[pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

.github/workflows/studio-gpu-offload-smoke.yml

@@ -0,0 +1,65 @@
+# Cross-platform GPU-offload validation smoke (no GPU required).
+#
+# Reproduces the silent CPU-only GGUF bug (#5807 / #5106 / #5830) with a fake
+# llama-server that "starts and serves HTTP 200" while its log reports CPU-only
+# or GPU offload, and asserts install_llama_prebuilt.py --smoke-test rejects the
+# CPU-only-tagged-GPU case (exit 2) and accepts the GPU case (exit 0). Runs on
+# GPU-less Windows / macOS / Linux runners, which is why this regressed
+# untested. Also runs the pure-Python selection + classifier unit tests.
+
+name: Studio GPU Offload Smoke
+
+on:
+  pull_request:
+    paths:
+      - 'studio/install_llama_prebuilt.py'
+      - 'studio/setup.sh'
+      - 'studio/setup.ps1'
+      - 'studio/backend/core/inference/llama_cpp.py'
+      - 'tests/studio/install/**'
+      - 'tests/sh/test_llama_gpu_smoke.sh'
+      - '.github/workflows/studio-gpu-offload-smoke.yml'
+  push:
+    branches: [main, pip]
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  gpu-offload-spoof:
+    name: GPU-offload spoof (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 15
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Install pytest
+        run: python -m pip install --upgrade pip pytest
+
+      - name: GPU-offload spoof (end to end, real subprocess + HTTP)
+        run: python tests/studio/install/run_smoke_spoof.py
+
+      - name: Selection + classifier unit tests
+        run: >
+          python -m pytest
+          tests/studio/install/test_validate_server_gpu_offload.py
+          tests/studio/install/test_selection_logic.py
+          tests/studio/install/test_gpu_offload_spoof.py
+          -q
+
+      - name: setup.sh smoke-exit classifier (POSIX)
+        if: runner.os != 'Windows'
+        run: bash tests/sh/test_llama_gpu_smoke.sh

studio/backend/core/inference/llama_cpp.py

@@ -59,6 +59,107 @@ from core.inference.tool_call_parser import (
 logger = get_logger(__name__)
 
 
+# ── GPU-offload log classifier (shared shape with install_llama_prebuilt) ──
+# llama-server can serve HTTP 200 while running entirely on CPU; detect that so
+# Studio can warn instead of silently running a "GPU" model on CPU (#5807 /
+# #5106 / #5830). Robust across llama.cpp log formats (the "model buffer size"
+# lines were dropped in recent builds): buffer-size markers, "offloaded N/M
+# layers to GPU", and the "device_info:" enumeration.
+_GPU_BUFFER_MARKERS = (
+    "CUDA",
+    "ROCm",
+    "ROCM",
+    "HIP",
+    "Metal",
+    "Vulkan",
+    "OpenCL",
+    "SYCL",
+    "MUSA",
+    "CANN",
+)
+_DEVICE_ROW_RE = re.compile(
+    r"-\s*(?P<dev>(?:CUDA|ROCm|ROCM|HIP|Metal|Vulkan|SYCL|OpenCL|MUSA|CANN|CPU)\w*)\s*:",
+    re.IGNORECASE,
+)
+_GPU_DEVICE_PREFIXES = (
+    "cuda",
+    "rocm",
+    "hip",
+    "metal",
+    "vulkan",
+    "sycl",
+    "opencl",
+    "musa",
+    "cann",
+)
+_OFFLOADED_LAYERS_RE = re.compile(
+    r"offloaded\s+(\d+)\s*/\s*(\d+)\s+layers?\s+to\s+gpu", re.IGNORECASE
+)
+_OFFLOADING_COUNT_RE = re.compile(
+    r"offloading\s+(\d+)\s+(?:repeating\s+|non-repeating\s+)?layers?\s+to\s+gpu",
+    re.IGNORECASE,
+)
+
+
+def classify_gpu_offload_lines(lines: list[str]) -> Optional[bool]:
+    """True if the model landed on a GPU, False if it stayed on CPU despite GPU
+    intent, None when the log has no usable signal. Priority: explicit
+    offloaded-layer counts (authoritative), then GPU model-buffer lines, then
+    device_info enumeration."""
+    # Signal 1: explicit offloaded counts win over everything (a KV/compute
+    # buffer can be on the GPU while 0 model layers are offloaded). Any counted
+    # line N>0 => True; all 0 => False (scan all; a draft model can log 0/k
+    # before the main model's 33/33). Uncounted "offloading output layer to GPU"
+    # is only a weak positive when no counted line exists.
+    saw_zero_count = False
+    saw_uncounted_offloading = False
+    for line in lines:
+        match = _OFFLOADED_LAYERS_RE.search(line) or _OFFLOADING_COUNT_RE.search(line)
+        if match:
+            if int(match.group(1)) > 0:
+                return True
+            saw_zero_count = True
+            continue
+        low = line.lower()
+        if "offloading" in low and "to gpu" in low:
+            saw_uncounted_offloading = True
+    if saw_zero_count:
+        return False
+    if saw_uncounted_offloading:
+        return True
+
+    # Signal 2: GPU marker on a *model* buffer (exclude host-pinned _Host).
+    saw_model_buffer = False
+    for line in lines:
+        if "model buffer size" not in line:
+            continue
+        saw_model_buffer = True
+        if "_Host" not in line and any(
+            marker in line for marker in _GPU_BUFFER_MARKERS
+        ):
+            return True
+
+    after_device_info = False
+    saw_device_row = False
+    for line in lines:
+        if "device_info:" in line:
+            after_device_info = True
+            continue
+        if not after_device_info:
+            continue
+        match = _DEVICE_ROW_RE.search(line)
+        if not match:
+            continue
+        saw_device_row = True
+        dev = match.group("dev").lower()
+        if any(dev.startswith(prefix) for prefix in _GPU_DEVICE_PREFIXES):
+            return True
+
+    if saw_model_buffer or saw_device_row:
+        return False
+    return None
+
+
 # ── Pre-compiled patterns for plan-without-action re-prompt ──
 # Forward-looking intent signals that indicate the model is
 # describing what it *will* do rather than giving a final answer.
@@ -3865,27 +3966,14 @@ class LlamaCppBackend:
         expected_gpu: bool,
         detected_gpus: list[tuple[int, int]],
     ) -> Optional[bool]:
-        """True if a GPU model buffer was allocated, False if only CPU
-        buffers landed despite GPU intent, None when there's no signal
-        (no GPU detected, no buffer-size lines, etc.)."""
+        """True if the model landed on a GPU, False if only CPU buffers landed
+        despite GPU intent, None when there's no signal (no GPU detected, no
+        usable log lines, etc.). Delegates to the shared classifier so it tracks
+        current llama.cpp logs (device_info / offloaded-layers), which dropped
+        the older ``model buffer size`` lines this used to key on."""
         if not detected_gpus or not expected_gpu:
             return None
-        # llama-server logs one ``... model buffer size = N MiB`` line
-        # per backend buffer; CUDA0 / ROCm0 / Metal / Vulkan0 /
-        # OpenCL0 / SYCL0 are GPU, CPU / CPU_Mapped are not.
-        gpu_markers = ("CUDA", "ROCm", "Metal", "Vulkan", "OpenCL", "SYCL")
-        saw_buffer_line = False
-        saw_gpu_buffer = False
-        for line in self._stdout_lines:
-            if "model buffer size" not in line:
-                continue
-            saw_buffer_line = True
-            if any(marker in line for marker in gpu_markers):
-                saw_gpu_buffer = True
-                break
-        if not saw_buffer_line:
-            return None
-        return saw_gpu_buffer
+        return classify_gpu_offload_lines(self._stdout_lines)
 
