diff --git a/docs/configuration/stage_configs.md b/docs/configuration/stage_configs.md
index 1deb7946c..66113a5f9 100644
--- a/docs/configuration/stage_configs.md
+++ b/docs/configuration/stage_configs.md
@@ -94,12 +94,14 @@ Each entry under `stages:` accepts any `StageDeployConfig` field directly (no ne
 | `max_num_batched_tokens` | int \| null | optional | `null` | Per-stage prefill/token budget; also contributes to the native maximum in-flight token limit. |
 | `max_model_len` | int \| null | optional | `null` | Per-sequence context or KV length; `-1` enables native cache-capacity auto-fitting, while values above the HF default auto-set `VLLM_ALLOW_LONG_MAX_MODEL_LEN=1`. |
 | `async_scheduling` | bool \| null | optional | `null` | Per-stage async scheduling toggle. |
-| `devices` | str \| null | optional | `null` | Device list assigned to this stage. |
+| `devices` | str \| null | optional | `null` | Device list assigned to this stage. The number of device ids must equal this stage's local world size (`tensor_parallel_size` × local data-parallel size × `pipeline_parallel_size`, or `num_replicas` × that product for a replica pool); a mismatch fails early — see the note below. |
 | `output_connectors` | dict \| null | optional | `null` | Keyed by `to_stage_<n>`; values are names registered under top-level `connectors:`. |
 | `input_connectors` | dict \| null | optional | `null` | Keyed by `from_stage_<n>`; values are names registered under top-level `connectors:`. |
 | `default_sampling_params` | dict \| null | optional | `null` | Baseline sampling params. Deep-merged with pipeline `sampling_constraints` (pipeline wins). |
 | `engine_extras` | dict | optional | `{}` | Catch-all for engine fields not listed above; deep-merged across overlays and forwarded to the stage engine. |
 
+**Note:** a stage's `devices` count must equal its local world size (`tensor_parallel_size` × `data_parallel_size_local` × `pipeline_parallel_size`, falling back to global `data_parallel_size` when the local size is unset), or `num_replicas` × that product for a replica pool. A mismatch fails early and names the offending stage. A top-level `--tensor-parallel-size` is broadcast to every stage, so it can make a single-GPU stage violate this contract ([issue #5003](gh-issue:5003)); fix that case with `--stage-overrides` (set `tensor_parallel_size` and `devices` together per stage) or set TP only on the multi-GPU stage.
+
 ### Connector schema
 
