diff --git a/tests/unit_tests/backends/diffusion/test_wan_vae_decoder.py b/tests/unit_tests/backends/diffusion/test_wan_vae_decoder.py
new file mode 100644
index 000000000..1c4d9c40c
--- /dev/null
+++ b/tests/unit_tests/backends/diffusion/test_wan_vae_decoder.py
@@ -0,0 +1,116 @@
+##########################################################################
+# Copyright (c) 2025, Advanced Micro Devices, Inc.
+#
+# See LICENSE for license information.
+###########################################################################
+
+import torch
+
+from primus.backends.diffusion.models.wan.vae2_2 import Decoder3d, count_conv3d
+
+
+def _make_decoder():
+    # A tiny decoder with a single temporal-upsample stage: enough to exercise
+    # cache indexing, the "Rep" sentinel, and first_chunk propagation without
+    # a channel-changing residual shortcut (which vae2_2's ResidualBlock does
+    # not route through the feat_cache, and would desync feat_idx from
+    # count_conv3d(decoder)).
+    # Seed only within a forked RNG scope so this doesn't mutate global RNG
+    # state and leak into other tests run later in the same process.
+    with torch.random.fork_rng():
+        torch.manual_seed(0)
+        decoder = Decoder3d(
+            dim=8,
+            z_dim=4,
+            dim_mult=[1, 1],
+            num_res_blocks=1,
+            attn_scales=[],
+            temperal_upsample=[True],
+            dropout=0.0,
+        )
+    decoder.eval()
+    return decoder
+
+
+def _decode_streaming(decoder, x, conv_num):
+    """Replay WanVAE_.decode's per-frame chunked-decode protocol: one latent
+    frame per call, a single feat_cache list reused across calls, feat_idx
+    reset to [0] for every frame, and first_chunk=True only on the first
+    call."""
+    feat_map = [None] * conv_num
+    outputs = []
+    for i in range(x.shape[2]):
+        feat_idx = [0]
+        frame_out = decoder(
+            x[:, :, i : i + 1, :, :],
+            feat_cache=feat_map,
+            feat_idx=feat_idx,
+            first_chunk=(i == 0),
+        )
+        # Every CausalConv3d on the cached path consumes exactly one slot.
+        assert feat_idx[0] == conv_num
+        outputs.append(frame_out)
+    return outputs, feat_map
+
+
+def test_forward_streaming_decode_matches_wan22_chunked_protocol():
+    # Direct Decoder3d output is 12 channels (patchified latent space); the
+    # conversion to 3 RGB channels happens later, in unpatchify.
+    decoder = _make_decoder()
+    conv_num = count_conv3d(decoder)
+    x = torch.randn(1, 4, 3, 4, 4)
+    feat_map = [None] * conv_num
+
+    outputs = []
+    with torch.no_grad():
+        for i in range(x.shape[2]):
+            feat_idx = [0]
+            frame_out = decoder(
+                x[:, :, i : i + 1, :, :],
+                feat_cache=feat_map,
+                feat_idx=feat_idx,
+                first_chunk=(i == 0),
+            )
+            # Every CausalConv3d on the cached path consumes exactly one slot.
+            assert feat_idx[0] == conv_num
+
+            if i == 0:
+                # After the first chunk every cache slot is populated; the
+                # temporal-upsample stage's slot holds the "Rep" sentinel
+                # until a second chunk gives it real history to work with.
+                assert all(slot is not None for slot in feat_map)
+                assert any(isinstance(slot, str) and slot == "Rep" for slot in feat_map)
+            else:
+                # Once real history is available, "Rep" must have been
+                # replaced by an actual cached tensor.
+                assert not any(isinstance(slot, str) and slot == "Rep" for slot in feat_map)
+
+            outputs.append(frame_out)
+
+    # First chunk has no cached history yet, so its upsample3d stage can only
+    # emit its own frame; later chunks have history and double their
+    # temporal contribution.
+    assert outputs[0].shape == (1, 12, 1, 8, 8)
+    assert outputs[1].shape == (1, 12, 2, 8, 8)
+    assert outputs[2].shape == (1, 12, 2, 8, 8)
+
+    out = torch.cat(outputs, dim=2)
+    assert out.shape == (1, 12, 5, 8, 8)
+    assert torch.isfinite(out).all()
+
+
+def test_forward_streaming_decode_is_deterministic_after_cache_reset():
+    decoder = _make_decoder()
+    conv_num = count_conv3d(decoder)
+    x = torch.randn(1, 4, 3, 4, 4)
+
+    with torch.no_grad():
+        outputs_1, _ = _decode_streaming(decoder, x, conv_num)
+        outputs_2, _ = _decode_streaming(decoder, x, conv_num)
+
+    torch.testing.assert_close(
+        torch.cat(outputs_1, dim=2),
+        torch.cat(outputs_2, dim=2),
+        rtol=0,
+        atol=1e-6,
+    )