     def unload_model(self) -> bool:
         """Terminate the llama-server subprocess and cancel any in-flight download."""

studio/backend/tests/test_llama_cpp_context_fit.py

@@ -577,3 +577,80 @@ class TestClassifyGpuOffload:
             ]
         )
         assert inst._classify_gpu_offload(True, [(0, 22805)]) is True
+
+    def test_current_device_info_cuda_returns_true(self):
+        # Recent llama.cpp dropped the buffer lines and prints device_info.
+        inst = self._backend(
+            [
+                "0.00 I device_info:",
+                "0.01 I   - CUDA0 : NVIDIA GeForce RTX 5070 (12282 MiB free)",
+                "0.01 I   - CPU : AMD Ryzen 7 9700X (32000 MiB free)",
+            ]
+        )
+        assert inst._classify_gpu_offload(True, [(0, 12282)]) is True
+
+    def test_current_device_info_cpu_only_returns_false(self):
+        # The #5830 symptom: device_info shows only CPU, CUDA0 missing.
+        inst = self._backend(
+            [
+                "0.00 I device_info:",
+                "0.00 I   - CPU : AMD Ryzen 7 9700X (32000 MiB free)",
+                "0.00 I srv llama_server: model loaded",
+            ]
+        )
+        assert inst._classify_gpu_offload(True, [(0, 12282)]) is False
+
+    def test_offloaded_layers_count_decides(self):
+        assert (
+            self._backend(
+                ["load_tensors: offloaded 33/33 layers to GPU"]
+            )._classify_gpu_offload(True, [(0, 22805)])
+            is True
+        )
+        assert (
+            self._backend(
+                ["load_tensors: offloaded 0/33 layers to GPU"]
+            )._classify_gpu_offload(True, [(0, 22805)])
+            is False
+        )
+
+    def test_system_info_only_returns_none(self):
+        # A compiled CUDA backend advertised in system_info is not proof of an
+        # available device; without device_info/buffer/offload signal -> None.
+        inst = self._backend(
+            ["system_info: n_threads = 8 | CUDA : ARCHS = 1200 | CPU : AVX2 = 1"]
+        )
+        assert inst._classify_gpu_offload(True, [(0, 22805)]) is None
+
+    def test_cuda_host_buffer_is_not_gpu(self):
+        # CUDA_Host is host-pinned CPU RAM; weights on CPU_Mapped means CPU only.
+        inst = self._backend(
+            [
+                "load_tensors:   CUDA_Host model buffer size = 21000.0 MiB",
+                "load_tensors:   CPU_Mapped model buffer size =     0.6 MiB",
+            ]
+        )
+        assert inst._classify_gpu_offload(True, [(0, 22805)]) is False
+
+    def test_draft_zero_before_main_offload_is_gpu(self):
+        inst = self._backend(
+            [
+                "load_tensors: offloaded 0/2 layers to GPU",
+                "load_tensors: offloaded 33/33 layers to GPU",
+            ]
+        )
+        assert inst._classify_gpu_offload(True, [(0, 22805)]) is True
+
+    def test_offloading_zero_repeating_does_not_mask_zero_total(self):
+        inst = self._backend(
+            [
+                "llm_load_tensors: offloading 0 repeating layers to GPU",
+                "llm_load_tensors: offloaded 0/33 layers to GPU",
+                "llm_load_tensors: CPU model buffer size = 7338.64 MiB",
+            ]
+        )
+        assert inst._classify_gpu_offload(True, [(0, 22805)]) is False
+
+    def test_hip_model_buffer_is_gpu(self):
+        inst = self._backend(["load_tensors:   HIP0 model buffer size = 21000.0 MiB"])
+        assert inst._classify_gpu_offload(True, [(0, 22805)]) is True

