diff --git a/scripts/serve-frank-primary.sh b/scripts/serve-frank-primary.sh
index d09b6c8..459b211 100755
--- a/scripts/serve-frank-primary.sh
+++ b/scripts/serve-frank-primary.sh
@@ -104,6 +104,7 @@ podman run -d --name "$FRANK_PRIMARY_CONTAINER" \
   --cap-add SYS_PTRACE \
   --device /dev/kfd --device /dev/dri --device /dev/mem \
   -e HIP_VISIBLE_DEVICES=0,1 -e VLLM_ROCM_USE_AITER=0 \
+  -e PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True \
   -v "$FRANK_PRIMARY_MODEL_DIR":/models:ro \
   "$FRANK_PRIMARY_IMAGE" /models \
   --served-model-name "$FRANK_PRIMARY_SERVED_NAME" --port 7770 \
diff --git a/tests/frank_primary_unit.bats b/tests/frank_primary_unit.bats
index a14b942..21a9bfc 100755
--- a/tests/frank_primary_unit.bats
+++ b/tests/frank_primary_unit.bats
@@ -106,6 +106,15 @@ DOWNLOAD="$REPO_ROOT/scripts/download-frank-primary.sh"
     grep -q '^FRANK_PRIMARY_REASONINGPARSER=glm47$' "$ENV_FILE"
 }
 
+@test "expandable segments is ON — the cache arena leaves no slack" {
+    # The expert-offload plan fills VRAM to the modelled headroom, so the
+    # profile/graph-capture phase has ~0.2 GiB to work with and the default
+    # allocator's fragmentation fails it: measured 2026-09-30 on the deployed
+    # unit, "Tried to allocate 662.00 MiB ... 240.00 MiB is free" from the
+    # engine core while the identical shape booted with this env set.
+    grep -q 'PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True' "$SERVE"
+}
+
 @test "expert offload is ON — the payload cannot fit VRAM without it" {
     # 170 GiB of int4 experts against 64 GiB of VRAM. Without the offload flag
     # the engine has no home for the expert tensors at all; the arena