diff --git a/python/sglang/multimodal_gen/test/unit/test_qwen_image21_cuda.py b/python/sglang/multimodal_gen/test/unit/test_qwen_image21_cuda.py
index b2746c236683..703915077760 100644
--- a/python/sglang/multimodal_gen/test/unit/test_qwen_image21_cuda.py
+++ b/python/sglang/multimodal_gen/test/unit/test_qwen_image21_cuda.py
@@ -455,11 +455,16 @@ def inputs_hd128(seed, edit):
     return kwargs
 
 
+@pytest.mark.parametrize("cuda_kernels", [False, True])
 @pytest.mark.parametrize("edit", [False, True])
 @torch.no_grad()
 def test_cuda_qk_rope_pack_matches_eager_prefill_and_cached_steps(
-    bf16_model_hd128, edit, monkeypatch
+    bf16_model_hd128, edit, cuda_kernels, monkeypatch
 ):
+    if not cuda_kernels:
+        monkeypatch.setattr(
+            model_module, "can_use_qknorm_complex_rope_cuda", lambda *args: False
+        )
     # The CUDA Q/K norm + RoPE + KV packing path must reproduce the Triton/eager
     # chain bit for bit on the prefill step, on cached steps and under BCG replay.
     actual_model = bf16_model_hd128
@@ -489,9 +494,12 @@ def test_cuda_qk_rope_pack_matches_eager_prefill_and_cached_steps(
         for timestep, output in zip((700, 300, 10), expected, strict=True):
             kwargs["timestep"].fill_(timestep)
             torch.testing.assert_close(actual_model(**kwargs), output, atol=0, rtol=0)
-    # The CUDA kernels are exact by construction and must have engaged.
+    # Unsupported CUDA kernels must leave the exact fallback and cache intact.
     for gate in (qk_gate, kv_gate):
-        assert gate.verified and not gate.disabled, gate.name
+        if cuda_kernels:
+            assert gate.verified and not gate.disabled, gate.name
+        else:
+            assert not gate.verified and not gate.disabled, gate.name
     # The packed and the direct-write projections are GEMM re-plumbings whose
     # first-sight compare depends on cuBLAS picking the same kernel for both
     # shapes; where it does not, the gate declines and the next tier takes over
diff --git a/python/sglang/multimodal_gen/test/unit/test_residual_gate_add_dispatch.py b/python/sglang/multimodal_gen/test/unit/test_residual_gate_add_dispatch.py
new file mode 100644
index 000000000000..417d6fc53082
--- /dev/null
+++ b/python/sglang/multimodal_gen/test/unit/test_residual_gate_add_dispatch.py
@@ -0,0 +1,58 @@
+# SPDX-License-Identifier: Apache-2.0
+"""Platform dispatch for the PTX-only diffusion residual fast path."""
+
+import pytest
+import torch
+
+from sglang.kernels.kda_kernels import residual_gate_add_jit as residual_ops
+
+pytestmark = pytest.mark.skipif(not torch.cuda.is_available(), reason="GPU required")
+
+
+def _inputs(dtype, layout):
+    torch.manual_seed(13)
+    residual = torch.randn(1, 16, 32, device="cuda", dtype=dtype)
+    if layout == "transposed_row":
+        residual = residual.transpose(1, 2).contiguous().transpose(1, 2)
+    update = torch.randn(residual.shape, device="cuda", dtype=dtype)
+    shape = {
+        "full": residual.shape,
+        "row": (1, 1, 32),
+        "token": (1, 16, 1),
+        "transposed_row": (1, 1, 32),
+    }[layout]
+    gate = torch.randn(shape, device="cuda", dtype=dtype)
+    return residual, update, gate
+
+
+@pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16, torch.float32])
+@pytest.mark.parametrize("layout", ["full", "row", "token", "transposed_row"])
+def test_hip_dispatch_never_attempts_ptx(monkeypatch, dtype, layout):
+    residual, update, gate = _inputs(dtype, layout)
+    expected = residual + update * gate
+    monkeypatch.setattr(torch.version, "hip", "test-hip")
+
+    def unsupported(*args):
+        pytest.fail("HIP must not attempt the PTX-only kernel")
+
+    monkeypatch.setattr(residual_ops, "_residual_gate_add_custom_op", unsupported)
+    assert not residual_ops.can_use_residual_gate_add_cuda(residual, update, gate)
+    torch.testing.assert_close(
+        residual_ops.residual_gate_add(residual, update, gate), expected, atol=0, rtol=0
+    )
+    with pytest.raises(RuntimeError, match="unsupported input"):
+        residual_ops.residual_gate_add_cuda(residual, update, gate)
+
+
+@pytest.mark.skipif(torch.version.hip is not None, reason="PTX kernel requires CUDA")
+@pytest.mark.parametrize("dtype", [torch.float16, torch.bfloat16])
+@pytest.mark.parametrize("layout", ["full", "row", "token", "transposed_row"])
+def test_cuda_fast_path_remains_exact(dtype, layout):
+    residual, update, gate = _inputs(dtype, layout)
+    assert residual_ops.can_use_residual_gate_add_cuda(residual, update, gate)
+    torch.testing.assert_close(
+        residual_ops.residual_gate_add_cuda(residual, update, gate),
+        residual + update * gate,
+        atol=0,
+        rtol=0,
+    )