 Each entry under top-level `connectors:` follows this shape:
diff --git a/tests/engine/test_stage_device_layout.py b/tests/engine/test_stage_device_layout.py
new file mode 100644
index 000000000..0473dc341
--- /dev/null
+++ b/tests/engine/test_stage_device_layout.py
@@ -0,0 +1,229 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
+
+"""Regression tests for issue #5003: a per-stage world size that its assigned
+``devices`` cannot satisfy must fail early in ``build_vllm_config`` with a clear
+message, rather than surfacing as an opaque worker-side ``local rank ... out of
+bounds`` assertion.
+
+Root cause: a top-level ``--tensor-parallel-size`` is broadcast to every stage,
+but each stage's ``devices`` is not adjusted, so a stage can end up with e.g.
+tensor_parallel_size=4 while still holding a single-GPU deploy default. Without
+``--strategy-config`` the strategy-path device check never runs.
+"""
+
+import json
+import re
+import types
+from unittest import mock
+
+import pytest
+
+from vllm_omni.engine import stage_init_utils
+from vllm_omni.engine.stage_init_utils import (
+    _check_stage_device_layout,
+    build_vllm_config,
+    compute_replica_layout,
+    get_stage_devices_per_replica,
+)
+
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
+
+def _stage(stage_id, devices, num_replicas=1, engine_args=None):
+    return types.SimpleNamespace(
+        stage_id=stage_id,
+        stage_type="llm",
+        engine_args=engine_args or {},
+        runtime=types.SimpleNamespace(devices=devices, num_replicas=num_replicas),
+    )
+
+
+def test_tp_broadcast_without_devices_fails_early():
+    """stage0 gets tensor_parallel_size=4 (broadcast) but only 1 device -> clear error."""
+    stage = _stage(0, devices="0")
+    engine_args = {
+        "tensor_parallel_size": 4,
+        "data_parallel_size": 1,
+        "pipeline_parallel_size": 1,
+    }
+    with pytest.raises(ValueError) as excinfo:
+        _check_stage_device_layout(stage, engine_args)
+    msg = str(excinfo.value)
+    # Message names the stage, the mismatch, and the actionable workaround.
+    assert "Stage 0" in msg
+    assert "1 device" in msg and "4" in msg
+    assert "--stage-overrides" in msg
+
+
+def test_workaround_example_sets_tp_on_every_stage():
+    """The JSON example in the error message must not reproduce the bug: every
+    stage entry it suggests has to set ``tensor_parallel_size`` (a single-GPU
+    stage that only overrides ``devices`` would still inherit the broadcast
+    tp and crash again — the original #5003 failure mode)."""
+    stage = _stage(0, devices="0")
+    with pytest.raises(ValueError) as excinfo:
+        _check_stage_device_layout(stage, {"tensor_parallel_size": 4})
+    msg = str(excinfo.value)
+
+    match = re.search(r"\{.*\}", msg, re.DOTALL)
+    assert match, f"no JSON example found in error message: {msg}"
+    example = json.loads(match.group(0))
+
+    assert example, "example must contain at least one stage"
+    for stage_id, overrides in example.items():
+        assert "tensor_parallel_size" in overrides, (
+            f"stage {stage_id} in the suggested workaround omits "
+            f"tensor_parallel_size, which reproduces #5003: {overrides}"
+        )
+
+
+def test_consistent_tp_and_devices_pass():
+    """tensor_parallel_size=4 with 4 assigned devices is valid."""
+    stage = _stage(0, devices="0,1,2,3")
+    _check_stage_device_layout(
+        stage,
+        {"tensor_parallel_size": 4, "data_parallel_size": 1, "pipeline_parallel_size": 1},
+    )
+
+
+def test_single_gpu_stage_passes():
+    """A TP=1 stage on a single device (talker/code2wav default) is valid."""
+    stage = _stage(1, devices="1")
+    _check_stage_device_layout(stage, {"tensor_parallel_size": 1})
+
+
+def test_missing_devices_is_skipped():
+    """No explicit devices -> vLLM assigns them; nothing to validate here."""
+    stage = _stage(0, devices=None)
+    _check_stage_device_layout(stage, {"tensor_parallel_size": 4})
+
+
+def test_replica_pool_layout_passes():
+    """Pool mode: num_replicas=2 x tp=2 => 4 devices is a valid pool shape."""
+    stage = _stage(0, devices="0,1,2,3", num_replicas=2)
+    _check_stage_device_layout(stage, {"tensor_parallel_size": 2})
+
+
+def test_multinode_dp_uses_local_width_for_device_validation():
+    """One local DP engine needs one device even when global DP spans four nodes."""
+    stage = _stage(0, devices="0")
+    _check_stage_device_layout(
+        stage,
+        {
+            "tensor_parallel_size": 1,
+            "data_parallel_size": 4,
+            "data_parallel_size_local": 1,
+            "pipeline_parallel_size": 1,
