diff --git a/manager/state/raft/transport/peer.go b/manager/state/raft/transport/peer.go
index 1c79eae54..7b5efe156 100644
--- a/manager/state/raft/transport/peer.go
+++ b/manager/state/raft/transport/peer.go
@@ -172,6 +172,10 @@ func splitSnapshotData(_ context.Context, m *raftpb.Message) []api.StreamRaftMes
 		}
 
 		raftMsg := *m
+		// Clone Snapshot so that re-slicing Snapshot.Data below
+		// does not mutate m.Snapshot.Data through the shared pointer.
+		snap := *m.Snapshot
+		raftMsg.Snapshot = &snap
 
 		// sub-slice for this snapshot chunk.
 		raftMsg.Snapshot.Data = m.Snapshot.Data[snapDataIndex : snapDataIndex+chunkSize]
diff --git a/manager/state/raft/transport/transport_test.go b/manager/state/raft/transport/transport_test.go
index 14e4550e5..46e376b7b 100644
--- a/manager/state/raft/transport/transport_test.go
+++ b/manager/state/raft/transport/transport_test.go
@@ -105,6 +105,51 @@ func testSend(ctx context.Context, c *mockCluster, from uint64, to []uint64, msg
 	}
 }
 
+// TestSplitSnapshotDataDoesNotMutateInput is a regression test for #3231.
+// Before the fix, splitSnapshotData did a shallow copy of the raft message
+// and re-sliced the shared Snapshot.Data on each iteration, shrinking the
+// original slice's capacity and eventually panicking with
+// "slice bounds out of range".
+func TestSplitSnapshotDataDoesNotMutateInput(t *testing.T) {
+	ctx := context.Background()
+
+	// Build a MsgSnap whose Snapshot.Data clearly exceeds GRPCMaxMsgSize so
+	// that the split loop runs multiple iterations (where the bug manifests).
+	const dataSize = 3 * GRPCMaxMsgSize
+	data := make([]byte, dataSize)
+	for i := range data {
+		data[i] = byte(i % (1 << 8))
+	}
+	m := raftpb.Message{
+		Type: raftpb.MsgSnap,
+		From: 1,
+		To:   2,
+		Snapshot: &raftpb.Snapshot{
+			Data: data,
+			Metadata: raftpb.SnapshotMetadata{
+				Index: uint64(len(data)),
+			},
+		},
+	}
+	origData := m.Snapshot.Data
+	origLen, origCap := len(origData), cap(origData)
+
+	msgs := splitSnapshotData(ctx, &m)
+	require.Greater(t, len(msgs), 1, "data larger than GRPCMaxMsgSize must split into multiple chunks")
+
+	// Chunks must reassemble to the original data.
+	var assembled []byte
+	for _, msg := range msgs {
+		assembled = append(assembled, msg.Message.Snapshot.Data...)
+	}
+	assert.Equal(t, data, assembled)
+
+	// The input message's Snapshot.Data must be untouched (regression guard).
+	assert.Equal(t, origLen, len(m.Snapshot.Data))
+	assert.Equal(t, origCap, cap(m.Snapshot.Data))
+	assert.Equal(t, data, m.Snapshot.Data)
+}
+
 func TestSend(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	c := newCluster()