studio/install_llama_prebuilt.py

@@ -452,6 +452,14 @@ class PrebuiltFallback(RuntimeError):
     pass
 
 
+class GpuOffloadFailure(PrebuiltFallback):
+    """The binary started and served a completion but loaded the model only on
+    CPU despite GPU offload being requested. A subclass of PrebuiltFallback so
+    the prebuilt resolver still advances to the next candidate, while the
+    --smoke-test CLI can tell this apart from an inconclusive start/serve
+    failure (definite CPU-only -> EXIT_FALLBACK; inconclusive -> EXIT_ERROR)."""
+
+
 class BusyInstallConflict(RuntimeError):
     pass
 
@@ -1437,7 +1445,14 @@ def direct_linux_release_plan(
         )
         if lemonade_choice is not None:
             attempts.append(lemonade_choice)
-    else:
+    elif not host.has_usable_nvidia:
+        # CPU-only host (or --cpu-fallback, which zeroes the GPU flags): ship the
+        # CPU bundle. Do NOT append it for a usable-NVIDIA host: if every CUDA
+        # bundle fails the GPU-offload check, let install raise PrebuiltFallback
+        # so setup.sh builds llama.cpp from source for the native arch (which
+        # offloads) instead of silently shipping a CPU-only "GPU" install
+        # (#5807). The CPU prebuilt stays the last resort after the source build,
+        # reached via setup.sh's --cpu-fallback path.
         cpu_choice = published_asset_choice_for_kind(bundle, "linux-cpu")
         if cpu_choice is not None:
             attempts.append(cpu_choice)
@@ -1530,19 +1545,26 @@ def direct_upstream_release_plan(
                         install_kind = "windows-hip",
                     )
                 )
-        cpu_asset = f"llama-{release_tag}-bin-win-cpu-x64.zip"
-        cpu_url = assets.get(cpu_asset)
-        if cpu_url:
-            attempts.append(
-                AssetChoice(
-                    repo = repo,
-                    tag = release_tag,
-                    name = cpu_asset,
-                    url = cpu_url,
-                    source_label = "upstream",
-                    install_kind = "windows-cpu",
+        # Append the CPU bundle only for a host with no usable GPU (or
+        # --cpu-fallback, which zeroes the GPU flags). On a GPU host, omitting it
+        # lets install raise PrebuiltFallback when every GPU bundle fails the
+        # offload check, so setup.ps1 builds from source / falls back to the CPU
+        # prebuilt as a labelled last resort instead of silently shipping a
+        # CPU-only "GPU" install (#5807).
+        if not host.has_usable_nvidia and not host.has_rocm:
+            cpu_asset = f"llama-{release_tag}-bin-win-cpu-x64.zip"
+            cpu_url = assets.get(cpu_asset)
+            if cpu_url:
+                attempts.append(
+                    AssetChoice(
+                        repo = repo,
+                        tag = release_tag,
+                        name = cpu_asset,
+                        url = cpu_url,
+                        source_label = "upstream",
+                        install_kind = "windows-cpu",
+                    )
                 )
-            )
     elif host.is_windows and host.is_arm64:
         # Upstream ggml-org/llama.cpp ships llama-bNNNN-bin-win-cpu-arm64.zip
         # (visible in the b9334 release manifest). Without this branch the
@@ -1589,7 +1611,12 @@ def direct_upstream_release_plan(
                     install_kind = "macos-x64",
                 )
             )
-    elif host.is_linux and host.is_x86_64 and not host.has_usable_nvidia:
+    elif (
+        host.is_linux
+        and host.is_x86_64
+        and not host.has_usable_nvidia
+        and not host.has_rocm
+    ):
         asset_name = f"llama-{release_tag}-bin-ubuntu-x64.tar.gz"
         asset_url = assets.get(asset_name)
         if asset_url:
@@ -1603,7 +1630,12 @@ def direct_upstream_release_plan(
                     install_kind = "linux-cpu",
                 )
             )
-    elif host.is_linux and host.is_arm64 and not host.has_usable_nvidia:
+    elif (
+        host.is_linux
+        and host.is_arm64
+        and not host.has_usable_nvidia
+        and not host.has_rocm
+    ):
         # Upstream ggml-org/llama.cpp ships llama-bNNNN-bin-ubuntu-arm64.tar.gz
         # (visible in the b9334 release manifest). Without this branch the
         # selector returned 0 attempts and the installer fell back to a
@@ -5577,6 +5609,179 @@ def validate_quantize(
         )
 