+        },
+    )
+
+
+def test_zero_local_dp_skips_local_device_validation():
+    """A head process with no local engines does not own a local device layout."""
+    stage = _stage(0, devices="0")
+    _check_stage_device_layout(
+        stage,
+        {
+            "tensor_parallel_size": 4,
+            "data_parallel_size": 4,
+            "data_parallel_size_local": 0,
+            "pipeline_parallel_size": 1,
+        },
+    )
+
+
+@pytest.mark.parametrize(
+    ("devices", "expected"),
+    [
+        ("0,1", ["0,1", "2,3"]),
+        ("0,1,2,3", ["0,1", "2,3"]),
+    ],
+)
+def test_replica_guard_and_splitter_share_local_world_size(devices, expected):
+    """PP and local DP contribute to the same per-replica size in both paths."""
+    engine_args = {
+        "tensor_parallel_size": 1,
+        "data_parallel_size": 4,
+        "data_parallel_size_local": 1,
+        "pipeline_parallel_size": 2,
+    }
+    stage = _stage(0, devices=devices, num_replicas=2, engine_args=engine_args)
+
+    _check_stage_device_layout(stage, engine_args)
+    assert get_stage_devices_per_replica(stage) == 2
+    _, replica_devices_map = compute_replica_layout([stage])
+    assert replica_devices_map == {0: expected}
+
+
+def test_non_tp_layout_error_uses_generic_guidance():
+    """A PP mismatch must not be reported as a top-level TP broadcast."""
+    stage = _stage(0, devices="0")
+    with pytest.raises(ValueError) as excinfo:
+        _check_stage_device_layout(
+            stage,
+            {
+                "tensor_parallel_size": 1,
+                "data_parallel_size": 1,
+                "pipeline_parallel_size": 2,
+            },
+        )
+
+    msg = str(excinfo.value)
+    assert "local world size" in msg
+    assert "top-level --tensor-parallel-size" not in msg
+
+
+def test_build_vllm_config_fails_before_engine_config_on_mismatch():
+    """Pin the production wiring: ``build_vllm_config`` must run the device-layout
+    guard *before* ``create_engine_config``. If the guard call is dropped or moved
+    after engine-config creation, #5003 regresses while the unit tests above stay
+    green — so assert here that a mismatched stage raises and that neither
+    ``create_engine_config`` nor ``Executor.get_class`` is reached."""
+    stage = _stage(0, devices="0")
+    with (
+        mock.patch.object(stage_init_utils.OmniEngineArgs, "create_engine_config") as create_engine_config,
+        mock.patch.object(stage_init_utils.Executor, "get_class") as get_class,
+    ):
+        with pytest.raises(ValueError) as excinfo:
+            build_vllm_config(
+                stage,
+                model="dummy-model",
+                engine_args_dict={
+                    "tensor_parallel_size": 4,
+                    "data_parallel_size": 1,
+                    "pipeline_parallel_size": 1,
+                },
+            )
+    msg = str(excinfo.value)
+    assert "Stage 0" in msg
+    assert "--stage-overrides" in msg
+    # Guard fired early: worker/config construction was never reached.
+    create_engine_config.assert_not_called()
+    get_class.assert_not_called()
+
+
+def test_build_vllm_config_proceeds_on_consistent_layout():
+    """The guard must not false-positive: a consistent single-GPU stage flows past
+    it and reaches ``create_engine_config`` / ``Executor.get_class`` as usual."""
+    stage = _stage(1, devices="1")
+    fake_config = types.SimpleNamespace(
+        quant_config=None,
+        model_config=types.SimpleNamespace(hf_config=types.SimpleNamespace()),
+    )
+    sentinel_executor = object()
+    with (
+        mock.patch.object(
+            stage_init_utils.OmniEngineArgs, "create_engine_config", return_value=fake_config
+        ) as create_engine_config,
+        mock.patch.object(stage_init_utils.Executor, "get_class", return_value=sentinel_executor),
+        mock.patch.object(stage_init_utils.OmniINCConfig, "maybe_upgrade", side_effect=lambda quant: quant),
+    ):
+        vllm_config, executor_class = build_vllm_config(
+            stage,
+            model="dummy-model",
+            engine_args_dict={"tensor_parallel_size": 1},
+        )
+    create_engine_config.assert_called_once()
+    assert vllm_config is fake_config
+    assert executor_class is sentinel_executor
diff --git a/vllm_omni/engine/stage_init_utils.py b/vllm_omni/engine/stage_init_utils.py
index 263804fd6..f24eda0a1 100644
--- a/vllm_omni/engine/stage_init_utils.py
+++ b/vllm_omni/engine/stage_init_utils.py
@@ -1,3 +1,6 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM-Omni project
+
 """
 Stage initialization helpers for vLLM-Omni multi-stage runtime.
 
