diff --git a/tests/comfyui/test_comfyui_integration.py b/tests/comfyui/test_comfyui_integration.py
index 5164f3b9acb..80e86d82412 100644
--- a/tests/comfyui/test_comfyui_integration.py
+++ b/tests/comfyui/test_comfyui_integration.py
@@ -523,7 +523,6 @@ def run_server():
             "Qwen/Qwen-Image-Edit",
             True,
             id="image-to-image-dalle-endpoint",
-            marks=pytest.mark.skip(reason="Temporarily disabled due to failure."),
         ),
         pytest.param(
             ServerCase(
diff --git a/tests/entrypoints/openai_api/test_image_server.py b/tests/entrypoints/openai_api/test_image_server.py
index 607b3eaa813..a2353f2f5d1 100644
--- a/tests/entrypoints/openai_api/test_image_server.py
+++ b/tests/entrypoints/openai_api/test_image_server.py
@@ -18,6 +18,7 @@
 from pytest_mock import MockerFixture
 from vllm import SamplingParams
 
+from vllm_omni.entrypoints.errors import InputValidationError
 from vllm_omni.entrypoints.openai.image_api_utils import (
     encode_image_base64,
     parse_size,
@@ -1038,6 +1039,48 @@ def _fail_load(*args, **kwargs):
     assert engine.captured_prompt is None
 
 
+def test_image_edit_ignores_non_concrete_diffusion_input_limits(async_omni_test_client):
+    engine = async_omni_test_client.app.state.engine_client
+    engine.get_diffusion_od_config = lambda: SimpleNamespace(
+        supports_multimodal_inputs=SimpleNamespace(),
+        max_multimodal_image_inputs=SimpleNamespace(),
+    )
+
+    response = async_omni_test_client.post(
+        "/v1/images/edits",
+        files=[("image", make_test_image_bytes((16, 16)))],
+        data={"prompt": "hello world."},
+    )
+
+    assert response.status_code == 200
+
+
+def test_image_edit_maps_worker_side_input_limit_error_to_400(async_omni_test_client):
+    engine = async_omni_test_client.app.state.engine_client
+    engine.get_diffusion_od_config = lambda: None
+
+    async def _fail_generate(*args, **kwargs):
+        raise InputValidationError("Received 5 input images. At most 4 images are supported by this model.")
+        yield
+
+    engine.generate = _fail_generate
+
+    response = async_omni_test_client.post(
+        "/v1/images/edits",
+        files=[
+            ("image", make_test_image_bytes((16, 16))),
+            ("image", make_test_image_bytes((16, 16))),
+            ("image", make_test_image_bytes((16, 16))),
+            ("image", make_test_image_bytes((16, 16))),
+            ("image", make_test_image_bytes((16, 16))),
+        ],
+        data={"prompt": "hello world."},
+    )
+
+    assert response.status_code == 400
+    assert response.json()["detail"] == "Received 5 input images. At most 4 images are supported by this model."
+
+
 def test_image_edit_parameter_pass(async_omni_test_client):
     img_bytes_1 = make_test_image_bytes((16, 16))
 
diff --git a/tests/entrypoints/test_async_omni_abort.py b/tests/entrypoints/test_async_omni_abort.py
index b34652162d0..0df017b9ed9 100644
--- a/tests/entrypoints/test_async_omni_abort.py
+++ b/tests/entrypoints/test_async_omni_abort.py
@@ -4,6 +4,7 @@
 import pytest
 
 from vllm_omni.entrypoints.async_omni import AsyncOmni
+from vllm_omni.entrypoints.errors import InputValidationError
 
 pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
 
@@ -83,3 +84,32 @@ async def collect_outputs(request_id):
         ]
 
