diff --git a/tests/compile/passes/distributed/test_fusion_all_reduce.py b/tests/compile/passes/distributed/test_fusion_all_reduce.py
index 17c4e30959b..b86018a7555 100644
--- a/tests/compile/passes/distributed/test_fusion_all_reduce.py
+++ b/tests/compile/passes/distributed/test_fusion_all_reduce.py
@@ -13,6 +13,7 @@ from vllm._custom_ops import cutlass_scaled_fp4_mm, scaled_fp4_quant
 from vllm.compilation.passes.fusion.allreduce_rms_fusion import (
     AllReduceFusionPass,
     RocmAiterAllReduceFusionPass,
+    _select_flashinfer_allreduce_use_oneshot,
 )
 from vllm.compilation.passes.fx_utils import find_op_nodes
 from vllm.compilation.passes.utility.fix_functionalization import (
@@ -48,6 +49,35 @@ from vllm.utils.torch_utils import set_random_seed
 DEVICE_TYPE = current_platform.device_type
 
 
+@pytest.mark.parametrize(
+    ("workspace_backend", "device_capability", "world_size", "tensor_size", "expected"),
+    [
+        ("mnnvl", 103, 8, 2 * 1024 * 1024, None),
+        ("trtllm", 103, 8, 2 * 1024 * 1024, True),
+        ("trtllm", 103, 8, 2 * 1024 * 1024 + 1, False),
+        ("trtllm", 100, 4, 4 * 1024 * 1024, True),
+        ("trtllm", 100, 4, 4 * 1024 * 1024 + 1, False),
+        ("trtllm", None, 8, 128 * 1024 * 1024, True),
+    ],
+)
+def test_select_flashinfer_allreduce_use_oneshot(
+    workspace_backend: str,
+    device_capability: int | None,
+    world_size: int,
+    tensor_size: int,
+    expected: bool | None,
+):
+    assert (
+        _select_flashinfer_allreduce_use_oneshot(
+            workspace_backend,
+            device_capability,
+            world_size,
+            tensor_size,
+        )
+        is expected
+    )
+
+
 class TestAllReduceRMSNormModel(torch.nn.Module):
     def __init__(
         self,
diff --git a/vllm/compilation/passes/fusion/allreduce_rms_fusion.py b/vllm/compilation/passes/fusion/allreduce_rms_fusion.py
index f4962424d7f..ab400028925 100644
--- a/vllm/compilation/passes/fusion/allreduce_rms_fusion.py
+++ b/vllm/compilation/passes/fusion/allreduce_rms_fusion.py
@@ -128,6 +128,29 @@ _FI_ALLREDUCE_ONE_SHOT_MAX_SIZES_MB: dict[int, dict[int, float]] = {
     },
 }
 
+MiB = 1024 * 1024
+
+
+def _select_flashinfer_allreduce_use_oneshot(
+    workspace_backend: str,
+    device_capability: int | None,
+    world_size: int,
+    current_tensor_size: int,
+) -> bool | None:
+    if workspace_backend == "mnnvl":
+        # FlashInfer sizes MNNVL workspaces around its own AUTO strategy.
+        # Forcing vLLM's per-rank threshold can request one-shot for tensors
+        # larger than the MNNVL one-shot workspace.
+        return None
+
+    if device_capability is None:
+        max_one_shot_size = None
+    else:
+        max_one_shot_size = _FI_ALLREDUCE_ONE_SHOT_MAX_SIZES_MB.get(
+            device_capability, {}
+        ).get(world_size)
+    return max_one_shot_size is None or current_tensor_size <= max_one_shot_size * MiB
+
 
 if flashinfer_comm is not None:
     from vllm.distributed.device_communicators.flashinfer_all_reduce import (
@@ -138,8 +161,6 @@ if flashinfer_comm is not None:
 
     ar_fusion_patterns = flashinfer_comm.AllReduceFusionPattern
 
-    MiB = 1024 * 1024
-
     def call_trtllm_fused_allreduce_norm(
         allreduce_in: torch.Tensor,
         residual: torch.Tensor,
@@ -174,16 +195,6 @@ if flashinfer_comm is not None:
         )
         curr_device = current_platform.get_device_capability()
         device_capability = curr_device.to_int() if curr_device is not None else None
-        # Get one shot input size limit for the current world size
-        # for the current device capability
-        max_one_shot_size = _FI_ALLREDUCE_ONE_SHOT_MAX_SIZES_MB.get(
-            device_capability,  # type: ignore[arg-type, unused-ignore]
-            {},
-        ).get(world_size, None)
-        # Use one shot if no max size is specified
-        use_oneshot = (
-            max_one_shot_size is None or current_tensor_size <= max_one_shot_size * MiB
-        )
 
         # Select workspace based on pattern: quant patterns use the
         # trtllm quant workspace, non-quant patterns use the primary workspace.
@@ -205,6 +216,12 @@ if flashinfer_comm is not None:
         assert workspace is not None, (
             "Flashinfer allreduce workspace must be initialized when using flashinfer"
         )
+        use_oneshot = _select_flashinfer_allreduce_use_oneshot(
+            workspace.backend,
+            device_capability,
+            world_size,
+            current_tensor_size,
+        )
         assert flashinfer_comm is not None
         if norm_out is None:
             norm_out = allreduce_in
@@ -248,7 +265,7 @@ if flashinfer_comm is not None:
             # the end for the one-shot path; the two-shot path is synchronized
             # and keeps the early completion. Related one-shot instability in
             # the same kernel: flashinfer-ai/flashinfer#1223.
-            trigger_completion_at_end=use_oneshot
+            trigger_completion_at_end=(use_oneshot is True)
             or num_tokens > PDL_ADVANCE_LAUNCH_TOKENS,
         )