 
+# GPU backend markers that appear in llama-server's "... model buffer size ..."
+# load lines (older llama.cpp) and in its "device_info:" enumeration (current
+# llama.cpp). CUDA0 / ROCm0 / Metal / Vulkan0 / OpenCL0 / SYCL0 / HIP0 / MUSA0
+# / CANN0 are GPU; CPU / CPU_Mapped are not. Kept broad so a single helper
+# covers every backend Studio ships.
+_GPU_MODEL_BUFFER_MARKERS = (
+    "CUDA",
+    "ROCm",
+    "ROCM",
+    "HIP",
+    "Metal",
+    "Vulkan",
+    "OpenCL",
+    "SYCL",
+    "MUSA",
+    "CANN",
+)
+
+# A device_info row looks like "<ts> I   - CUDA0   : NVIDIA B200 (...)" or
+# "<ts> I   - CPU : ...". Matched on the "- <backend> :" shape (the leading
+# "- " distinguishes it from the system_info line, which prints
+# "| CUDA : ARCHS = ..." for a *compiled* backend even when the CUDA runtime
+# failed to initialize at load time). Scanned only after a "device_info:" line.
+_DEVICE_ROW_RE = re.compile(
+    r"-\s*(?P<dev>(?:CUDA|ROCm|ROCM|HIP|Metal|Vulkan|SYCL|OpenCL|MUSA|CANN|CPU)\w*)\s*:",
+    re.IGNORECASE,
+)
+# Keep in sync with _GPU_MODEL_BUFFER_MARKERS: every GPU backend matched by
+# _DEVICE_ROW_RE must have a prefix here, or its device row counts as CPU-only
+# and a working GPU build is wrongly rejected (OpenCL was the gap).
+_GPU_DEVICE_PREFIXES = (
+    "cuda",
+    "rocm",
+    "hip",
+    "metal",
+    "vulkan",
+    "sycl",
+    "opencl",
+    "musa",
+    "cann",
+)
+
+# "load_tensors: offloaded 33/33 layers to GPU".
+_OFFLOADED_LAYERS_RE = re.compile(
+    r"offloaded\s+(\d+)\s*/\s*(\d+)\s+layers?\s+to\s+gpu", re.IGNORECASE
+)
+# "offloading 0 repeating layers to GPU" / "offloading 1 non-repeating ...".
+# A counted form, so a zero here is a definite CPU-only signal (it usually
+# precedes "offloaded 0/N"); kept separate from the uncounted
+# "offloading output layer to GPU" phrasing, which carries no number.
+_OFFLOADING_COUNT_RE = re.compile(
+    r"offloading\s+(\d+)\s+(?:repeating\s+|non-repeating\s+)?layers?\s+to\s+gpu",
+    re.IGNORECASE,
+)
+
+# install_kind values that ship a real GPU backend and are launched with
+# --n-gpu-layers 1 during validation, so a real Metal / CUDA / HIP backend is
+# loaded and a broken GPU dylib/DLL surfaces. CPU kinds (windows-cpu,
+# linux-cpu, ...) are exempt.
+_GPU_INSTALL_KINDS = frozenset(
+    {"linux-cuda", "linux-rocm", "windows-cuda", "windows-hip", "macos-arm64"}
+)
+
+# Subset whose CPU-only load is a *fixable* fault -- a missing cudart64_* /
+# cublas64_* DLL, a PTX-only build on an older driver, or a HIP backend that
+# did not initialize -- that a different bundle or a source build can repair,
+# so validation rejects it to advance the resolver (#5807, #5106). macOS Metal
+# is excluded on purpose: on Apple Silicon the macos-arm64 prebuilt is already
+# the correct artifact, and a CPU-only load means Metal is unavailable in this
+# environment (a headless or virtualized host such as a CI runner), which no
+# rebuild can fix. Rejecting it would force a source build that also runs on
+# CPU and needlessly breaks the install. macOS is still launched with
+# --n-gpu-layers (it is in _GPU_INSTALL_KINDS) so a broken libggml-metal.dylib
+# still surfaces; it is only exempt from the CPU-only *rejection*.
+_GPU_OFFLOAD_REQUIRED_KINDS = _GPU_INSTALL_KINDS - frozenset({"macos-arm64"})
+
+
+def server_log_shows_gpu_offload(log_text: str) -> bool | None:
+    """Classify whether llama-server put the model on the GPU.
+
+    Robust across llama.cpp log formats (the "model buffer size" lines were
+    dropped in recent builds), in priority order:
+
+    1. ``offloaded N/M layers to GPU`` / ``offloading N ... layers to GPU`` --
+       the explicit layer count is authoritative: N>0 => True, all 0 => False.
+       It must win over auxiliary GPU buffers (a KV/compute buffer can sit on
+       the GPU while 0 model layers are offloaded -- still CPU inference).
+    2. ``<backend> model buffer size = N`` lines -- a GPU marker on a *model*
+       buffer (not host-pinned ``CUDA_Host``) => True (older llama.cpp).
+    3. ``device_info:`` enumeration -- a GPU device row (``- CUDA0 :`` etc.)
+       => True; only a ``- CPU :`` row => False. This is the version-stable
+       signal and matches how the CPU-only-binary bug is diagnosed in the
+       field (``device_info`` shows only CPU, CUDA0 missing -- #5830).
+
+    Returns True (GPU offload confirmed), False (started but stayed on CPU
+    despite GPU intent -- the silent fallback in #5807 / #5106), or None when
+    the log carries no usable signal so callers never reject on no evidence.
+    """
+    lines = log_text.splitlines()
+
+    # Signal 1: explicit offloaded-layers counts (authoritative). Any counted
+    # line with N>0 => True; if every counted line is 0 => False (a draft model
+    # can log "offloaded 0/k" before the main model's "offloaded 33/33", so scan
+    # all before deciding). The uncounted "offloading output layer to GPU"
+    # phrasing carries no number, so it is only a weak positive used when no
+    # counted line exists at all.
+    saw_zero_count = False
+    saw_uncounted_offloading = False
+    for line in lines:
+        match = _OFFLOADED_LAYERS_RE.search(line) or _OFFLOADING_COUNT_RE.search(line)
+        if match:
+            if int(match.group(1)) > 0:
+                return True
+            saw_zero_count = True
+            continue
+        low = line.lower()
+        if "offloading" in low and "to gpu" in low:
+            saw_uncounted_offloading = True
+    if saw_zero_count:
+        return False
+    if saw_uncounted_offloading:
+        return True
+
+    # Signal 2: per-backend model-buffer lines. A GPU marker on a *model* buffer
+    # means model weights live on the GPU. Exclude host-pinned buffers
+    # ("CUDA_Host" / "ROCm_Host" ...): those are CPU RAM the GPU backend pinned,
+    # not device memory. KV/compute buffers are ignored here -- they can be on
+    # the GPU even when all weights are on CPU.
+    saw_model_buffer = False
+    for line in lines:
+        if "model buffer size" not in line:
+            continue
+        saw_model_buffer = True
+        if "_Host" not in line and any(
+            marker in line for marker in _GPU_MODEL_BUFFER_MARKERS
+        ):
+            return True
+
+    # Signal 3: device_info enumeration. Only trust device rows once the
+    # "device_info:" header has appeared, so the compiled-backend system_info
+    # line ("| CUDA : ARCHS = ...") is never mistaken for an available device.
+    after_device_info = False
+    saw_device_row = False
+    for line in lines:
+        if "device_info:" in line:
+            after_device_info = True
+            continue
+        if not after_device_info:
+            continue
+        match = _DEVICE_ROW_RE.search(line)
+        if not match:
+            continue
+        saw_device_row = True
+        dev = match.group("dev").lower()
+        if any(dev.startswith(prefix) for prefix in _GPU_DEVICE_PREFIXES):
+            return True
+
+    if saw_model_buffer or saw_device_row:
+        # We had a concrete signal, but every buffer/device was CPU.
+        return False
+    return None
+
+
+def read_full_log(log_path: Path, *, max_chars: int = 1_000_000) -> str:
+    """Read a server log from the start (model-load buffer lines appear early,
+    so unlike read_log_excerpt this keeps the head rather than the tail)."""
+    try:
+        content = log_path.read_text(encoding = "utf-8", errors = "replace")
+    except FileNotFoundError:
+        return ""
+    return content[:max_chars]
+
+
 def validate_server(
     server_path: Path,
     probe_path: Path,
@@ -5585,6 +5790,7 @@ def validate_server(
     *,
     runtime_line: str | None = None,
     install_kind: str | None = None,
+    require_gpu_signal: bool = False,
 ) -> None:
     last_failure: PrebuiltFallback | None = None
     for port_attempt in range(1, SERVER_PORT_BIND_ATTEMPTS + 1):
@@ -5615,15 +5821,10 @@ def validate_server(
         # validation. Use the resolved install_kind as the source of
         # truth and fall back to host detection when the caller did not
         # pass one (keeps backwards compatibility with older call sites).
-        _gpu_kinds = {
-            "linux-cuda",
-            "linux-rocm",
-            "windows-cuda",
-            "windows-hip",
-            "macos-arm64",
-        }
+        _gpu_kinds = _GPU_INSTALL_KINDS
         if install_kind is not None:
             _enable_gpu_layers = install_kind in _gpu_kinds
+            _require_gpu_offload = install_kind in _GPU_OFFLOAD_REQUIRED_KINDS
         else:
             # Older call sites that don't pass install_kind: keep ROCm
             # hosts in the GPU-validation path so an AMD-only Linux host
@@ -5634,6 +5835,10 @@ def validate_server(
                 or host.has_rocm
                 or (host.is_macos and host.is_arm64)
             )
+            # macOS Metal is exercised but never *required* (a CPU-only Metal
+            # load is an environment limitation, not a fixable binary fault;
+            # see _GPU_OFFLOAD_REQUIRED_KINDS).
+            _require_gpu_offload = host.has_usable_nvidia or host.has_rocm
         if _enable_gpu_layers:
             command.extend(["--n-gpu-layers", "1"])
 
