diff --git a/python/sglang/test/kits/attention_unittest/attention_methods/dsv4_attention.py b/python/sglang/test/kits/attention_unittest/attention_methods/dsv4_attention.py
index 282fc847c..d1b29939d 100644
--- a/python/sglang/test/kits/attention_unittest/attention_methods/dsv4_attention.py
+++ b/python/sglang/test/kits/attention_unittest/attention_methods/dsv4_attention.py
@@ -19,6 +19,7 @@ from typing import Any
 import torch
 from torch import nn
 
+from sglang.srt.environ import envs
 from sglang.srt.layers.attention.attention_registry import ATTENTION_BACKENDS
 from sglang.srt.layers.attention.dsv4.quant_k_cache import (
     quant_to_nope_fp8_rope_bf16_pack_triton,
@@ -594,6 +595,9 @@ class DSV4AttentionFixture:
     forward_batch: ForwardBatch
     prefix_hidden: list[torch.Tensor]
     input_hidden: torch.Tensor
+    # Selects dense vs sparse-prefill C4 seeding; lives on the fixture because
+    # the reference re-seeds after rebuilding metadata (`_seed_c4_if_needed`).
+    seed_c4_for_sparse_prefill: bool = False
 
 
 @dataclass
@@ -1096,14 +1100,20 @@ def prepare_dsv4_runner_inputs(
         _populate_extra_kv_cache(fixture, layer_id=0, num_entries=_DSV4_EXTRA_ENTRIES)
 
 
-def _seed_c4_if_needed(fixture: DSV4AttentionFixture) -> None:
-    """For compress_ratio=4, seed `c4_sparse_page_indices` to the entries the
-    fixture wrote via `_populate_extra_kv_cache` (the C4Indexer would normally
-    populate this; the smoke fixture skips the indexer). No-op for other
-    compress_ratios.
+def _seed_c4_if_needed(
+    fixture: DSV4AttentionFixture, *, num_entries: int = _DSV4_EXTRA_ENTRIES
+) -> None:
+    """For compress_ratio=4, seed the C4 metadata the exercised path consumes
+    (the C4Indexer would normally populate it; the smoke fixture skips the
+    indexer): `c4_sparse_page_indices` for the dense extend path,
+    `c4_sparse_raw_indices` for sparse prefill. No-op for other compress_ratios.
     """
-    if fixture.case.compress_ratio == 4:
-        _seed_c4_sparse_indices(fixture, num_entries=_DSV4_EXTRA_ENTRIES)
+    if fixture.case.compress_ratio != 4:
+        return
+    if fixture.seed_c4_for_sparse_prefill:
+        _seed_c4_sparse_prefill_indices(fixture, num_entries=num_entries)
+    else:
+        _seed_c4_sparse_indices(fixture, num_entries=num_entries)
 
 
 def run_dsv4_fixture_eager(fixture: DSV4AttentionFixture) -> torch.Tensor:
@@ -1401,6 +1411,47 @@ def _seed_c4_sparse_indices(
     )
 
 
+def _seed_c4_sparse_prefill_indices(
+    fixture: DSV4AttentionFixture,
+    *,
+    num_entries: int,
+) -> None:
+    """Seed C4 metadata for the sparse prefill extend path.
+
+    `_forward_prefill_sparse` reads `c4_sparse_raw_indices` (request-local
+    compressed positions, normally the indexer's output) and derives per-query
+    lengths as `(pos + 1) // 4`. Seed the sequential positions the indexer
+    emits for short sequences and mirror the same causal set into
+    `c4_sparse_page_indices` / `c4_sparse_topk_lengths` so the reference
+    attends identical entries. The mirror relies on raw position `k` mapping
+    to physical extra-cache id `k` (page 0 of a fresh single-request layout);
+    asserted below.
+    """
+    md = fixture.backend.forward_metadata.core_metadata
+    raw_indices = md.c4_sparse_raw_indices
+    assert raw_indices is not None, "requires init_flashmla_related(is_prefill=True)"
+    num_q, width = raw_indices.shape
+    lens = (md.positions_casual + 1) // 4
+    max_len = int(lens.max().item())
+    pool = fixture.runner.token_to_kv_pool
+    c4_page_size = pool.get_extra_key_page_size(layer_id=0)
+    assert max_len <= min(
+        num_entries, c4_page_size
+    ), f"case attends {max_len} c4 entries; only {min(num_entries, c4_page_size)} populated"
+    assert (
+        md.page_table[:, 0] == 0
+    ).all(), "sparse seeding requires the raw==physical identity (first page 0)"
+    seq = (
+        torch.arange(width, dtype=raw_indices.dtype, device=raw_indices.device)
+        .unsqueeze(0)
+        .expand(num_q, -1)
+    )
+    seeded = torch.where(seq < lens.unsqueeze(1), seq, seq.new_full((), -1))
+    md.c4_sparse_raw_indices = seeded
+    md.c4_sparse_page_indices = seeded.clone()
+    md.c4_sparse_topk_lengths = lens.to(md.c4_sparse_topk_lengths.dtype)
+
+
 def run_dsv4_target_verify_attention_case(
     testcase,
     case: DSV4AttentionCase,
@@ -1540,6 +1591,7 @@ def run_dsv4_compress_attention_case(
     case: DSV4AttentionCase,
     *,
     extra_entries: int = 32,
+    sparse_prefill: bool = False,
     dtype: torch.dtype = torch.bfloat16,
     device: str = "cuda",
 ) -> None:
@@ -1549,17 +1601,24 @@ def run_dsv4_compress_attention_case(
     Pre-writes random packed K into both the SWA cache and the extra
     (C4/C128) cache via the production pack+set paths, lets
     `init_forward_metadata` populate the compression metadata, manually seeds
-    `c4_sparse_page_indices` for the C4 case (so the flash_mla `extra_k_cache`
-    path actually attends to entries we wrote rather than the all-`-1` initial
-    value that the un-run indexer would leave), then dispatches `forward(
-    compress_ratio=case.compress_ratio)` and compares against an independent
-    pure-PyTorch SWA + extra reference that reads the SAME cache bytes and
-    metadata indices.
+    the C4 metadata the exercised path consumes (see `_seed_c4_if_needed`; the
+    un-run indexer would otherwise leave it at `-1` / uninitialized), then
+    dispatches `forward(compress_ratio=case.compress_ratio)` and compares
+    against an independent pure-PyTorch SWA + extra reference that reads the
+    SAME cache bytes and metadata indices.
+
+    `sparse_prefill` pins `SGLANG_OPT_FLASHMLA_SPARSE_PREFILL`, selecting the
+    dense `flash_mla_with_kvcache` extend path or `_forward_prefill_sparse`;
+    the C4 seeding dispatches on the same flag.
     """
     assert case.compress_ratio in (
         4,
         128,
     ), f"smoke runner requires compress_ratio in (4, 128); got {case.compress_ratio}"
+    if sparse_prefill:
+        assert (
+            case.forward_mode.is_extend_without_speculative()
+        ), f"sparse prefill only serves extend; got {case.forward_mode}"
     fixture = build_dsv4_attention_fixture(
         testcase,
         case,
@@ -1567,6 +1626,7 @@ def run_dsv4_compress_attention_case(
         device=device,
         compression_ratios=[case.compress_ratio],
     )
+    fixture.seed_c4_for_sparse_prefill = sparse_prefill
     runner = fixture.runner
     max_context_len = runner.req_to_token_pool.req_to_token.shape[1]
 
