoints/async_omni.py
@@ -906,17 +906,27 @@ class AsyncOmni(EngineClient, OmniBase):
                         await self.duplex_lifecycle_events.put(msg)
                         continue
 
-                    if isinstance(msg, ErrorMessage) and not msg.fatal:
-                        req_state = self.request_states.get(msg.request_id)
-                        if req_state is not None:
-                            await req_state.queue.put(msg)
-                        else:
-                            logger.warning(
-                                "[%s] dropping non-fatal error for unknown req %s",
-                                self._name,
-                                msg.request_id,
-                            )
-                        continue
+                    if isinstance(msg, ErrorMessage):
+                        # Route request-scoped errors to that request's queue and
+                        # keep the loop alive. A request whose stage replica died
+                        # and was evicted gets a fatal error delivered here; only
+                        # that request fails (its consumer raises), the server
+                        # stays up for other stages/requests (#4285). A fatal
+                        # error without a request_id is a genuine engine-wide
+                        # death and falls through to the except handler below.
+                        if msg.request_id is not None:
+                            req_state = self.request_states.get(msg.request_id)
+                            if req_state is not None:
+                                await req_state.queue.put(msg)
+                            else:
+                                logger.warning(
+                                    "[%s] dropping error for unknown req %s",
+                                    self._name,
+                                    msg.request_id,
+                                )
+                            continue
+                        if not msg.fatal:
+                            continue
 
                     should_continue, _, stage_id, req_state = self._handle_output_message(msg)
                     if should_continue:
@@ -1317,10 +1327,11 @@ class AsyncOmni(EngineClient, OmniBase):
 
     @property
     def errored(self) -> bool:
-        """Whether the engine is in a non-recoverable error state.
+        """Whether the engine is in a process-fatal error state.
 
-        Delegates to ``OmniBase.errored`` which checks the orchestrator
-        thread and all stage clients.  Redeclared here to satisfy the
+        Delegates to ``OmniBase.errored``, which is true only when the
+        orchestrator thread is dead; per-stage liveness is reported via
+        ``check_health`` instead.  Redeclared here to satisfy the
         ``EngineClient`` abstract-property requirement (Python's ABC
         mechanism does not resolve abstract methods from sibling MRO
         entries).
diff --git a/vllm_omni/entrypoints/omni_base.py b/vllm_omni/entrypoints/omni_base.py
index 3cae33661..587e0e73c 100644
--- a/vllm_omni/entrypoints/omni_base.py
+++ b/vllm_omni/entrypoints/omni_base.py
@@ -4,7 +4,7 @@ import os
 import time
 import weakref
 from collections.abc import Sequence
-from typing import Any, Literal
+from typing import TYPE_CHECKING, Any, Literal
 
 import huggingface_hub
 import vllm.envs as envs
@@ -31,6 +31,9 @@ from vllm_omni.model_executor.model_loader.weight_utils import download_weights_
 from vllm_omni.outputs import OmniRequestOutput
 from vllm_omni.utils.tracking_parser import TrackingNamespace
 
+if TYPE_CHECKING:
+    from vllm_omni.engine.stage_pool import StagePool, StagePoolClient
+
 logger = init_logger(__name__)
 
 
@@ -237,6 +240,42 @@ class OmniBase(PDDisaggregationMixin):
         """Expose engine stage configs for PD disaggregation detection and validation."""
         return self.engine.stage_configs
 