@@ -5657,6 +5862,7 @@ def validate_server(
                 startup_started = time.time()
                 response_body = ""
                 last_error: Exception | None = None
+                completion_succeeded = False
                 while time.time() < deadline:
                     if process.poll() is not None:
                         process.wait(timeout = 5)
@@ -5697,7 +5903,11 @@ def validate_server(
                             status_code = response.status
                             response_body = response.read().decode("utf-8", "replace")
                             if status_code == 200:
-                                return
+                                # Served a completion. Confirm real GPU offload
+                                # below (outside this try, so GpuOffloadFailure is
+                                # not swallowed by the except clauses).
+                                completion_succeeded = True
+                                break
                             last_error = RuntimeError(
                                 f"unexpected HTTP status {status_code}"
                             )
@@ -5717,6 +5927,45 @@ def validate_server(
                         + output
                         + ("\n" + response_body if response_body else "")
                     )
+                if completion_succeeded:
+                    # The server served a completion. When GPU offload is
+                    # *required* (CUDA / ROCm / HIP -- see
+                    # _GPU_OFFLOAD_REQUIRED_KINDS), confirm the model actually
+                    # landed on the GPU: a binary whose GPU backend failed to
+                    # load (CPU-only build, unresolved cudart64_*/cublas64_*
+                    # DLLs on Windows, or a PTX-only build on an older driver)
+                    # still serves HTTP 200 from CPU, and accepting it ships a
+                    # silently CPU-only install (#5807, #5106). Reject so the
+                    # resolver advances to the next bundle / source build. macOS
+                    # Metal is intentionally excluded: a CPU-only Metal load is
+                    # an unfixable environment limitation, not a bad binary.
+                    if _require_gpu_offload:
+                        log_handle.flush()
+                        offload = server_log_shows_gpu_offload(read_full_log(log_path))
+                        if offload is False:
+                            raise GpuOffloadFailure(
+                                "llama-server served a completion but loaded the "
+                                "model entirely on CPU despite GPU offload being "
+                                f"requested (install_kind={install_kind}). The "
+                                "binary's GPU backend did not initialize -- on "
+                                "Windows this is usually unresolved "
+                                "cudart64_*.dll / cublas64_*.dll, or a build that "
+                                "does not match the driver (PTX-only). Rejecting "
+                                "so a GPU-capable bundle is selected "
+                                "(unslothai/unsloth#5807, #5106):\n"
+                                + read_log_excerpt(log_path)
+                            )
+                        # No GPU signal: install validation stays conservative
+                        # (never reject on no evidence), but --smoke-test sets
+                        # require_gpu_signal so its "0 = offload confirmed"
+                        # contract does not pass an unproven log.
+                        if offload is None and require_gpu_signal:
+                            raise PrebuiltFallback(
+                                "llama-server served a completion but its startup "
+                                "log carried no GPU-offload signal; smoke-test "
+                                "result is inconclusive:\n" + read_log_excerpt(log_path)
+                            )
+                    return
         finally:
             if process is not None and process.poll() is None:
                 process.terminate()
@@ -6496,6 +6745,47 @@ def validate_prebuilt_attempts(
     raise PrebuiltFallback("no prebuilt bundle passed validation")
 
 
+def existing_gpu_install_offloads(
+    install_dir: Path,
+    host: HostInfo,
+    plan: InstallReleasePlan,
+    probe_path: Path,
+) -> bool:
+    """For a GPU plan whose metadata matches the existing install, smoke-test
+    the already-installed llama-server's offload before reusing it. Returns True
+    to keep the existing install (non-GPU kind, passed, or inconclusive) and
+    False on a definite CPU-only load so the caller reinstalls. This closes the
+    "reinstall/restart keeps the silently CPU-only binary" path (#5807): without
+    it, a metadata match short-circuits the new offload validation entirely."""
+    choice = plan.attempts[0]
+    # Only re-validate kinds whose CPU-only load is a fixable fault. macOS Metal
+    # is exempt (a CPU-only load is unfixable, see _GPU_OFFLOAD_REQUIRED_KINDS),
+    # so it keeps the existing install without a per-restart smoke test.
+    if choice.install_kind not in _GPU_OFFLOAD_REQUIRED_KINDS:
+        return True
+    server_name = "llama-server.exe" if host.is_windows else "llama-server"
+    try:
+        server_path = discover_installed_executable(install_dir, server_name)
+    except PrebuiltFallback:
+        return True
+    try:
+        validate_server(
+            server_path,
+            probe_path,
+            host,
+            install_dir,
+            runtime_line = choice.runtime_line,
+            install_kind = choice.install_kind,
+        )
+    except GpuOffloadFailure:
+        return False
+    except PrebuiltFallback:
+        # Inconclusive (failed to start, etc.): keep the existing install rather
+        # than reinstall on uncertain evidence.
+        return True
+    return True
+
+
 def install_prebuilt(
     install_dir: Path,
     llama_tag: str,
@@ -6539,8 +6829,15 @@ def install_prebuilt(
                     published_repo,
                     published_release_tag,
                 )
-            if release_plans and existing_install_matches_plan(
-                install_dir, host, release_plans[0]
+            # Non-(offload-required) match: keep the fast path (no probe
+            # download). A CUDA/ROCm/HIP match must still be smoke-tested below,
+            # so it does not short-circuit here; macOS Metal (not offload
+            # required) keeps the fast path.
+            if (
+                release_plans
+                and existing_install_matches_plan(install_dir, host, release_plans[0])
+                and release_plans[0].attempts[0].install_kind
+                not in _GPU_OFFLOAD_REQUIRED_KINDS
             ):
                 current = release_plans[0]
                 log(
@@ -6557,9 +6854,27 @@ def install_prebuilt(
                 release_count = len(release_plans)
                 for release_index, plan in enumerate(release_plans):
                     choice = plan.attempts[0]
-                    if existing_install_matches_plan(install_dir, host, plan):
+                    # A metadata match used to skip straight to reuse; now a
+                    # matching GPU install is smoke-tested first so a previously
+                    # installed CPU-only "GPU" binary is rebuilt instead of kept
+                    # forever across reruns/restarts (#5807).
+                    matched = existing_install_matches_plan(install_dir, host, plan)
+                    existing_install_dir: Path | None = install_dir
+                    if matched and not existing_gpu_install_offloads(
+                        install_dir, host, plan, probe_path
+                    ):
                         log(
-                            "existing llama.cpp install already matches fallback release "
+                            "existing GPU llama.cpp install served a completion but "
+                            "loaded the model on CPU; reinstalling to recover GPU "
+                            "offload (unslothai/unsloth#5807)"
+                        )
+                        matched = False
+                        # Don't let validate_prebuilt_attempts short-circuit on
+                        # the same bad install we just rejected.
+                        existing_install_dir = None
+                    if matched:
+                        log(
+                            "existing llama.cpp install already matches release "
                             f"{plan.release_tag} upstream_tag={plan.llama_tag}; skipping reinstall"
                         )
                         return
@@ -6580,7 +6895,7 @@ def install_prebuilt(
                             release_tag = plan.release_tag,
                             approved_checksums = plan.approved_checksums,
                             initial_fallback_used = release_index > 0,
-                            existing_install_dir = install_dir,
+                            existing_install_dir = existing_install_dir,
                         )
                     except ExistingInstallSatisfied:
                         return
@@ -6616,6 +6931,81 @@ def install_prebuilt(
         raise SystemExit(EXIT_FALLBACK) from exc
 
 
+def resolve_smoke_test_install_kind(host: HostInfo) -> str:
+    """Best-effort install_kind for a post-build GPU smoke test, derived from
+    the host. setup.sh / setup.ps1 can override with --install-kind. Offload-
+    required kinds (in _GPU_OFFLOAD_REQUIRED_KINDS) make the smoke test require
+    real GPU offload; CPU kinds and macOS Metal skip that check."""
+    if host.is_macos and host.is_arm64:
+        return "macos-arm64"
+    if host.has_rocm:
+        return "windows-hip" if host.is_windows else "linux-rocm"
+    if host.has_usable_nvidia:
+        return "windows-cuda" if host.is_windows else "linux-cuda"
+    if host.is_windows:
+        return "windows-cpu"
+    if host.is_macos:
+        return "macos-cpu"
+    return "linux-cpu"
+
+
+def smoke_test_server_binary(
+    server_binary: str,
+    host: HostInfo,
+    *,
+    install_dir: str | None = None,
+    probe: str | None = None,
+    install_kind: str | None = None,
+) -> str:
+    """Run validate_server against an already-built llama-server binary.
+
+    Raises PrebuiltFallback when the binary fails to start, fails the completion
+    probe, or (on a GPU host) loads the model only on CPU. Returns the resolved
+    install_kind on success.
+    """
+    server_path = Path(server_binary).expanduser().resolve()
+    if not server_path.exists():
+        raise PrebuiltFallback(
+            f"smoke-test: llama-server binary not found at {server_path}"
+        )
+    resolved_install_dir = (
+        Path(install_dir).expanduser().resolve() if install_dir else server_path.parent
+    )
+    resolved_kind = install_kind or resolve_smoke_test_install_kind(host)
+    # For an offload-required kind (CUDA/ROCm/HIP), the CLI contract is
+    # "0 = offload confirmed", so an inconclusive (no-signal) log must not pass
+    # -- require a positive signal. macOS Metal is exempt (see
+    # _GPU_OFFLOAD_REQUIRED_KINDS): it only needs to load and serve.
+    require_signal = resolved_kind in _GPU_OFFLOAD_REQUIRED_KINDS
+    if probe:
+        probe_path = Path(probe).expanduser().resolve()
+        if not probe_path.exists():
+            raise PrebuiltFallback(f"smoke-test: probe model not found at {probe_path}")
+        validate_server(
+            server_path,
+            probe_path,
+            host,
+            resolved_install_dir,
+            install_kind = resolved_kind,
+            require_gpu_signal = require_signal,
+        )
+    else:
+        with tempfile.TemporaryDirectory(prefix = "unsloth-llama-smoke-") as tmp:
+            probe_path = Path(tmp) / "stories260K.gguf"
+            download_validation_model(
+                probe_path, validation_model_cache_path(resolved_install_dir)
+            )
+            validate_server(
+                server_path,
+                probe_path,
+                host,
+                resolved_install_dir,
+                install_kind = resolved_kind,
+                require_gpu_signal = require_signal,
+            )
+    return resolved_kind
+
+
 def parse_args() -> argparse.Namespace:
     parser = argparse.ArgumentParser(
         description = "Install and validate a prebuilt llama.cpp bundle for Unsloth Studio."
@@ -6647,6 +7037,33 @@ def parse_args() -> argparse.Namespace:
         action = "store_true",
         help = "Use the simplified platform-specific prebuilt selection policy.",
     )
+    parser.add_argument(
+        "--smoke-test",
+        metavar = "LLAMA_SERVER",
+        help = (
+            "Validate an already-built llama-server binary instead of "
+            "installing. Launches it against a tiny probe model and, on a GPU "
+            "host, verifies the model actually offloaded to the GPU. Exits 0 on "
+            "success, 2 (EXIT_FALLBACK) when the binary only ran on CPU, 1 "
+            "(EXIT_ERROR) when inconclusive -- used by setup.sh / setup.ps1 to "
+            "validate source builds."
+        ),
+    )
+    parser.add_argument(
+        "--probe",
+        help = (
+            "Optional probe GGUF for --smoke-test. Defaults to the bundled "
+            "validation model (downloaded if absent)."
+        ),
+    )
+    parser.add_argument(
+        "--install-kind",
+        help = (
+            "Override the install_kind used by --smoke-test (e.g. linux-cuda, "
+            "windows-cuda, linux-rocm, windows-hip, macos-arm64). Auto-derived "
+            "from the host when omitted."
+        ),
+    )
     parser.add_argument(
         "--has-rocm",
         action = "store_true",
@@ -6788,6 +7205,48 @@ def main() -> int:
         )
         return EXIT_SUCCESS
 