@@ -1574,10 +1634,13 @@ def run_dsv4_compress_attention_case(
     _populate_extra_kv_cache(fixture, layer_id=0, num_entries=extra_entries)
 
     q_input, _ = fixture.actual_module.project(fixture.input_hidden)
-    with torch.no_grad(), forward_context(ForwardContext(attn_backend=fixture.backend)):
+    with (
+        torch.no_grad(),
+        forward_context(ForwardContext(attn_backend=fixture.backend)),
+        envs.SGLANG_OPT_FLASHMLA_SPARSE_PREFILL.override(sparse_prefill),
+    ):
         fixture.backend.init_forward_metadata(fixture.forward_batch)
-        if case.compress_ratio == 4:
-            _seed_c4_sparse_indices(fixture, num_entries=extra_entries)
+        _seed_c4_if_needed(fixture, num_entries=extra_entries)
         actual = fixture.backend.forward(
             q=q_input,
             k=q_input,
@@ -1588,6 +1651,17 @@ def run_dsv4_compress_attention_case(
             save_kv_cache=False,
             attn_sink=fixture.actual_module.attn_sink,
         )
+        # Only `_forward_prefill_sparse` populates `sparse_prefill_cache`;
+        # verify the intended path ran before the reference rebuilds metadata.
+        sparse_cache = fixture.backend.forward_metadata.sparse_prefill_cache
+        if sparse_prefill:
+            testcase.assertIsNotNone(
+                sparse_cache, f"{case.name} did not take _forward_prefill_sparse"
+            )
+        else:
+            testcase.assertIsNone(
+                sparse_cache, f"{case.name} did not take the dense extend path"
+            )
         expected = _pure_torch_dsv4_combined_reference(fixture, q_input)
 
     torch.testing.assert_close(
diff --git a/test/registered/attention/unittests/dsv4/test_deepseek_v4.py b/test/registered/attention/unittests/dsv4/test_deepseek_v4.py
index dbd949869..3f0737df7 100644
--- a/test/registered/attention/unittests/dsv4/test_deepseek_v4.py
+++ b/test/registered/attention/unittests/dsv4/test_deepseek_v4.py
@@ -183,13 +183,27 @@ class TestDSV4AttentionBackendCorrectness(CustomTestCase):
     )
 
     def test_compress_attention_cases(self):
+        # Pinned to the dense extend path; the sparse prefill path is covered
+        # by test_compress_attention_cases_sparse_prefill below.
         for case in self.COMPRESS_CASES:
             with self.subTest(
                 case=case.name,
                 backend=case.backend,
                 compress_ratio=case.compress_ratio,
             ):
-                run_dsv4_compress_attention_case(self, case)
+                run_dsv4_compress_attention_case(self, case, sparse_prefill=False)
+
+    def test_compress_attention_cases_sparse_prefill(self):
+        # `_forward_prefill_sparse` extend path; decode never reaches it.
+        for case in self.COMPRESS_CASES:
+            if not case.forward_mode.is_extend_without_speculative():
+                continue
+            with self.subTest(
+                case=case.name,
+                backend=case.backend,
+                compress_ratio=case.compress_ratio,
+            ):
+                run_dsv4_compress_attention_case(self, case, sparse_prefill=True)
 
     def test_eagle_target_verify_chain_cases(self):
         for case in self.TARGET_VERIFY_CASES: