diff --git a/src/exo/worker/engines/mlx/patches/qwen3_5_moe_split/dflash_split.py b/src/exo/worker/engines/mlx/patches/qwen3_5_moe_split/dflash_split.py
index 6b08c4530..8733b06bd 100644
--- a/src/exo/worker/engines/mlx/patches/qwen3_5_moe_split/dflash_split.py
+++ b/src/exo/worker/engines/mlx/patches/qwen3_5_moe_split/dflash_split.py
@@ -152,12 +152,19 @@ def make_split_speculative_next(group):  # type: ignore[no-untyped-def]
             )
             n_accepted = int(gathered_n[MOE_RANK].item())
 
-        # 7. Rollback — same n_accepted on both ranks keeps caches consistent
+        # 7. Rollback — same n_accepted on both ranks keeps caches consistent.
+        # For BatchKVCache, both `offset` AND `_idx` must be decremented.
+        # `_idx` is the buffer-length tracker that `update_and_fetch` uses to
+        # decide where to write next and `fetch` uses to slice returned keys.
+        # Stock DFlash only decrements offset, which leaves stale rejected-draft
+        # entries in the cache that corrupt subsequent verifies.
         rollback = verify_len - n_accepted
         if rollback > 0:
             for c in batch.cache:
                 if hasattr(c, "offset"):
                     c.offset -= rollback
+                    if hasattr(c, "_idx"):
+                        c._idx -= rollback
                 elif hasattr(c, "rollback"):
                     c.rollback(n_accepted)