diff --git a/c/Makefile b/c/Makefile
index e794d4b..b04cf63 100644
--- a/c/Makefile
+++ b/c/Makefile
@@ -105,10 +105,30 @@ INSTALL    ?= install
 #                   clean. See backend_loader.c and README "cuda-dll" below.
 CUDA      ?= 0
 CUDA_DLL  ?= 0
+ifneq ($(IS_WIN),)
+# the CUDA installer sets CUDA_PATH system-wide (e.g. C:\Program Files\NVIDIA
+# GPU Computing Toolkit\CUDA\v13.2); fall back to it before the POSIX default.
+# NVCC defaults to plain `nvcc` from PATH: CUDA_PATH contains spaces, which the
+# unquoted recipe checks cannot survive — and the cuda-dll recipe already
+# requires an MSVC environment (vcvars64) on PATH, so requiring the CUDA bin
+# directory too is symmetric. The installer adds it by default.
+CUDA_HOME ?= $(subst \,/,$(CUDA_PATH))
+NVCC      ?= nvcc
+else
 CUDA_HOME ?= /usr/local/cuda
 NVCC      ?= $(CUDA_HOME)/bin/nvcc
+endif
 CUDA_ARCH ?= native
+ifneq ($(IS_WIN),)
+# nvcc's host compiler on Windows is MSVC cl.exe: the GCC-style
+# -Xcompiler=-Wall,-Wextra is rejected ("D8021 invalid numeric argument
+# '/Wextra'"), which made `make cuda-dll` unbuildable as shipped. -W3 is
+# the MSVC warning level (dash form, NOT /W3: MSYS make would mangle the
+# slash-form into a filesystem path).
+NVCCFLAGS ?= -O3 -std=c++17 -arch=$(CUDA_ARCH) -Xcompiler=-W3
+else
 NVCCFLAGS ?= -O3 -std=c++17 -arch=$(CUDA_ARCH) -Xcompiler=-Wall,-Wextra
+endif
 ifeq ($(IS_WIN),)
 PYTHON    ?= python3
 else
@@ -169,7 +189,7 @@ backend_loader.o: backend_loader.c backend_cuda.h compat.h
 # compiler, required by nvcc on Windows) into coli_cuda.dll. Run this from a
 # shell that has the MSVC environment set (e.g. after vcvars64.bat, or from a
 # "x64 Native Tools Command Prompt"). COLI_CUDA_BUILDING_DLL enables
-# __declspec(dllexport) so the 11 API symbols are exported.
+# __declspec(dllexport) so the 15 API symbols are exported.
 cuda-dll: backend_cuda.cu backend_cuda.h
 	@command -v $(NVCC) >/dev/null 2>&1 || { echo "nvcc not found: set CUDA_HOME or NVCC" >&2; exit 1; }
 	@command -v cl >/dev/null 2>&1 || { echo "cl.exe (MSVC) not in PATH — run vcvars64.bat first" >&2; exit 1; }
diff --git a/c/glm.c b/c/glm.c
index 8b061db..498a927 100644
--- a/c/glm.c
+++ b/c/glm.c
@@ -1497,7 +1497,12 @@ static void expert_host_release(Model *m, ESlot *s){
     if(s->fslab) compat_munlock(s->fslab,(size_t)s->fslab_cap*sizeof(float));
 #endif
     int64_t bytes=qt_bytes(&s->g)+qt_bytes(&s->u)+qt_bytes(&s->d);
-    free(s->slab); free(s->fslab); s->slab=NULL; s->fslab=NULL; s->slab_cap=s->fslab_cap=0;
+    /* slab is posix_memalign'd: on Windows that is _aligned_malloc, and plain
+     * free() corrupts the CRT heap (0xC0000374) — same bug the compat.h audit
+     * fixed at the original expert_load site. fslab is plain malloc/falloc
+     * on the CPU path, so its free() stays plain (Metal path frees it before
+     * re-alloc and never reaches here with an aligned fslab on _WIN32). */
+    compat_aligned_free(s->slab); free(s->fslab); s->slab=NULL; s->fslab=NULL; s->slab_cap=s->fslab_cap=0;
     QT *q[3]={&s->g,&s->u,&s->d};
     for(int k=0;k<3;k++){ q[k]->qf=NULL; q[k]->q8=NULL; q[k]->q4=NULL; q[k]->s=NULL; }
     m->resident_bytes-=bytes; if(m->resident_bytes<0) m->resident_bytes=0;
diff --git a/c/tests/test_backend_cuda.cu b/c/tests/test_backend_cuda.cu
index 58ef250..a9eb48f 100644
--- a/c/tests/test_backend_cuda.cu
+++ b/c/tests/test_backend_cuda.cu
@@ -5,6 +5,14 @@
 #include <cstdint>
 #include <cstdlib>
 
+#ifdef _WIN32
+/* MSVC has no POSIX setenv/unsetenv */
+static int setenv(const char *name, const char *value, int overwrite) {
+    (void)overwrite; return _putenv_s(name, value);
+}
+static int unsetenv(const char *name) { return _putenv_s(name, ""); }
+#endif
+
 static int close_enough(const float *got, const float *want, int n) {
     for (int i = 0; i < n; i++) {
         if (std::fabs(got[i] - want[i]) > 1e-4f) {