+    @staticmethod
+    def _replica_is_dead(client: StagePoolClient | None) -> bool:
+        """Whether a replica slot is evicted (``None``) or its engine is dead.
+
+        ``check_health()`` is the source of truth: it raises ``EngineDeadError``
+        when dead (inspecting the ``engine_dead`` flags internally), and the
+        diffusion client additionally runs a synchronous ``proc.is_alive()``
+        probe that catches a silent SIGKILL/segfault the monitor thread has not
+        flagged yet — so ``/health`` does not report 200 for a stage whose
+        subprocess is already dead. A client exposing no ``check_health`` cannot
+        confirm liveness, so it is treated as dead (fail closed).
+        """
+        if client is None:
+            return True
+        check_health = getattr(client, "check_health", None)
+        if not callable(check_health):
+            return True
+        try:
+            check_health()
+        except EngineDeadError:
+            return True
+        return False
+
+    def _live_replica_count(self, pool: StagePool) -> int:
+        """Number of replicas in ``pool`` that are neither evicted nor dead.
+
+        A dead replica is evicted from its pool (slot set to ``None`` in
+        ``StagePool.clients``); a replica that died but has not been evicted
+        yet still carries an ``engine_dead`` flag. Both are excluded.
+        """
+        return sum(1 for client in pool.clients if not self._replica_is_dead(client))
+
+    def _stage_has_no_live_replica(self, pool: StagePool) -> bool:
+        """True when a non-empty stage pool has lost all of its replicas."""
+        return len(pool.clients) > 0 and self._live_replica_count(pool) == 0
+
     def _consumed_metric_message_ids(self, request_id: str) -> set[int]:
         consumed_by_request = getattr(self, "_consumed_metric_messages", None)
         if consumed_by_request is None:
@@ -244,38 +283,33 @@ class OmniBase(PDDisaggregationMixin):
             self._consumed_metric_messages = consumed_by_request
         return consumed_by_request.setdefault(request_id, set())
 
-    def _has_dead_stage(self) -> bool:
-        for stage_client in self.engine.stage_clients:
-            if getattr(stage_client, "_engine_dead", False):
-                return True
-            resources = getattr(stage_client, "resources", None)
-            if resources is not None and getattr(resources, "engine_dead", False):
-                return True
-        return False
-
     @property
     def is_running(self) -> bool:
-        return self.engine.is_alive() and not self._has_dead_stage()
+        return self.engine.is_alive()
 
     @property
     def errored(self) -> bool:
-        """Whether the engine is in a non-recoverable error state.
+        """Whether the engine is in a process-fatal error state.
 
-        True when the orchestrator thread is dead **or** any stage client
-        has been marked dead (e.g. diffusion worker OOM / process death).
-
-        Checks both ``_engine_dead`` (StageDiffusionClient) and
-        ``resources.engine_dead`` (StageEngineCoreClient / AsyncMPClient)
-        since the two client types store the flag differently.
+        True only when the orchestrator thread is dead. Per-stage liveness is
+        deliberately excluded: the OpenAI serving paths precheck ``errored``
+        before request routing, so including it would reject requests that do
+        not touch the dead stage (e.g. text-only chat when the talker stage is
+        down). A fully dead stage instead fails only the requests routed
+        through it (dispatch guards) and flips readiness to 503 via
+        :meth:`check_health` (per-replica fault isolation, #4285).
         """
-        return not self.engine.is_alive() or self._has_dead_stage()
+        return not self.engine.is_alive()
 
     def check_health(self) -> None:
         if not self.engine.is_alive():
             raise EngineDeadError("Orchestrator process is not alive")
-        for stage_client in self.engine.stage_clients:
-            if hasattr(stage_client, "check_health"):
-                stage_client.check_health()
+        pools = getattr(self.engine, "stage_pools", None)
+        if pools is None:
+            return
+        for pool in pools:
+            if self._stage_has_no_live_replica(pool):
+                raise EngineDeadError(f"Stage-{pool.stage_id} has no live replica")
 
     def resolve_sampling_params_list(
         self,
diff --git a/vllm_omni/model_executor/models/step_audio2/step_audio2_thinker.py b/vllm_omni/model_executor/models/step_audio2/step_audio2_thinker.py
index c92f23978..b56229cf6 100644
--- a/vllm_omni/model_executor/models/step_audio2/step_audio2_thinker.py
+++ b/vllm_omni/model_executor/models/step_audio2/step_audio2_thinker.py
@@ -758,7 +758,11 @@ class StepAudio2ThinkerForConditionalGeneration(nn.Module, SupportsMultiModal, S
 
         audio_feature_lens = (audio_lens - 1) // 2 + 1
 
-        audio_feature_list = [audio_features[i, : audio_feature_lens[i]] for i in range(audio_features.size(0))]
+        # Single host sync; slicing with per-item 0-d cuda tensors syncs once per item.
+        # Safe: _process_audio_input runs in the eager multimodal-encoder prefill path,
+        # never inside a decode CUDA-graph capture, so this .tolist() host sync is fine.
+        audio_feature_lens_cpu = audio_feature_lens.tolist()
+        audio_feature_list = [audio_features[i, : audio_feature_lens_cpu[i]] for i in range(audio_features.size(0))]
 