     asyncio.run(run_test())
+
+
+def test_process_orchestrator_results_restores_input_validation_error():
+    async def run_test():
+        omni = object.__new__(AsyncOmni)
+        req_state = SimpleNamespace(queue=asyncio.Queue(), stage_id=0)
+        omni.request_states = {"req-1": req_state}
+
+        await req_state.queue.put(
+            {
+                "request_id": "req-1",
+                "stage_id": 0,
+                "error": "Received 5 input images. At most 4 images are supported by this model.",
+                "error_type": "InputValidationError",
+            }
+        )
+
+        with pytest.raises(InputValidationError, match="At most 4 images are supported by this model"):
+            async for _ in AsyncOmni._process_orchestrator_results(
+                omni,
+                "req-1",
+                metrics=None,
+                final_stage_id_for_e2e=0,
+                req_start_ts={},
+                wall_start_ts=0.0,
+            ):
+                pass
+
+    asyncio.run(run_test())
diff --git a/vllm_omni/diffusion/data.py b/vllm_omni/diffusion/data.py
index 0a19eb11974..bb90146b38e 100644
--- a/vllm_omni/diffusion/data.py
+++ b/vllm_omni/diffusion/data.py
@@ -759,6 +759,7 @@ class DiffusionOutput:
     trajectory_log_probs: torch.Tensor | dict | None = None
     trajectory_decoded: list[Image.Image] | None = None
     error: str | None = None
+    error_type: str | None = None
     aborted: bool = False
     abort_message: str | None = None
 
diff --git a/vllm_omni/diffusion/diffusion_engine.py b/vllm_omni/diffusion/diffusion_engine.py
index fe940d623e5..d5fc7df9a84 100644
--- a/vllm_omni/diffusion/diffusion_engine.py
+++ b/vllm_omni/diffusion/diffusion_engine.py
@@ -30,6 +30,7 @@
 from vllm_omni.diffusion.sched import RequestScheduler, SchedulerInterface, StepScheduler
 from vllm_omni.diffusion.sched.interface import DiffusionRequestStatus
 from vllm_omni.diffusion.worker.utils import RunnerOutput
+from vllm_omni.entrypoints.errors import get_serialized_error_type, restore_serialized_error
 from vllm_omni.inputs.data import OmniDiffusionSamplingParams, OmniTextPrompt
 from vllm_omni.outputs import OmniRequestOutput
 
@@ -122,7 +123,7 @@ def step(self, request: OmniDiffusionRequest) -> list[OmniRequestOutput]:
         if output.aborted:
             raise DiffusionRequestAbortedError(output.abort_message or "Diffusion request aborted.")
         if output.error:
-            raise RuntimeError(f"{output.error}")
+            raise restore_serialized_error(output.error, output.error_type)
         logger.info("Generation completed successfully.")
 
         if output.output is None:
@@ -364,7 +365,7 @@ def add_req_and_wait_for_response(self, request: OmniDiffusionRequest) -> Diffus
                         req_id=sched_req_id,
                         step_index=None,
                         finished=True,
-                        result=DiffusionOutput(error=str(exc)),
+                        result=DiffusionOutput(error=str(exc), error_type=get_serialized_error_type(exc)),
                     )
 
                 self._process_aborts_queue()
diff --git a/vllm_omni/diffusion/inline_stage_diffusion_client.py b/vllm_omni/diffusion/inline_stage_diffusion_client.py
index a33a3e95619..5f026bf37d4 100644
--- a/vllm_omni/diffusion/inline_stage_diffusion_client.py
+++ b/vllm_omni/diffusion/inline_stage_diffusion_client.py
@@ -20,6 +20,7 @@
 from vllm_omni.diffusion.diffusion_engine import DiffusionEngine
 from vllm_omni.diffusion.request import OmniDiffusionRequest
 from vllm_omni.engine.stage_init_utils import StageMetadata
+from vllm_omni.entrypoints.errors import get_serialized_error_type
 from vllm_omni.inputs.data import OmniDiffusionSamplingParams
 from vllm_omni.outputs import OmniRequestOutput
 
