diff --git a/tests/v1/worker/test_mamba_utils.py b/tests/v1/worker/test_mamba_utils.py
new file mode 100644
index 000000000..38eb250fb
--- /dev/null
+++ b/tests/v1/worker/test_mamba_utils.py
@@ -0,0 +1,67 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from unittest.mock import MagicMock, patch
+
+from vllm.v1.core.sched.output import CachedRequestData, SchedulerOutput
+from vllm.v1.worker.mamba_utils import preprocess_mamba
+
+
+def _make_scheduler_output(
+    finished_req_ids: set[str],
+    preempted_req_ids: set[str] | None,
+    resumed_req_ids: set[str],
+) -> SchedulerOutput:
+    cached = CachedRequestData.make_empty()
+    cached.resumed_req_ids = resumed_req_ids
+    return SchedulerOutput(
+        scheduled_new_reqs=[],
+        scheduled_cached_reqs=cached,
+        num_scheduled_tokens={},
+        total_num_scheduled_tokens=0,
+        scheduled_spec_decode_tokens={},
+        scheduled_encoder_inputs={},
+        num_common_prefix_blocks=[],
+        finished_req_ids=finished_req_ids,
+        free_encoder_mm_hashes=[],
+        preempted_req_ids=preempted_req_ids,
+    )
+
+
+def test_resumed_req_ids_cleared_from_mamba_state_idx():
+    """When a request is force-preempted (e.g. reset_prefix_cache),
+    it appears in resumed_req_ids but NOT in preempted_req_ids.
+    preprocess_mamba must still clear its mamba_state_idx entry,
+    otherwise stale indices can point beyond the new block allocation.
+    """
+    spec = MagicMock(block_size=64, num_speculative_blocks=0)
+    cache_config = MagicMock(enable_prefix_caching=True)
+    input_batch = MagicMock(req_ids=[])
+
+    mamba_state_idx = {
+        "finished": 1,
+        "preempted": 2,
+        "resumed": 3,  # only in resumed_req_ids, NOT in preempted
+        "keep": 99,
+    }
+    sched = _make_scheduler_output(
+        finished_req_ids={"finished"},
+        preempted_req_ids={"preempted"},
+        resumed_req_ids={"resumed"},
+    )
+
+    with patch(
+        "vllm.v1.worker.mamba_utils.get_mamba_groups",
+        return_value=([0], spec),
+    ):
+        preprocess_mamba(
+            sched,
+            MagicMock(),
+            cache_config,
+            mamba_state_idx,
+            input_batch,
+            {},
+            {},
+            (),
+        )
+
+    assert mamba_state_idx == {"keep": 99}
diff --git a/vllm/v1/worker/mamba_utils.py b/vllm/v1/worker/mamba_utils.py
index a22b0eeb0..4f8a3bd05 100644
--- a/vllm/v1/worker/mamba_utils.py
+++ b/vllm/v1/worker/mamba_utils.py
@@ -129,7 +129,13 @@ def preprocess_mamba(
     block_size = mamba_spec.block_size
     finished_req_ids = scheduler_output.finished_req_ids
     preempted_req_ids = scheduler_output.preempted_req_ids or set()
-    for req_id in itertools.chain(finished_req_ids, preempted_req_ids):
+    # We need to clear mamba_state_idx for resumed requests. When requests are
+    # force-preempted (e.g., during reset_prefix_cache / KV cache flush),
+    # they appear in resumed_req_ids without a corresponding entry in
+    # preempted_req_ids, leaving stale mamba_state_idx entries that can
+    # point to block indices beyond the new (smaller) block allocation.
+    resumed_req_ids = scheduler_output.scheduled_cached_reqs.resumed_req_ids
+    for req_id in itertools.chain(finished_req_ids, preempted_req_ids, resumed_req_ids):
         mamba_state_idx.pop(req_id, None)
 
     src_state_list: list[int] = []