         return audio_feature_list
 
diff --git a/vllm_omni/model_executor/models/step_audio2/step_audio2_token2wav.py b/vllm_omni/model_executor/models/step_audio2/step_audio2_token2wav.py
index b116b6bcd..9666a4f5a 100644
--- a/vllm_omni/model_executor/models/step_audio2/step_audio2_token2wav.py
+++ b/vllm_omni/model_executor/models/step_audio2/step_audio2_token2wav.py
@@ -364,7 +364,7 @@ class StepAudio2Token2WavCore(nn.Module):
 
     def stream_chunk_for(
         self,
-        audio_tokens: list[int],
+        audio_tokens: list[int] | torch.Tensor,
         prompt_wav: str,
         last_chunk: bool,
         state: _StreamState,
@@ -377,7 +377,14 @@ class StepAudio2Token2WavCore(nn.Module):
             self.cache[prompt_wav] = self._prepare_prompt(prompt_wav)
         _, _, spk_emb, prompt_mels, _ = self.cache[prompt_wav]
 
-        token_tensor = torch.tensor([audio_tokens], dtype=torch.int32, device=self.device)
+        if isinstance(audio_tokens, torch.Tensor):
+            # Already on device; avoids the D2H->H2D round-trip of rebuilding from a list.
+            # clone() so token_tensor never aliases the caller's input_ids: when the ids
+            # already arrive int32-on-device the .to() is a no-op and would share storage,
+            # so an in-place write inside inference_chunk could corrupt input_ids.
+            token_tensor = audio_tokens.reshape(1, -1).to(device=self.device, dtype=torch.int32).clone()
+        else:
+            token_tensor = torch.tensor([audio_tokens], dtype=torch.int32, device=self.device)
 
         with torch.amp.autocast(
             str(self.device.type),
@@ -651,10 +658,11 @@ class StepAudio2Token2WavForConditionalGeneration(nn.Module, SupportsPP):
         state = self._stream_states[0]
 
         # --- Extract audio tokens ---
-        audio_tokens = input_ids.flatten().cpu().tolist()
+        # Kept on device; stream_chunk_for takes the tensor directly.
+        audio_tokens = input_ids.flatten()
 
         # Empty chunk (e.g. EOF with no audio tokens)
-        if not audio_tokens:
+        if audio_tokens.numel() == 0:
             if state.setup_done:
                 self.token2wav.reset_stream_for(state)
             else:
diff --git a/vllm_omni/model_executor/models/voxtral_tts/cuda_graph_acoustic_transformer_wrapper.py b/vllm_omni/model_executor/models/voxtral_tts/cuda_graph_acoustic_transformer_wrapper.py
index 5898136c9..c9cc2a3d9 100644
--- a/vllm_omni/model_executor/models/voxtral_tts/cuda_graph_acoustic_transformer_wrapper.py
+++ b/vllm_omni/model_executor/models/voxtral_tts/cuda_graph_acoustic_transformer_wrapper.py
@@ -74,6 +74,12 @@ class CUDAGraphAcousticTransformerWrapper:
 
         # Pre-create persistent buffers
         self.timesteps = torch.linspace(0, 1, self.n_steps + 1, device=device, dtype=dtype)
+        # Precompute schedule constants (dt, projected time embeddings) so the captured
+        # graph indexes a table instead of re-running them on every replay.
+        self.dts = self.timesteps[1:] - self.timesteps[:-1]
+        with torch.no_grad():
+            t_emb_table = self.acoustic_transformer.time_embedding(self.timesteps.view(-1, 1)).to(dtype)
+            self.t_proj_table = self.acoustic_transformer.time_projection(t_emb_table)
         self.fake_eos_one = torch.tensor(1.0, dtype=dtype, device=device)
         self.fake_eos_zero = torch.tensor(0.0, dtype=dtype, device=device)
 