@@ -123,6 +124,7 @@ async def _dispatch_request(
                 images=[],
             )
             error_output.error = str(e)
+            error_output.error_type = get_serialized_error_type(e)
             self._output_queue.put_nowait(error_output)
         finally:
             self._tasks.pop(request_id, None)
@@ -223,6 +225,7 @@ async def _dispatch_batch(
                 images=[],
             )
             error_output.error = str(e)
+            error_output.error_type = get_serialized_error_type(e)
             self._output_queue.put_nowait(error_output)
         finally:
             self._tasks.pop(request_id, None)
diff --git a/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit_plus.py b/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit_plus.py
index 2e25d0fe6b2..2a2202d2335 100644
--- a/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit_plus.py
+++ b/vllm_omni/diffusion/models/qwen_image/pipeline_qwen_image_edit_plus.py
@@ -48,6 +48,7 @@
     normalize_min_aligned_size,
 )
 from vllm_omni.diffusion.utils.tf_utils import get_transformer_config_kwargs
+from vllm_omni.entrypoints.errors import InputValidationError
 from vllm_omni.inputs.data import OmniTextPrompt
 from vllm_omni.model_executor.model_loader.weight_utils import (
     download_weights_from_hf_specific,
@@ -101,7 +102,7 @@ def pre_process_func(
             if not isinstance(raw_image, list):
                 raw_image = [raw_image]
             if len(raw_image) > MAX_QWEN_IMAGE_EDIT_PLUS_INPUT_IMAGES:
-                raise ValueError(
+                raise InputValidationError(
                     f"Received {len(raw_image)} input images. "
                     f"At most {MAX_QWEN_IMAGE_EDIT_PLUS_INPUT_IMAGES} images are supported by this model."
                 )
diff --git a/vllm_omni/diffusion/stage_diffusion_client.py b/vllm_omni/diffusion/stage_diffusion_client.py
index 480d113d192..181b02ba3ed 100644
--- a/vllm_omni/diffusion/stage_diffusion_client.py
+++ b/vllm_omni/diffusion/stage_diffusion_client.py
@@ -178,6 +178,7 @@ def _drain_responses(self) -> None:
                         images=[],
                     )
                     error_output.error = error_msg
+                    error_output.error_type = msg.get("error_type")
                     self._output_queue.put_nowait(error_output)
 
     # Fields that are subprocess-local and cannot be serialized across
diff --git a/vllm_omni/diffusion/stage_diffusion_proc.py b/vllm_omni/diffusion/stage_diffusion_proc.py
index eced444fd32..8525d82efe0 100644
--- a/vllm_omni/diffusion/stage_diffusion_proc.py
+++ b/vllm_omni/diffusion/stage_diffusion_proc.py
@@ -30,6 +30,7 @@
     OmniMsgpackDecoder,
     OmniMsgpackEncoder,
 )
+from vllm_omni.entrypoints.errors import get_serialized_error_type
 from vllm_omni.inputs.data import OmniDiffusionSamplingParams
 from vllm_omni.outputs import OmniRequestOutput
 
@@ -339,6 +340,7 @@ async def _dispatch_request(
                             "type": "error",
                             "request_id": request_id,
                             "error": str(e),
+                            "error_type": get_serialized_error_type(e),
                         }
                     )
                 )
@@ -394,6 +396,7 @@ async def _dispatch_batch(
                                         "type": "error",
                                         "request_id": rid,
                                         "error": str(e),
+                                        "error_type": get_serialized_error_type(e),
                                     }
                                 )
                             )
@@ -443,6 +446,7 @@ async def _dispatch_batch(
                                     "type": "error",
                                     "rpc_id": rpc_id,
                                     "error": str(e),
+                                    "error_type": get_serialized_error_type(e),
                                 }
                             )
                         )
diff --git a/vllm_omni/diffusion/worker/diffusion_worker.py b/vllm_omni/diffusion/worker/diffusion_worker.py
index 160309e0d8d..9a27b0bd175 100644
--- a/vllm_omni/diffusion/worker/diffusion_worker.py
+++ b/vllm_omni/diffusion/worker/diffusion_worker.py
@@ -41,6 +41,7 @@
 from vllm_omni.diffusion.sched.interface import DiffusionSchedulerOutput
 from vllm_omni.diffusion.worker.diffusion_model_runner import DiffusionModelRunner
 from vllm_omni.diffusion.worker.utils import RunnerOutput
+from vllm_omni.entrypoints.errors import get_serialized_error_type
 from vllm_omni.lora.request import LoRARequest
 from vllm_omni.platforms import current_omni_platform
 from vllm_omni.profiler import OmniTorchProfilerWrapper, create_omni_profiler
@@ -501,7 +502,7 @@ def worker_busy_loop(self) -> None:
                 except Exception as e:
                     logger.error(f"Error processing RPC: {e}", exc_info=True)
                     if self.result_mq is not None:
-                        self.return_result(DiffusionOutput(error=str(e)))
+                        self.return_result(DiffusionOutput(error=str(e), error_type=get_serialized_error_type(e)))
 
