diff --git a/DEPLOY.md b/DEPLOY.md
index 108047a..4f5495f 100644
--- a/DEPLOY.md
+++ b/DEPLOY.md
@@ -161,17 +161,19 @@ Override the base directory with `BGFILTER_WEIGHTS_DIR`. `models/` is gitignored
 The pipeline runs a second, high-resolution matting model
 (`ZhengPeng7/BiRefNet_HR-matting`, ~425 MB) as a "second opinion" that clears
 background residue trapped between hair strands. It is **ON by default** (both CLI
-and HTTP service) and, with reuse-as-seg, that same forward doubles as the
-segmentation mask on the birefnet backend — so it replaces the primary seg model
-rather than adding to it. **You must provision this model** (Option A or B above):
+and HTTP service). **You must provision this model** (Option A or B above):
 
 - ⚠️ **On an offline server (`HF_HUB_OFFLINE=1`) it must be present before the
   first request** — otherwise every default request fails, since it cannot
   download the model at runtime.
 - Turn it off service-wide with `BGFILTER_CROSS_CHECK=0`, or override per request
   with the `cross_check` form field (`true`/`false`).
-- The service preloads it at startup with the rest of the warm set; on the
-  birefnet backend the redundant primary segmenter is skipped.
+- The service preloads it at startup with the rest of the warm set.
+- Optional: `cross_check.reuse_as_seg: true` in the config (default **off**) makes
+  that same forward double as the segmentation mask on the birefnet backend —
+  replacing the primary seg model instead of adding to it (~20 s/image faster on
+  CPU, ~1 GB less VRAM; the redundant primary segmenter is then skipped at
+  startup). Off by default so the veto stays an independent signal.
 
 ### Downloading behind a firewall (CN networks)
 
@@ -289,19 +291,18 @@ service has **no authentication**. To reach it from other hosts:
 - For throughput: use a GPU (`BGFILTER_DEVICE=cuda`, install a CUDA torch build) —
   roughly an order of magnitude faster — and/or run multiple instances behind a load
   balancer (each with `--workers 1`).
-- **GPU deployment (measured, RTX 5070 Ti 16 GB, full default pipeline —
-  cross-check on, reused as seg, bf16):** ~1–2 s/image (2 s at 4 MP), peak VRAM
-  ~10.1 GB allocated. The number that must fit on the card is the allocator's
-  *reserved* footprint; set this in the service environment on Linux:
-  ```ini
-  Environment=PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True
-  ```
-  Measured effect: reserved 15.3 GB → **12.0 GB** (fragmentation 5.2 → 1.9 GB) and
-  ~10 % *faster* — with it, the full pipeline fits a 16 GB card with ~3.5 GB
-  headroom even next to a desktop session. Without it (or on native Windows, where
-  the option is silently unsupported), reserved is ~15.3 GB: a headless 16 GB card
-  still fits, but a desktop-attached one is borderline. Precision `bf16` is what
-  makes 16 GB viable at all — fp32 reserved ~17 GB overflows.
+- **GPU deployment (measured, RTX 5070 Ti 16 GB, full pipeline — cross-check on,
+  bf16):** ~1–2 s/image (2 s at 4 MP), peak VRAM ~10.9 GB allocated (10.1 GB with
+  `reuse_as_seg: true`). The number that must fit on the card is the allocator's
+  *reserved* footprint. `bgfilter` defaults
+  `PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True` on Linux/WSL2 (set before
+  torch loads; native Windows does not support it, and an explicit env value
+  always wins). Measured effect: reserved 15.3 GB → **12.0 GB** with reuse
+  (13.3 GB without, the default) and ~10 % *faster* — the full pipeline fits a
+  16 GB card with ~2–3.5 GB headroom even next to a desktop session. Without
+  expandable segments (e.g. native Windows) reserved is ~15.3–16.2 GB:
+  borderline-to-overflow on a 16 GB card. Precision `bf16` is what makes 16 GB
+  viable at all — fp32 reserved ~17 GB overflows.
 - Memory: one worker holds all three models (~1 GB weights + runtime). `MemoryMax=8G`
   is a safe ceiling for a single CPU worker.
 - **Keeping RSS flat (glibc).** After each image the service calls glibc
diff --git a/DEPLOY_ZH.md b/DEPLOY_ZH.md
index 077fe0a..0a4c7ea 100644
--- a/DEPLOY_ZH.md
+++ b/DEPLOY_ZH.md
@@ -150,14 +150,17 @@ hf download ZhengPeng7/BiRefNet_HR-matting       --local-dir models/BiRefNet_HR-
 ### cross-check 否决（默认开启）
 