@@ -142,21 +148,21 @@ class CUDAGraphAcousticTransformerWrapper:
 
         x = noise
 
-        # Pre-compute zero hidden states for unconditional CFG branch
-        hidden_states_zero = torch.zeros_like(hidden_states)
+        # Hoist the step-invariant concat + llm_projection out of the loop; inside the
+        # captured graph these would otherwise be baked in and replayed every step.
+        llm_batched = torch.cat([hidden_states, torch.zeros_like(hidden_states)], dim=0)  # (2B, D)
+        llm_proj_batched = at.llm_projection(llm_batched)  # (2B, D)
 
         timesteps = self.timesteps
         for i in range(len(timesteps) - 1):
-            t = timesteps[i]
-            dt = timesteps[i + 1] - timesteps[i]
+            dt = self.dts[i]
 
-            # Batch conditional + unconditional velocity in a single forward pass
-            t_emb = at.time_embedding(t.view(-1, 1).repeat(B, 1)).to(hidden_states.dtype)
+            # Batch cond + uncond in a single pass; reuse the cached projected-t_emb row.
+            t_proj = self.t_proj_table[i].unsqueeze(0).expand(B, -1)
             x_batched = torch.cat([x, x], dim=0)  # (2B, C)
-            llm_batched = torch.cat([hidden_states, hidden_states_zero], dim=0)  # (2B, D)
-            t_emb_batched = t_emb.repeat(2, 1)  # (2B, D)
+            t_proj_batched = t_proj.repeat(2, 1)  # (2B, D)
 
-            v_all = at._predict_velocity(x_t=x_batched, llm_output=llm_batched, t_emb=t_emb_batched)
+            v_all = at._predict_velocity(x_t=x_batched, llm_proj=llm_proj_batched, t_proj=t_proj_batched)
             v_t, uncond_v_t = v_all[:B], v_all[B:]
 
             # CFG combination (cfg_alpha is (B, 1), v_t is (B, C))
@@ -239,6 +245,10 @@ class CUDAGraphAcousticTransformerWrapper:
         if not self.enabled or not self._warmed_up:
             return self.model.compute_mm_logits(hidden_states, cfg_alpha=cfg_alpha)
 
+        # Inner graph replay is illegal during an outer stream capture.
+        if torch.cuda.is_current_stream_capturing():
+            return self.model.compute_mm_logits(hidden_states, cfg_alpha=cfg_alpha)
+
         padded_size = self._get_padded_size(actual_size)
         if padded_size is None or padded_size not in self.graphs:
             return self.model.compute_mm_logits(hidden_states, cfg_alpha=cfg_alpha)
diff --git a/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts.py b/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts.py
index 40d4ec620..69cfa6d26 100644
--- a/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts.py
+++ b/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts.py
@@ -45,10 +45,12 @@ def parse_batched_audio_input(input_ids: torch.Tensor, num_codebooks: int) -> tu
     """
     all_audio_tokens: list[torch.Tensor] = []
     all_ctx_frames: list[int] = []
+    # One D2H copy up-front avoids 2 syncs per request inside the loop.
+    header_view = input_ids.cpu() if input_ids.is_cuda else input_ids
     offset = 0
     while offset < input_ids.numel():
-        ctx_frames = int(input_ids[offset].item())
-        context_length = int(input_ids[offset + 1].item())
+        ctx_frames = int(header_view[offset])
+        context_length = int(header_view[offset + 1])
         offset += 2
 
         req_input_ids_length = (ctx_frames + context_length) * num_codebooks
@@ -155,6 +157,9 @@ class VoxtralTTSForConditionalGeneration(
                 architectures=["VoxtralTTSAudioTokenizer"],
             )
             self.model = self.audio_tokenizer
+            # forward() returns a runtime-length per-request list; FULL CUDAGraphWrapper
+            # would freeze that length at capture.
+            self.supports_cudagraph_full = False
         else:
             raise ValueError("Invalid model stage")
 
diff --git a/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts_audio_generation.py b/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts_audio_generation.py
index 370639fb0..4f63bbec1 100644
--- a/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts_audio_generation.py
+++ b/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts_audio_generation.py
@@ -441,6 +441,8 @@ class FlowMatchingAudioTransformer(nn.Module):
             torch.linspace(0, 1, self._n_steps + 1),
             persistent=False,
         )
+        # Lazy per-dtype cache of schedule constants: (timesteps, t_proj_table, dts) on device.
+        self._timesteps_cache: dict[torch.dtype, tuple[torch.Tensor, torch.Tensor, torch.Tensor]] = {}
 
     def load_weight(self, weight: tuple[str, torch.Tensor]) -> str:
         params_dict = dict(self.named_parameters())
@@ -514,12 +516,25 @@ class FlowMatchingAudioTransformer(nn.Module):
         # Skip decoding if codebook 0 is [END_AUDIO] token.
         should_decode = semantic_code != self._end_audio_token_id
 
-        # acoustic_codes starts from x_0
-        x_0 = torch.randn(B, self.model_args.n_acoustic_codebook).to(dtype=llm_hidden.dtype, device=llm_hidden.device)
+        # acoustic_codes starts from x_0; generate directly on device to skip H2D.
+        x_0 = torch.randn(B, self.model_args.n_acoustic_codebook, dtype=llm_hidden.dtype, device=llm_hidden.device)
         x_0 = self._noise_scale * x_0
 
-        timesteps = self._timesteps.to(dtype=llm_hidden.dtype)
-        llm_hidden_zero = torch.zeros_like(llm_hidden)
+        # Build the schedule constants once per dtype and reuse them every frame.
+        cache = self._timesteps_cache.get(llm_hidden.dtype)
+        if cache is None:
+            timesteps_d = self._timesteps.to(dtype=llm_hidden.dtype)
+            t_emb_table = self.time_embedding(timesteps_d.view(-1, 1)).to(llm_hidden.dtype)
+            t_proj_table = self.time_projection(t_emb_table)
+            dts = timesteps_d[1:] - timesteps_d[:-1]
+            self._timesteps_cache[llm_hidden.dtype] = (timesteps_d, t_proj_table, dts)
+            cache = self._timesteps_cache[llm_hidden.dtype]
+        timesteps, t_proj_table, dts = cache
+
+        # Hoist the step-invariant concat + llm_projection out of the loop; inside the
+        # captured graph these would otherwise replay every step.
+        llm_batched = torch.cat([llm_hidden, torch.zeros_like(llm_hidden)], dim=0)
+        llm_proj_batched = self.llm_projection(llm_batched)
 
         # Reshape cfg_alpha for broadcasting: (B,) -> (B, 1)
         cfg_alpha = cfg_alpha.to(dtype=llm_hidden.dtype, device=llm_hidden.device)
@@ -528,20 +543,19 @@ class FlowMatchingAudioTransformer(nn.Module):
         # Euler integration with batched conditional + unconditional velocity
         sampled = x_0
         for i in range(len(timesteps) - 1):
-            t = timesteps[i]
-            dt = timesteps[i + 1] - timesteps[i]
+            dt = dts[i]  # precomputed constant step size
 
-            t_emb = self.time_embedding(t.view(-1, 1).repeat(B, 1)).to(llm_hidden.dtype)
+            # Reuse cached projected-t_emb row, expanded along batch dim.
+            t_proj = t_proj_table[i].unsqueeze(0).expand(B, -1)
 
             # Batch cond + uncond into a single forward pass (2B batch)
             x_batched = torch.cat([sampled, sampled], dim=0)
-            llm_batched = torch.cat([llm_hidden, llm_hidden_zero], dim=0)
-            t_emb_batched = torch.cat([t_emb, t_emb], dim=0)
+            t_proj_batched = torch.cat([t_proj, t_proj], dim=0)
 
             v_all = self._predict_velocity(
                 x_t=x_batched,
-                llm_output=llm_batched,
-                t_emb=t_emb_batched,
+                llm_proj=llm_proj_batched,
+                t_proj=t_proj_batched,
             )
             v_t, uncond_v_t = v_all[:B], v_all[B:]
             v_t = cfg_alpha * v_t + (1 - cfg_alpha) * uncond_v_t
@@ -558,18 +572,16 @@ class FlowMatchingAudioTransformer(nn.Module):
     def _predict_velocity(
         self,
         x_t: torch.Tensor,  # BxC
-        llm_output: torch.Tensor,  # BxD
-        t_emb: torch.Tensor,  # BxD
+        llm_proj: torch.Tensor,  # BxD, already through llm_projection
+        t_proj: torch.Tensor,  # BxD, already through time_projection
     ) -> torch.Tensor:
-        x_t = x_t.to(llm_output.dtype)
-
-        t_emb = self.time_projection(t_emb)
-        llm_output = self.llm_projection(llm_output)
+        # llm_proj/t_proj arrive pre-projected (hoisted out of the per-step loop by callers).
+        x_t = x_t.to(llm_proj.dtype)
 