@@ -565,7 +568,7 @@ def _maybe_set_qwen3_omni_moe_env(engine_args_dict: dict[str, Any]) -> None:
 def split_devices_for_replicas(
     devices_str: str | None,
     num_replicas: int,
-    tp_size: int,
+    devices_per_replica: int,
     stage_id: int,
 ) -> list[str | None]:
     """Split a devices string into per-replica subsets.
@@ -578,19 +581,19 @@ def split_devices_for_replicas(
     When ``num_replicas`` is 1, returns ``[devices_str]`` unchanged.
     Otherwise, two YAML shapes are accepted:
 
-    1. **Legacy / pool mode** — ``len(devices) == num_replicas * tp_size``:
+    1. **Legacy / pool mode** — ``len(devices) == num_replicas * devices_per_replica``:
        the string enumerates the full per-stage pool. Each replica gets
-       ``tp_size`` consecutive entries. The values are logical indices
+       ``devices_per_replica`` consecutive entries. The values are logical indices
        into the launcher's ``CUDA_VISIBLE_DEVICES``.
 
        ``split_devices_for_replicas("1,2,3,4", 2, 2, 1) → ["1,2", "3,4"]``
 
-    2. **Template mode** — ``len(devices) == tp_size``: the YAML declares
+    2. **Template mode** — ``len(devices) == devices_per_replica``: the YAML declares
        a single per-replica template (the same shape one replica would
-       use), and is **dp-independent**. Each replica r gets the offsets
-       ``[r*tp_size + a for a in template]`` of the launcher's
+       use), and is **replica-count-independent**. Each replica r gets the offsets
+       ``[r*devices_per_replica + a for a in template]`` of the launcher's
        ``CUDA_VISIBLE_DEVICES``. The template's entries must lie in
-       ``[0, tp_size)``.
+       ``[0, devices_per_replica)``.
 
        ``split_devices_for_replicas("0,1", 2, 2, 1) → ["0,1", "2,3"]``
        ``split_devices_for_replicas("0,1", 4, 2, 1) → ["0,1", "2,3", "4,5", "6,7"]``
@@ -613,27 +616,29 @@ def split_devices_for_replicas(
 
     device_list = [d.strip() for d in devices_str.split(",") if d.strip()]
 
-    if len(device_list) == num_replicas * tp_size:
-        return [",".join(device_list[r * tp_size : (r + 1) * tp_size]) for r in range(num_replicas)]
+    if len(device_list) == num_replicas * devices_per_replica:
+        return [
+            ",".join(device_list[r * devices_per_replica : (r + 1) * devices_per_replica]) for r in range(num_replicas)
+        ]
 
-    if len(device_list) == tp_size:
+    if len(device_list) == devices_per_replica:
         try:
             offsets = [int(a) for a in device_list]
         except ValueError as e:
             raise ValueError(f"Stage {stage_id}: template-mode devices must be ints, got {devices_str!r}") from e
-        bad = [a for a in offsets if not (0 <= a < tp_size)]
+        bad = [a for a in offsets if not (0 <= a < devices_per_replica)]
         if bad:
             raise ValueError(
                 f"Stage {stage_id}: template-mode device offset(s) {bad} "
-                f"out of range [0, {tp_size}); devices={devices_str!r}"
+                f"out of range [0, {devices_per_replica}); devices={devices_str!r}"
             )
-        return [",".join(str(r * tp_size + a) for a in offsets) for r in range(num_replicas)]
+        return [",".join(str(r * devices_per_replica + a) for a in offsets) for r in range(num_replicas)]
 
     raise ValueError(
         f"Stage {stage_id}: devices={devices_str!r} has {len(device_list)} id(s); "
-        f"need either {tp_size} (template, dp-independent) or "
-        f"{num_replicas * tp_size} (pool / legacy). "
-        f"num_replicas={num_replicas}, tensor_parallel_size={tp_size}."
+        f"need either {devices_per_replica} (per-replica template) or "
+        f"{num_replicas * devices_per_replica} (pool / legacy). "
+        f"num_replicas={num_replicas}, devices_per_replica={devices_per_replica}."
     )
 
 
@@ -645,9 +650,32 @@ def get_stage_tp_size(stage_cfg: Any) -> int:
     return int(getattr(engine_args, "tensor_parallel_size", 1) or 1)
 