             elif isinstance(msg, dict) and msg.get("type") == "shutdown":
                 logger.info("Worker %s: Received shutdown message", self.gpu_id)
@@ -517,7 +518,7 @@ def worker_busy_loop(self) -> None:
                         f"Error executing forward in event loop: {e}",
                         exc_info=True,
                     )
-                    output = DiffusionOutput(error=str(e))
+                    output = DiffusionOutput(error=str(e), error_type=get_serialized_error_type(e))
 
                 try:
                     self.return_result(output)
diff --git a/vllm_omni/engine/orchestrator.py b/vllm_omni/engine/orchestrator.py
index a5d7ad032e9..f7a8b7c61f3 100644
--- a/vllm_omni/engine/orchestrator.py
+++ b/vllm_omni/engine/orchestrator.py
@@ -281,6 +281,7 @@ async def _orchestration_loop(self) -> None:
                                         "request_id": parent_id,
                                         "stage_id": stage_id,
                                         "error": output.error,
+                                        "error_type": output.error_type,
                                     }
                                 )
                                 role_map = self._companion_map.get(parent_id, {})
diff --git a/vllm_omni/entrypoints/async_omni.py b/vllm_omni/entrypoints/async_omni.py
index 9606cc80d0d..99ceb5177f0 100644
--- a/vllm_omni/entrypoints/async_omni.py
+++ b/vllm_omni/entrypoints/async_omni.py
@@ -25,6 +25,7 @@
 from vllm.v1.engine.exceptions import EngineDeadError
 
 from vllm_omni.entrypoints.client_request_state import ClientRequestState
+from vllm_omni.entrypoints.errors import restore_serialized_error
 from vllm_omni.entrypoints.omni_base import OmniBase
 from vllm_omni.metrics.stats import OrchestratorAggregator as OrchestratorMetrics
 from vllm_omni.outputs import OmniRequestOutput
@@ -441,7 +442,7 @@ async def _process_orchestrator_results(
                     stage_id,
                     result["error"],
                 )
-                raise RuntimeError(result)
+                raise restore_serialized_error(result["error"], result.get("error_type"))
 
             # Process the result (constructs OmniRequestOutput)
             output_to_yield = self._process_single_result(
diff --git a/vllm_omni/entrypoints/errors.py b/vllm_omni/entrypoints/errors.py
new file mode 100644
index 00000000000..e29a917a96d
--- /dev/null
+++ b/vllm_omni/entrypoints/errors.py
@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: Apache-2.0
+
+
+class InputValidationError(ValueError):
+    def __init__(self, message: str = "Invalid input.") -> None:
+        super().__init__(message)
+
+
+def get_serialized_error_type(exc: BaseException) -> str | None:
+    if isinstance(exc, InputValidationError):
+        return InputValidationError.__name__
+    return None
+
+
+def restore_serialized_error(message: str, error_type: str | None) -> Exception:
+    if error_type == InputValidationError.__name__:
+        return InputValidationError(message)
+    return RuntimeError(message)
diff --git a/vllm_omni/entrypoints/openai/api_server.py b/vllm_omni/entrypoints/openai/api_server.py
index 745b719d5b2..fb1b4ad20d2 100644
--- a/vllm_omni/entrypoints/openai/api_server.py
+++ b/vllm_omni/entrypoints/openai/api_server.py
@@ -114,7 +114,7 @@
 from vllm_omni.entrypoints.openai.serving_video import OmniOpenAIServingVideo, ReferenceImage
 from vllm_omni.entrypoints.openai.storage import STORAGE_MANAGER
 from vllm_omni.entrypoints.openai.stores import VIDEO_STORE, VIDEO_TASKS
-from vllm_omni.entrypoints.openai.utils import get_stage_type, parse_lora_request
+from vllm_omni.entrypoints.openai.utils import get_stage_type, normalize_optional_bool, parse_lora_request
 from vllm_omni.entrypoints.openai.video_api_utils import decode_input_reference
 from vllm_omni.inputs.data import OmniDiffusionSamplingParams, OmniSamplingParams, OmniTextPrompt
 
@@ -1746,10 +1746,22 @@ def _get_max_edit_input_images(raw_request: Request, engine_client: Any) -> int
         # config is not exposed on the serving surface.
         return None
 
-    if not bool(getattr(od_config, "supports_multimodal_inputs", False)):
+    supports_multimodal_inputs = normalize_optional_bool(getattr(od_config, "supports_multimodal_inputs", None))
+    if supports_multimodal_inputs is None:
+        return None
+
+    if not supports_multimodal_inputs:
         return 1
 
-    return getattr(od_config, "max_multimodal_image_inputs", None)
+    return _normalize_positive_int(getattr(od_config, "max_multimodal_image_inputs", None))
+
+
+def _normalize_positive_int(value: Any) -> int | None:
+    if isinstance(value, bool) or not isinstance(value, int):
+        return None
+    if value < 1:
+        return None
+    return value
 
 
 def _get_lora_from_json_str(lora_body):
diff --git a/vllm_omni/entrypoints/openai/serving_chat.py b/vllm_omni/entrypoints/openai/serving_chat.py
index 8cddac6a6c5..c627ee689c3 100644
--- a/vllm_omni/entrypoints/openai/serving_chat.py
+++ b/vllm_omni/entrypoints/openai/serving_chat.py
@@ -88,6 +88,7 @@
 from vllm_omni.entrypoints.openai.utils import (
     get_stage_type,
     get_supported_speakers_from_hf_config,
+    normalize_optional_bool,
     parse_lora_request,
     validate_requested_speaker,
 )
@@ -2255,8 +2256,12 @@ async def generate_diffusion_images(
                 gen_prompt["multi_modal_data"] = {"image": pil_images[0]}
             else:
                 od_config = getattr(engine, "od_config", None)
-                supports_multimodal_inputs = getattr(od_config, "supports_multimodal_inputs", False)
-                if od_config is None:
+                supports_multimodal_inputs = (
+                    True
+                    if od_config is None
+                    else normalize_optional_bool(getattr(od_config, "supports_multimodal_inputs", None))
+                )
+                if supports_multimodal_inputs is None:
                     supports_multimodal_inputs = True
                 if supports_multimodal_inputs:
                     gen_prompt["multi_modal_data"] = {"image": pil_images}
@@ -2459,8 +2464,12 @@ async def _create_diffusion_chat_completion(
                     gen_prompt["multi_modal_data"]["image"] = pil_images[0]
                 else:
                     od_config = getattr(self._diffusion_engine, "od_config", None)
-                    supports_multimodal_inputs = getattr(od_config, "supports_multimodal_inputs", False)
-                    if od_config is None:
+                    supports_multimodal_inputs = (
+                        True
+                        if od_config is None
+                        else normalize_optional_bool(getattr(od_config, "supports_multimodal_inputs", None))
+                    )
+                    if supports_multimodal_inputs is None:
                         # TODO: entry is asyncOmni. We hack the od config here.
                         supports_multimodal_inputs = True
                     if supports_multimodal_inputs:
diff --git a/vllm_omni/entrypoints/openai/utils.py b/vllm_omni/entrypoints/openai/utils.py
index f411526fdb2..0742fb9d396 100644
--- a/vllm_omni/entrypoints/openai/utils.py
+++ b/vllm_omni/entrypoints/openai/utils.py
@@ -83,3 +83,9 @@ def validate_requested_speaker(speaker: str | None, supported_speakers: set[str]
     if supported_speakers and normalized not in supported_speakers:
         raise ValueError(f"Invalid speaker '{speaker}'. Supported: {', '.join(sorted(supported_speakers))}")
     return normalized
+
+
+def normalize_optional_bool(value: Any) -> bool | None:
+    if isinstance(value, bool):
+        return value
+    return None
diff --git a/vllm_omni/outputs.py b/vllm_omni/outputs.py
index c02c0c1427c..bdffa733418 100644
--- a/vllm_omni/outputs.py
+++ b/vllm_omni/outputs.py
@@ -102,6 +102,7 @@ class OmniRequestOutput:
 
     # error handling
     error: str | None = None
+    error_type: str | None = None
 
     @classmethod
     def from_pipeline(