         acoustic_and_semantic_embeddings = [
             self.input_projection(x_t.unsqueeze(1)),  # Bx1xD
-            t_emb.unsqueeze(1),
-            llm_output.unsqueeze(1),
+            t_proj.unsqueeze(1),
+            llm_proj.unsqueeze(1),
         ]
         acoustic_transformer_inputs = torch.concatenate(acoustic_and_semantic_embeddings, dim=1)
 
@@ -954,6 +966,8 @@ class VoxtralTTSAudioGenerationForConditionalGeneration(nn.Module, SupportsMulti
         self.audio_tok_id = audio_encoder.audio_token
         self.eos_tok_id = self.tokenizer.instruct.tokenizer.eos_id
         self.vocab_size = config.text_config.vocab_size
+        self._end_audio_token_id = AudioSpecialTokens.id(AudioSpecialTokens.end_audio)
+        self._fake_eos_consts: dict[torch.device, tuple[torch.Tensor, torch.Tensor]] = {}
 
     def get_language_model(self) -> torch.nn.Module:
         return self.language_model
@@ -1052,11 +1066,15 @@ class VoxtralTTSAudioGenerationForConditionalGeneration(nn.Module, SupportsMulti
             llm_hidden=hidden_states,
             cfg_alpha=cfg_alpha,
         )
-        fake_eos = torch.where(
-            audio_codes[:, 0] == AudioSpecialTokens.id(AudioSpecialTokens.end_audio),
-            torch.tensor(1.0, dtype=torch.bfloat16),
-            torch.tensor(0.0, dtype=torch.bfloat16),
-        )
+        # Cache device-resident 1.0/0.0 scalars to avoid a per-call H2D transfer.
+        consts = self._fake_eos_consts.get(audio_codes.device)
+        if consts is None:
+            consts = (
+                torch.tensor(1.0, dtype=torch.bfloat16, device=audio_codes.device),
+                torch.tensor(0.0, dtype=torch.bfloat16, device=audio_codes.device),
+            )
+            self._fake_eos_consts[audio_codes.device] = consts
+        fake_eos = torch.where(audio_codes[:, 0] == self._end_audio_token_id, consts[0], consts[1])
         # BxC -> Bx1xC since this is per-step
         # Make it a list for vllm-omni processing
         audio_list = list(torch.split(audio_codes.unsqueeze(1), 1, dim=0))
diff --git a/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts_audio_tokenizer.py b/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts_audio_tokenizer.py
index 57a86a5e9..9be16c0f1 100644
--- a/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts_audio_tokenizer.py
+++ b/vllm_omni/model_executor/models/voxtral_tts/voxtral_tts_audio_tokenizer.py
@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+import logging
 import math
 from copy import deepcopy
 from dataclasses import dataclass
@@ -9,6 +10,7 @@ import numpy as np
 import torch
 import torch.nn as nn
 import torch.nn.functional as F
+from torch.nn.utils.rnn import pad_sequence
 from vllm.config import VllmConfig
 
 from vllm_omni.model_executor.models.voxtral_tts.voxtral_tts_audio_generation import (
@@ -485,9 +487,10 @@ class Attention(nn.Module):
                 )
             return slopes  # shape [n_heads], values in (2^-8, 1]
 
+        # Keep alibi slopes in fp32 so `forward` can skip the per-call `.to(fp32)` cast.
         self.register_buffer(
             "alibi_slopes",
-            get_alibi_slopes(self.n_local_heads),
+            get_alibi_slopes(self.n_local_heads).to(torch.float32).contiguous(),
             persistent=False,
         )
 
@@ -552,8 +555,8 @@ class Attention(nn.Module):
         # rel_pos[i, j] = j - i
         rel_pos = positions.unsqueeze(0) - positions.unsqueeze(1)  # (S, S)
 