 
-def get_stage_devices_per_replica(stage_cfg: Any) -> int:
+def _get_local_llm_parallel_sizes(
+    stage_cfg: Any,
+    engine_args: Any | None = None,
+) -> tuple[int, int, int]:
+    """Return ``(tp, local_dp, pp)`` for one local LLM replica.
+
+    ``data_parallel_size`` is cluster-wide, whereas ``runtime.devices`` is
+    local to this process.  Prefer an explicitly resolved
+    ``data_parallel_size_local`` (including zero for a head process that owns
+    no local engines), and only fall back to the global DP width when it is
+    unset.
+    """
+    if engine_args is None:
+        engine_args = getattr(stage_cfg, "engine_args", {})
+    tp_size = int(_get_attr_or_item(engine_args, "tensor_parallel_size", 1) or 1)
+    pp_size = int(_get_attr_or_item(engine_args, "pipeline_parallel_size", 1) or 1)
+    local_dp_size = _get_attr_or_item(engine_args, "data_parallel_size_local", None)
+    if local_dp_size is None:
+        local_dp_size = _get_attr_or_item(engine_args, "data_parallel_size", 1)
+    return tp_size, int(local_dp_size if local_dp_size is not None else 1), pp_size
+
+
+def get_stage_devices_per_replica(stage_cfg: Any, engine_args: Any | None = None) -> int:
     """Return the number of devices consumed by one replica of *stage_cfg*."""
-    engine_args = getattr(stage_cfg, "engine_args", {})
+    if engine_args is None:
+        engine_args = getattr(stage_cfg, "engine_args", {})
     if getattr(stage_cfg, "stage_type", "llm") == "diffusion":
         parallel_config = _get_attr_or_item(engine_args, "parallel_config")
         if parallel_config is None:
@@ -664,14 +692,15 @@ def get_stage_devices_per_replica(stage_cfg: Any) -> int:
         except Exception:
             return 1
 
-    return get_stage_tp_size(stage_cfg)
+    tp_size, local_dp_size, pp_size = _get_local_llm_parallel_sizes(stage_cfg, engine_args)
+    return tp_size * max(1, local_dp_size) * pp_size
 
 
 def compute_replica_layout(
     stage_configs: Sequence[Any],
     *,
     allow_zero: bool = False,
-) -> tuple[list[int], dict[int, list[str]]]:
+) -> tuple[list[int], dict[int, list[str | None]]]:
     """Compute per-stage replica counts and device assignments.
 
     Args:
@@ -820,7 +849,7 @@ def _project_upstream_config_fields(
     field_map: Mapping[str, str],
 ) -> dict[str, Any]:
     """Project every explicit upstream input, including newly added fields."""
-    explicit_fields = getattr(config, "_omni_explicit_fields", frozenset())
+    explicit_fields: frozenset[str] = getattr(config, "_omni_explicit_fields", frozenset())
     unprojected_fields = explicit_fields - frozenset(field_map)
     if unprojected_fields:
         names = ", ".join(sorted(unprojected_fields))
@@ -838,9 +867,9 @@ def _project_omni_stage_engine_args(
     """Read backend inputs from one structured stage config."""
     engine_args: dict[str, Any] = {}
     is_diffusion = isinstance(stage_config, VllmOmniDiffusionStageConfig)
-
     if is_diffusion:
-        engine_args.update(_project_omni_config_fields(stage_config.diffusion_config))
+        diffusion_stage = cast(VllmOmniDiffusionStageConfig, stage_config)
+        engine_args.update(_project_omni_config_fields(diffusion_stage.diffusion_config))
 
     for config, excluded_fields in (
         (
@@ -1112,6 +1141,74 @@ def build_engine_args_dict_from_omni_stage_config(
     )
 
 
+def _count_stage_devices(devices: Any) -> int | None:
+    if devices is None:
+        return None
+    if isinstance(devices, (list, tuple)):
+        return len(devices)
+    values = [device for device in str(devices).split(",") if device.strip()]
+    return len(values) or None
+
+
+def _check_stage_device_layout(stage_config: Any, engine_args_dict: dict[str, Any]) -> None:
+    """Fail early when a stage's world size cannot fit its assigned ``devices``.
+
+    Re-runs :func:`check_device_layout` (normally only reached on the
+    ``--strategy-config`` path) against the fully resolved per-stage layout, so
+    an inconsistent ``tensor_parallel_size`` vs ``devices`` (issue #5003) is
+    reported here with a clear message instead of surfacing later as an opaque
+    worker-side ``local rank ... out of bounds`` assertion.
+    """
+    from vllm_omni.config.composable_parallel import StrategyApplyError, check_device_layout
+
+    runtime = getattr(stage_config, "runtime", None)
+    devices = _get_attr_or_item(runtime, "devices", None) if runtime is not None else None
+    if devices is None:
+        # No explicit placement -> vLLM assigns devices itself; nothing to check.
+        return
+
+    num_replicas = _get_attr_or_item(runtime, "num_replicas", 1) if runtime is not None else 1
+    stage_id = getattr(stage_config, "stage_id", "?")
+    tp_size, local_dp_size, pp_size = _get_local_llm_parallel_sizes(stage_config, engine_args_dict)
+    if local_dp_size == 0:
+        # This process hosts no local DP engines, so its local device list does
+        # not describe the cluster-wide DP layout and must not be validated.
+        return
+
+    try:
+        check_device_layout(
+            devices,
+            tensor_parallel_size=tp_size,
+            data_parallel_size=local_dp_size,
+            pipeline_parallel_size=pp_size,
+            num_replicas=int(num_replicas or 1),
+            role=f"stage-{stage_id}",
+        )
+    except StrategyApplyError as e:
+        message = (
+            f"Stage {stage_id}: device layout is inconsistent — {e} "
+            "Set devices and the per-stage TP, local DP, PP, and replica counts "
+            "so the declared device count matches the local world size."
+        )
+        device_count = _count_stage_devices(devices)
+        world_without_tp = local_dp_size * pp_size
+        valid_without_tp = {world_without_tp, int(num_replicas or 1) * world_without_tp}
+        if tp_size > 1 and device_count in valid_without_tp:
+            message += (
+                " This layout is consistent with issue #5003: a top-level "
+                "--tensor-parallel-size is applied to every stage, but each stage's "
+                "`devices` is not adjusted automatically. Pass --stage-overrides "
+                "to set tensor_parallel_size and devices together on every stage, "
+                "so single-GPU stages get tensor_parallel_size=1, e.g. "
+                '\'{"0": {"tensor_parallel_size": 4, "devices": "0,1,2,3"}, '
+                '"1": {"tensor_parallel_size": 1, "devices": "0"}, '
+                '"2": {"tensor_parallel_size": 1, "devices": "1"}}\'. '
+                "Or omit the top-level --tensor-parallel-size and set it only in "
+                "stage-0's override."
+            )
+        raise ValueError(message) from e
+
+
 def build_vllm_config(
     stage_config: Any,
     model: str,
@@ -1147,6 +1244,16 @@ def build_vllm_config(
 
     omni_engine_args = OmniEngineArgs(**filtered_engine_args_dict)
 
+    # Guard against a per-stage world size that its assigned ``devices`` cannot
+    # satisfy (issue #5003). A top-level ``--tensor-parallel-size`` is broadcast
+    # to every stage, but ``devices`` is not, so a stage can end up with e.g.
+    # tensor_parallel_size=4 while still holding a single-GPU deploy default.
+    # Without --strategy-config the strategy-path device check never runs, so
+    # the mismatch used to surface only as an opaque worker-side assertion
+    # ("DP adjusted local rank N is out of bounds for M devices."). Re-run the
+    # same check here, before workers spawn, to fail early with a clear message.
+    _check_stage_device_layout(stage_config, filtered_engine_args_dict)
+
     # Multi-stage pipelines (qwen3_tts code2wav, etc.) set max_model_len
     # larger than HF max_position_embeddings by design. vLLM's validator
     # rejects that without the env flag.
@@ -1473,6 +1580,7 @@ def build_diffusion_config(
     num_devices_per_stage = od_config.parallel_config.world_size
     device_control_env = current_omni_platform.device_control_env_var
     visible_devices_str = os.environ.get(device_control_env) if device_control_env else None
+    physical_devices: list[str | int]
     if visible_devices_str:
         physical_devices = [device.strip() for device in visible_devices_str.split(",") if device.strip()]
     else: