diff --git a/scripts/frank-default-large.env b/scripts/frank-default-large.env
index 0629418..f1f6353 100644
--- a/scripts/frank-default-large.env
+++ b/scripts/frank-default-large.env
@@ -11,17 +11,22 @@ PORT=7770
 # contextWindow=262144 for frank-default-large. Model is 29GB; ROCm has
 # ~70GB headroom, so 2x262k quantized KV fits. Verify at startup.
 CTX_SIZE=524288
-PARALLEL=2
+# Parallel slots. 2026-08-15: two concurrent 16k prose prompts (high
+# prompt-cache reuse) crashed DSpark with 'ROCm error: an illegal memory
+# access was encountered' inside per-sequence block sampling
+# (ggml_cuda_synchronize in common_speculative_impl_draft_dflash::draft,
+# status 6/ABRT, systemd restarted).  DSpark multi-slot is NOT safe on this
+# box, same class as the DFlash shared-draft-context bug.  Pinned to 1 slot
+# per user instruction; revisit after an upstream multi-slot DSpark fix.
+PARALLEL=1
 # Spec: DSpark sidecar draft (magnitudedev/Qwen3.8-27B-DSpark-GGUF Q8_0,
 # 1.36B, block_size 7, Markov confidence head).  Switched from bundled-MTP
 # 2026-08-15 at the user's request.  DSpark is the semi-autoregressive
 # extension of DFlash (DeepSeek, arXiv 2607.05147): parallel diffusion
 # backbone + sequential Markov head + confidence-scheduled verification.
 # n_max clamps to the trained block_size 7 (server warning if set higher).
-# DSpark extends DFlash (which had a shared-draft-context premature-EOS bug)
-# but adds per-sequence Markov/confidence machinery; parallel safety on this
-# box is still pending the two-16k-prompt test 2026-08-15. Revert to
-# bundled-MTP or set PARALLEL=1 if the parallel test fails.
+# Parallel safety FAILED on this box (see the PARALLEL=1 note above);
+# single-slot only until an upstream multi-slot DSpark fix lands.
 # DRAFT_FILE names the sidecar GGUF in MODEL_DIR (downloaded 2026-08-15,
 # sha256 b007a76b2ce57c1a3ecf36046543aebf60a763861624647d653f16d336c781d2).
 SPEC_TYPE=draft-dspark