-        # Alibi bias: slope * (j - i), shape (H, S, S)
-        alibi_slopes = self.alibi_slopes.to(dtype=xq.dtype, device=xq.device)
+        # Alibi bias: slope * (j - i), shape (H, S, S). Cast only to xq dtype (already on device).
+        alibi_slopes = self.alibi_slopes.to(dtype=xq.dtype)
         attn_bias = alibi_slopes.view(H, 1, 1) * rel_pos.unsqueeze(0).to(xq.dtype)
 
         # Causal mask: mask out j > i (future positions)
@@ -584,7 +587,7 @@ class Attention(nn.Module):
         xv = xv.view(bsz, seqlen, self.n_local_kv_heads, self.args.head_dim)
 
         if HAS_FLASH_ATTN:
-            alibi_slopes = self.alibi_slopes.to(torch.float32)
+            # alibi_slopes already fp32 — avoid per-call cast.
             output = flash_attn_func(
                 xq,
                 xk,
@@ -594,7 +597,7 @@ class Attention(nn.Module):
                     self.sliding_window,
                     0 if self.args.causal else self.sliding_window,
                 ),
-                alibi_slopes=alibi_slopes,
+                alibi_slopes=self.alibi_slopes,
             )
         else:
             output = self._native_attention(xq, xk, xv)
@@ -1052,14 +1055,15 @@ class VoxtralTTSAudioTokenizer(nn.Module):
         """
         chunk_size = 375  # TODO(chenyos): Hardcode. Fix it
 
-        # Pre-process: find EOA and unshift tokens
-        processed = []
-        for codes in codes_list:
-            eoa_mask = codes[:, 0] == 1
-            eoa_indices = eoa_mask.nonzero(as_tuple=False)
-            cutting_point = eoa_indices[0].item() if len(eoa_indices) > 0 else len(codes)
-            audio_tokens = codes[:cutting_point] - 2
-            processed.append(audio_tokens)
+        # Compute per-request EOA cutting points in a single D2H instead of one sync per request.
+        flags = []
+        for c in codes_list:
+            eoa_mask = c[:, 0] == 1
+            flags.append(torch.stack([eoa_mask.any().to(torch.long), eoa_mask.long().argmax()]))
+        resolved = torch.stack(flags).cpu().tolist() if flags else []
+        cutting_points = [int(idx) if has else c.shape[0] for (has, idx), c in zip(resolved, codes_list)]
+
+        processed = [c[:cp] - 2 for c, cp in zip(codes_list, cutting_points)]
 
         # Separate empty and non-empty
         results: list[torch.Tensor | None] = [None] * len(processed)
@@ -1087,25 +1091,19 @@ class VoxtralTTSAudioTokenizer(nn.Module):
             chunk_map.append((orig_idx, req_chunk_indices))
 
         # Pad chunks to max length and batch decode
-        max_chunk_len = max(chunk_lengths)
-        K = all_chunks[0].shape[1]
-        padded = torch.zeros(
-            len(all_chunks),
-            max_chunk_len,
-            K,
-            dtype=all_chunks[0].dtype,
-            device=all_chunks[0].device,
-        )
-        for i, chunk in enumerate(all_chunks):
-            padded[i, : len(chunk)] = chunk
+        padded = pad_sequence(all_chunks, batch_first=True, padding_value=0.0)
 
         audio_codes = padded.to(device=current_omni_platform.device_type)  # [B, T, K]
         audio_values = self.decode(audio_codes.transpose(1, 2), dtype=torch.bfloat16)  # [B, 1, T_out]
         audio_values = audio_values.detach().cpu().float().squeeze(1)  # [B, T_out]
-        if torch.min(audio_values) < -1.0:
-            logger.warning("Min value of decoded waveform signal is %s", torch.min(audio_values))
-        if torch.max(audio_values) > 1.0:
-            logger.warning("Max value of decoded waveform signal is %s", torch.max(audio_values))
+        # Gate min/max reductions behind logger level.
+        if logger.isEnabledFor(logging.WARNING):
+            amin = float(audio_values.min())
+            amax = float(audio_values.max())
+            if amin < -1.0:
+                logger.warning("Min value of decoded waveform signal is %s", amin)
+            if amax > 1.0:
+                logger.warning("Max value of decoded waveform signal is %s", amax)
 
         # Trim padding and reassemble per request
         for orig_idx, chunk_indices in chunk_map: