diff --git a/inst/tinytest/test_flux_staging.R b/inst/tinytest/test_flux_staging.R
index 6e7bbc0..192f082 100644
--- a/inst/tinytest/test_flux_staging.R
+++ b/inst/tinytest/test_flux_staging.R
@@ -47,12 +47,30 @@ expect_equal(diffuseR:::.pinned_set_gb("flux1", "bf16"), 34)
 # --- flux tier text_device: flux1 GPU-encodes only where T5 fits ------------------
 
 # fp8/bf16 tiers (>=14 GB) onload T5; nf4 tiers keep CPU-fp32 encode.
-expect_equal(recommend("flux1", vram_gb = 24)$text_device, "cuda")
-expect_equal(recommend("flux1", vram_gb = 16)$text_device, "cuda")   # fp8 tier
-expect_equal(recommend("flux1", vram_gb = 10)$text_device, "cpu")    # nf4 tier
-expect_equal(recommend("flux1", vram_gb = 0)$text_device, "cpu")     # cpu tier
-# The small flux2/zimage encoder rides the GPU on every GPU tier.
-expect_equal(recommend("flux2", vram_gb = 16)$text_device, "cuda")
+#
+# st_caps is pinned explicitly here. Left NULL, recommend() probes the
+# *installed* safetensors, so the fp8 tier only exists on a machine whose
+# build can read float8 -- which made this block pass on a dev box with
+# the fork and fail on Windows R-devel, and would have failed on CRAN,
+# where flux1 at 16 GB resolves to nf4 and its CPU encode instead.
+st_fork <- list(bfloat16 = TRUE, float8_e4m3fn = TRUE)
+st_cran <- list(bfloat16 = TRUE, float8_e4m3fn = FALSE)
+expect_equal(recommend("flux1", vram_gb = 24, st_caps = st_fork)$text_device,
+             "cuda")
+expect_equal(recommend("flux1", vram_gb = 16, st_caps = st_fork)$text_device,
+             "cuda") # fp8 tier
+expect_equal(recommend("flux1", vram_gb = 16, st_caps = st_cran)$text_device,
+             "cpu") # same card, stock safetensors: nf4 tier
+expect_equal(recommend("flux1", vram_gb = 10, st_caps = st_fork)$text_device,
+             "cpu") # nf4 tier
+expect_equal(recommend("flux1", vram_gb = 0, st_caps = st_fork)$text_device,
+             "cpu") # cpu tier
+# The small flux2/zimage encoder rides the GPU on every GPU tier,
+# whichever safetensors is installed.
+expect_equal(recommend("flux2", vram_gb = 16, st_caps = st_fork)$text_device,
+             "cuda")
+expect_equal(recommend("flux2", vram_gb = 16, st_caps = st_cran)$text_device,
+             "cuda")
 
 # --- .flux_build_staging opt-out paths return NULL (no torch needed) --------------
 
diff --git a/inst/tinytest/test_fp8_ltx23.R b/inst/tinytest/test_fp8_ltx23.R
index fb86506..936f275 100644
--- a/inst/tinytest/test_fp8_ltx23.R
+++ b/inst/tinytest/test_fp8_ltx23.R
@@ -8,14 +8,9 @@ if (!requireNamespace("safetensors", quietly = TRUE)) {
   exit_file("safetensors not installed")
 }
 # fp8 needs the F8-capable safetensors build
-f8_ok <- tryCatch({
-  x <- torch::torch_randn(2, 2)$to(dtype = torch::torch_float8_e4m3fn())
-  tmp <- tempfile(fileext = ".safetensors")
-  safetensors::safe_save_file(list(w = x), tmp)
-  y <- safetensors::safe_load_file(tmp, framework = "torch")
-  unlink(tmp)
-  TRUE
-}, error = function(e) FALSE)
+# Same gate the quantizer uses, so the diffuseR.st_caps override
+# reaches this guard too; a private probe would disagree with it.
+f8_ok <- diffuseR:::.st_can_write("float8_e4m3fn")
 if (!f8_ok) exit_file("safetensors build lacks F8 support")
 
 library(diffuseR)
diff --git a/inst/tinytest/test_quantize_flux.R b/inst/tinytest/test_quantize_flux.R
index 03b8e02..32fe6a2 100644
--- a/inst/tinytest/test_quantize_flux.R
+++ b/inst/tinytest/test_quantize_flux.R
@@ -87,14 +87,11 @@ ltx23_release_dequant_buffers()
 
 # --- fp8 round trip (needs an F8-capable safetensors build) ---------------------------
 
-f8_ok <- tryCatch({
-  x <- torch::torch_randn(2, 2)$to(dtype = torch::torch_float8_e4m3fn())
-  tmp <- tempfile(fileext = ".safetensors")
-  safetensors::safe_save_file(list(w = x), tmp)
-  y <- safetensors::safe_load_file(tmp, framework = "torch")
-  unlink(tmp)
-  TRUE
-}, error = function(e) FALSE)
+# Ask the same gate flux_quantize() asks, rather than re-probing here: a
+# private probe can disagree with .st_can_write() whenever the
+# diffuseR.st_caps override is set, and then this block runs only to hit
+# the hard error the override was meant to simulate.
+f8_ok <- diffuseR:::.st_can_write("float8_e4m3fn")
 
 if (f8_ok) {
   fp8_dir <- file.path(tempdir(), "flux-tiny-fp8")
diff --git a/inst/tinytest/test_quantize_flux2.R b/inst/tinytest/test_quantize_flux2.R
index 162f7e9..1a3c52d 100644
--- a/inst/tinytest/test_quantize_flux2.R
+++ b/inst/tinytest/test_quantize_flux2.R
@@ -78,14 +78,9 @@ ltx23_release_dequant_buffers()
 
 # --- fp8 round trips (streamed and resident) ------------------------------------------
 
-f8_ok <- tryCatch({
-  x <- torch::torch_randn(2, 2)$to(dtype = torch::torch_float8_e4m3fn())
-  tmp <- tempfile(fileext = ".safetensors")
-  safetensors::safe_save_file(list(w = x), tmp)
-  y <- safetensors::safe_load_file(tmp, framework = "torch")
-  unlink(tmp)
-  TRUE
-}, error = function(e) FALSE)
+# Same gate the quantizer uses, so the diffuseR.st_caps override
+# reaches this guard too; a private probe would disagree with it.
+f8_ok <- diffuseR:::.st_can_write("float8_e4m3fn")
 
 if (f8_ok) {
   fp8_dir <- file.path(tempdir(), "flux2-tiny-fp8")
diff --git a/inst/tinytest/test_quantize_zimage.R b/inst/tinytest/test_quantize_zimage.R
index 75b96de..e47b102 100644
--- a/inst/tinytest/test_quantize_zimage.R
+++ b/inst/tinytest/test_quantize_zimage.R
@@ -76,14 +76,9 @@ ltx23_release_dequant_buffers()
 
 # --- fp8 round trips (streamed and resident) ------------------------------------------
 
-f8_ok <- tryCatch({
-  x <- torch::torch_randn(2, 2)$to(dtype = torch::torch_float8_e4m3fn())
-  tmp <- tempfile(fileext = ".safetensors")
-  safetensors::safe_save_file(list(w = x), tmp)
-  y <- safetensors::safe_load_file(tmp, framework = "torch")
-  unlink(tmp)
-  TRUE
-}, error = function(e) FALSE)
+# Same gate the quantizer uses, so the diffuseR.st_caps override
+# reaches this guard too; a private probe would disagree with it.
+f8_ok <- diffuseR:::.st_can_write("float8_e4m3fn")
 
 if (f8_ok) {
   fp8_dir <- file.path(tempdir(), "zimage-tiny-fp8")