 管线会再跑一个高分辨率 matting 模型（`ZhengPeng7/BiRefNet_HR-matting`，约 425 MB）
-作为"第二意见"，清理发丝间残留的背景色。**CLI 和 HTTP 服务都默认开启**，而且开了
-reuse-as-seg 之后，在 birefnet 后端这同一次前向还兼作分割掩码——它是**替换**了 primary
-分割模型，而不是额外叠加。因此**必须备好这个模型**（上面方案 A 或 B）：
+作为"第二意见"，清理发丝间残留的背景色。**CLI 和 HTTP 服务都默认开启**，因此**必须
+备好这个模型**（上面方案 A 或 B）：
 
 - ⚠️ **离线服务器（`HF_HUB_OFFLINE=1`）必须在第一个请求前就备好它**，否则每个默认请求都会
   失败——运行时它无法联网下载模型。
 - 服务级关闭：`BGFILTER_CROSS_CHECK=0`；或按请求覆盖：表单字段 `cross_check`（`true`/`false`）。
-- 服务启动时会随暖机集一起预载它；在 birefnet 后端，冗余的 primary 分割模型会被跳过。
+- 服务启动时会随暖机集一起预载它。
+- 可选：配置里 `cross_check.reuse_as_seg: true`（默认**关**）可让这次前向在 birefnet
+  后端兼作分割掩码——**替换**而非叠加 primary 分割模型（CPU 快约 20 秒/张、GPU 省约
+  1 GB 显存；此时冗余的 primary 分割模型在启动时会被跳过）。默认关闭是为了让否决
+  保持为独立信号。
 
 ### 墙内下载（国内网络）
 
@@ -270,16 +273,15 @@ curl -sS -F "file=@input.png" -F "screen_color=#CFEFFF" \
   串行，所以单实例约 1–2 张/分钟。瓶颈是全分辨率的 ViTMatte 和 pymatting 前景估计。
 - 要提吞吐：上 GPU（`BGFILTER_DEVICE=cuda`，装 CUDA 版 torch），大约快一个数量级；
   和/或多实例 + 负载均衡（每个都 `--workers 1`）。
-- **GPU 部署（实测，RTX 5070 Ti 16 GB，完整默认管线——cross-check 开 + reuse-as-seg，
-  bf16）：** 约 1–2 秒/张（4 MP 也只要 2 秒），峰值 VRAM 分配约 10.1 GB。真正决定
-  「装不装得下」的是分配器的 *reserved* 占用；Linux 上在服务环境里加：
-  ```ini
-  Environment=PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True
-  ```
-  实测效果：reserved 15.3 GB → **12.0 GB**（碎片 5.2 → 1.9 GB），而且还**快约 10%**——
-  加上它之后，完整管线在 16 GB 卡上即使旁边挂着桌面会话也有约 3.5 GB 余量。不加
-  （或在原生 Windows 上——该选项会被静默忽略）reserved 约 15.3 GB：无头 16 GB 卡勉强
-  能装，带桌面输出的卡就很悬。另外 `bf16` 是 16 GB 可行的前提——fp32 的 reserved
+- **GPU 部署（实测，RTX 5070 Ti 16 GB，完整管线——cross-check 开，bf16）：**
+  约 1–2 秒/张（4 MP 也只要 2 秒），峰值 VRAM 分配约 10.9 GB（开 `reuse_as_seg`
+  为 10.1 GB）。真正决定「装不装得下」的是分配器的 *reserved* 占用。`bgfilter`
+  在 Linux/WSL2 上会**默认设置** `PYTORCH_CUDA_ALLOC_CONF=expandable_segments:True`
+  （在 torch 加载前;原生 Windows 不支持该选项;显式设过的环境变量优先）。
+  实测效果：reserved 15.3 GB → **12.0 GB**（开 reuse;默认不开为 13.3 GB），且还
+  **快约 10%**——完整管线在 16 GB 卡上即使旁边挂着桌面会话也有约 2–3.5 GB 余量。
+  没有 expandable segments（如原生 Windows）时 reserved 约 15.3–16.2 GB:在
+  16 GB 卡上从很悬到直接溢出。另外 `bf16` 是 16 GB 可行的前提——fp32 的 reserved
   约 17 GB，直接溢出。
 - 内存：单个 worker 持有全部三个模型（约 1 GB 权重 + 运行时）。单 CPU worker 用
   `MemoryMax=8G` 作上限比较稳妥。
diff --git a/README.md b/README.md
index 94efd49..a57e5d9 100644
--- a/README.md
+++ b/README.md
@@ -38,15 +38,15 @@ download (~0.9 GB) and one inference pass per image; disable with
 `--no-cross-check` (see the `cross_check` config section, and
 `docs/hair_gap_artifacts.md` for the analysis behind it).
 
-When the cross-check is on, that same HR-matting forward is **reused as the
-segmentation mask by default**, skipping the primary seg model entirely (one
-less model to load, ~20 s faster per image on CPU). Pilot-validated
-(TestImage3 / FixImage1): trimap 99.8% identical, no structural change to
-fingers, hair wisps or thin lines. Disable with `--no-cross-check-as-seg` to
-run the dedicated seg model instead. The reuse is a same-family swap, so it
+When the cross-check is on, that same HR-matting forward can optionally be
+**reused as the segmentation mask** (`--cross-check-as-seg`), skipping the
+primary seg model entirely (one less model to load, ~20 s faster per image on
+CPU, ~1 GB less VRAM on GPU). Pilot-validated (TestImage3 / FixImage1): trimap
+99.8% identical, no structural change to fingers, hair wisps or thin lines.
+**Off by default**: the dedicated segmenter keeps its own forward and the veto
+stays an independent second signal. The reuse is a same-family swap, so it
 applies to the `birefnet` backend only: with `--seg-backend anime-seg` the
-anime segmenter keeps its own forward and the cross-check veto still runs
-independently on top.
+anime segmenter always keeps its own forward.
 
 The segmentation trimap defaults to `directional` mode (chroma + seg + a hue-direction
 split: it keeps a background-coloured garment such as a white shirt while dropping a
@@ -119,8 +119,10 @@ D:\MiniConda\envs\lightML\python.exe -m bgfilter.cli `
 
 ## Performance (CPU reference numbers)
 
-Measured on a Ryzen 9700X (Zen 5, native bf16), 32 GB RAM, default settings
-(cross-check on, reused as seg; chunked global attention on):
+Measured on a Ryzen 9700X (Zen 5, native bf16), 32 GB RAM, with cross-check on,
+**`--cross-check-as-seg` enabled** and chunked global attention on. (Reuse is
+off by default on this branch; without it, add the primary BiRefNet@1024
+forward — roughly +20 s/image at these sizes.)
 
 | input | precision | warm / image | peak memory |
 |---|---|---|---|
@@ -218,12 +220,12 @@ caps input size (default ~4MP → `413`); `BGFILTER_PRELOAD=1` loads the default
 startup. Run a single worker (`--workers 1`) — each worker loads its own copy of the models.
 
 Like the CLI, the service runs the **cross-model veto by default** (config
-default: on). With reuse-as-seg it replaces the primary seg forward rather than
-adding to it, so it is not the latency hit it once was; its HR-matting model is
-preloaded with the rest of the warm set (and, on the birefnet backend, the now-
-redundant primary segmenter is skipped at startup). Set `BGFILTER_CROSS_CHECK=0`
-to turn it off service-wide, or use the per-request `cross_check` form field to
-override either way; `/healthz` reports the default as `crossCheck`.
+default: on); its HR-matting model is preloaded with the rest of the warm set.
+Set `BGFILTER_CROSS_CHECK=0` to turn it off service-wide, or use the per-request
+`cross_check` form field to override either way; `/healthz` reports the default
+as `crossCheck`. With `cross_check.reuse_as_seg: true` in the config (off by
+default) the HR-matting forward replaces the primary seg forward instead of
+adding to it, and the then-redundant primary segmenter is skipped at startup.
 
 ## Quality Check
 
diff --git a/bgfilter/__init__.py b/bgfilter/__init__.py
index 16a274e..324148f 100644
--- a/bgfilter/__init__.py
+++ b/bgfilter/__init__.py
@@ -1,6 +1,7 @@
 """Flat-background character matting pipeline."""
 
 import os
+import sys
 
 # Windows PyTorch allocates CPU tensors through a bundled mimalloc whose lazy
 # purge keeps ~10 GB of already-freed forward activations committed after a
@@ -11,4 +12,13 @@ import os
 # bgfilter.memtune.release_freed_memory(); see DEPLOY for jemalloc as an alt.
 os.environ.setdefault("MIMALLOC_PURGE_DELAY", "0")
 
+# Expandable segments defuse the CUDA caching allocator's fragmentation: the
+# full bf16 pipeline's reserved VRAM drops 15.3 -> 12.0 GB on a 16 GB card and
+# runs ~10% faster (measured, RTX 5070 Ti). Read at CUDA-allocator init, so it
+# must be set before torch loads; a no-op for CPU-only runs. Native Windows
+# does not support it (torch warns and ignores), so it is only defaulted where
+# it works (Linux/WSL2); setdefault keeps any explicit user setting in charge.
+if not sys.platform.startswith("win"):
+    os.environ.setdefault("PYTORCH_CUDA_ALLOC_CONF", "expandable_segments:True")
+
 __version__ = "0.1.0"
diff --git a/bgfilter/cli.py b/bgfilter/cli.py
index d326f41..8b27865 100644
--- a/bgfilter/cli.py
+++ b/bgfilter/cli.py
@@ -32,7 +32,7 @@ def main(
     fg_safe_radius_ratio: float | None = typer.Option(None, "--fg-safe-radius-ratio", min=0.0),
     despill: bool | None = typer.Option(None, "--despill/--no-despill"),
     cross_check: bool | None = typer.Option(None, "--cross-check/--no-cross-check", help="Second-opinion veto of background-hued residue between hair strands (default: on; costs one extra model inference)"),
-    cross_check_as_seg: bool | None = typer.Option(None, "--cross-check-as-seg/--no-cross-check-as-seg", help="Reuse the cross-check forward as the segmentation mask, skipping the primary seg model (default: on; saves ~20s on CPU; needs cross-check on)"),
+    cross_check_as_seg: bool | None = typer.Option(None, "--cross-check-as-seg/--no-cross-check-as-seg", help="Reuse the cross-check forward as the segmentation mask, skipping the primary seg model (default: off; saves ~20s on CPU; needs cross-check on)"),
     trimap_mode: str | None = typer.Option(None, "--trimap-mode", help="Trimap mode (segmentation pipeline): directional | seg | directional-hard-bg"),
     seg_backend: str | None = typer.Option(None, "--seg-backend", help="Segmentation backend: birefnet (default) | anime-seg"),
 ) -> None:
diff --git a/bgfilter/settings.py b/bgfilter/settings.py
index ea72a88..ac818b9 100644
--- a/bgfilter/settings.py
+++ b/bgfilter/settings.py
@@ -76,12 +76,13 @@ class CrossCheckSettings:
     input_size: int = 2048
     # Reuse the cross-check forward as the segmentation mask: one HR-matting
     # inference serves both trimap topology and the second opinion, skipping the
-    # primary seg model entirely (saves its load + forward). Pilot-validated on
-    # TestImage3/FixImage1: trimap 99.8% identical, no structural change. Only
-    # applies to the birefnet segmentation backend (a same-family swap); with
-    # anime-seg the chosen segmenter keeps its own forward and the cross-check
-    # runs independently. Needs cross_check and segmentation both enabled.
-    reuse_as_seg: bool = True
+    # primary seg model entirely (saves its load + forward; measured ~20 s/image
+    # on CPU, ~0.9-1.3 GB VRAM on GPU). Pilot-validated on TestImage3/FixImage1:
+    # trimap 99.8% identical. Default OFF on server-edition: the dedicated
+    # segmenter keeps its own forward and the veto stays an independent signal;
+    # opt in with --cross-check-as-seg. Only applies to the birefnet backend
+    # (a same-family swap); needs cross_check and segmentation both enabled.
+    reuse_as_seg: bool = False
     # Compute precision of the cross-check forward: "fp32" or "bf16" (autocast;
     # needs bf16-capable hardware, else falls back to fp32 with a warning).
     precision: str = "fp32"
diff --git a/configs/default.yaml b/configs/default.yaml
index 69a71e0..0c11370 100644
--- a/configs/default.yaml
+++ b/configs/default.yaml
@@ -74,8 +74,9 @@ cross_check:
   # trimap 99.8% identical, no structural change (fingers/wisps/bowstring OK).
   # Only applies to the birefnet backend (same-family swap); anime-seg keeps
   # its own forward, with the cross-check running independently on top.
-  # Disable with --no-cross-check-as-seg. No effect when cross_check is off.
-  reuse_as_seg: true
+  # Default OFF (server-edition): the dedicated segmenter keeps its own forward
+  # and the veto stays independent; enable with --cross-check-as-seg.
+  reuse_as_seg: false
   # Cross-check forward precision, same semantics as segmentation.precision
   # (bf16 via autocast: 2048 pass ~35s -> ~26s on a native-bf16 CPU).
   precision: fp32