+    if args.smoke_test is not None:
+        # Honor the same host overrides as the install path so a caller that
+        # forwards --has-rocm / --rocm-gfx (setup.sh/setup.ps1 do, because the
+        # installer's own probe can miss amd-smi-only hosts) is held to the GPU
+        # offload check instead of silently resolving a CPU kind.
+        host = _apply_host_overrides(
+            detect_host(),
+            override_has_rocm = args.has_rocm,
+            override_rocm_gfx = args.rocm_gfx,
+            force_cpu = args.cpu_fallback,
+        )
+        try:
+            resolved_kind = smoke_test_server_binary(
+                args.smoke_test,
+                host,
+                install_dir = args.install_dir,
+                probe = args.probe,
+                install_kind = args.install_kind,
+            )
+        except GpuOffloadFailure as exc:
+            # Definitive: GPU was requested but the model loaded on CPU. Exit
+            # EXIT_FALLBACK so the caller (setup.sh / setup.ps1) rebuilds CPU.
+            log(f"smoke-test failed (CPU-only): {exc}")
+            return EXIT_FALLBACK
+        except PrebuiltFallback as exc:
+            # Inconclusive: the binary could not be exercised (failed to start,
+            # missing probe, ...). Exit EXIT_ERROR so the caller keeps the GPU
+            # build rather than downgrading it on uncertain evidence.
+            log(f"smoke-test inconclusive: {exc}")
+            return EXIT_ERROR
+        if resolved_kind in _GPU_OFFLOAD_REQUIRED_KINDS:
+            log(
+                f"smoke-test passed: {args.smoke_test} "
+                f"(install_kind={resolved_kind}) offloaded to the GPU"
+            )
+        else:
+            log(
+                f"smoke-test passed: {args.smoke_test} "
+                f"(install_kind={resolved_kind})"
+            )
+        return EXIT_SUCCESS
+
     if not args.install_dir:
         raise SystemExit(
             "install_llama_prebuilt.py: --install-dir is required unless --resolve-llama-tag, --resolve-install-tag, or --resolve-source-build is used"

studio/setup.ps1

@@ -2633,6 +2633,16 @@ if (Test-Path -LiteralPath $LlamaServerBin) {
             Write-Host "   Existing llama-server was built with CUDA but no GPU detected -- rebuilding" -ForegroundColor Yellow
             $NeedRebuild = $true
         }
+        # The cache check catches a CPU build on a GPU host; this catches a CUDA
+        # build that still loads on CPU at runtime (PTX / runtime-init failure,
+        # #5807). Smoke-test before reusing it on a GPU host.
+        if (-not $NeedRebuild -and $HasNvidiaSmi -and $cachedCuda) {
+            & python "$PSScriptRoot\install_llama_prebuilt.py" --smoke-test "$LlamaServerBin" --install-dir "$LlamaCppDir" --install-kind "windows-cuda" 2>&1 | Out-String | Write-Host
+            if ($LASTEXITCODE -eq 2) {
+                Write-Host "   Existing CUDA llama-server runs on CPU only -- rebuilding" -ForegroundColor Yellow
+                $NeedRebuild = $true
+            }
+        }
     }
 }
 
@@ -2657,6 +2667,14 @@ if (-not $NeedLlamaSourceBuild) {
     substep "GGUF inference and export will not be available." "Yellow"
     substep "Install CMake from https://cmake.org/download/ and re-run setup." "Yellow"
     $script:LlamaCppDegraded = $true
+} elseif ($HasROCm -and -not $HasNvidiaSmi) {
+    # Windows has no HIP source-build path, so a ROCm host whose HIP prebuilt was
+    # missing or rejected must not silently CPU-source-build. Mark degraded so
+    # the CPU-prebuilt last resort below installs a clearly labelled CPU build
+    # instead of a "built" install that runs on CPU (#5807).
+    Write-Host ""
+    step "llama.cpp" "no usable HIP prebuilt and no Windows HIP source build; using CPU prebuilt" "Yellow"
+    $script:LlamaCppDegraded = $true
 } else {
     # A source build is committed here. The CUDA toolkit is only needed now, so
     # resolve (and winget-install if needed) it lazily, failing fast if no
@@ -2952,39 +2970,53 @@ if (-not $NeedLlamaSourceBuild) {
             $CmakeArgs += '-DLLAMA_CURL=OFF'
         }
         $CmakeArgs += '-DCMAKE_EXE_LINKER_FLAGS=/NODEFAULTLIB:LIBCMT'
-        # CUDA flags -- only if GPU available, otherwise explicitly disable
+        # CUDA flags -- only if GPU available, otherwise explicitly disable.
+        # $LlamaCudaBuild gates the post-build GPU smoke test and CUDA->CPU
+        # retry below.
+        $LlamaCudaBuild = $false
         if ($HasNvidiaSmi -and $NvccPath) {
-            $CmakeArgs += '-DGGML_CUDA=ON'
-            # Accept a host MSVC newer than nvcc's whitelist; a fresh toolkit
-            # (e.g. CUDA 13.3) otherwise aborts with "#error -- unsupported
-            # Microsoft Visual Studio version!". Mirrors the Linux fix. Via env
-            # (covers the configure probe + build), after Refresh-Environment, idempotent.
-            $nvccAllowFlag = '-allow-unsupported-compiler'
-            if ([string]::IsNullOrEmpty($env:NVCC_PREPEND_FLAGS)) {
-                $env:NVCC_PREPEND_FLAGS = $nvccAllowFlag
-            } elseif ($env:NVCC_PREPEND_FLAGS -notlike "*$nvccAllowFlag*") {
-                $env:NVCC_PREPEND_FLAGS = "$($env:NVCC_PREPEND_FLAGS) $nvccAllowFlag"
-            }
-            substep "NVCC_PREPEND_FLAGS = $env:NVCC_PREPEND_FLAGS"
-            $CmakeArgs += "-DCUDAToolkit_ROOT=$CudaToolkitRoot"
-            $CmakeArgs += "-DCUDA_TOOLKIT_ROOT_DIR=$CudaToolkitRoot"
-            $CmakeArgs += "-DCMAKE_CUDA_COMPILER=$NvccPath"
+            # Resolve a concrete CUDA architecture FIRST. A CUDA build with no
+            # -DCMAKE_CUDA_ARCHITECTURES is PTX-only and can fail at runtime on a
+            # driver older than the toolkit ("the provided PTX was compiled with
+            # an unsupported toolchain", #5854). If we cannot resolve a supported
+            # arch, build CPU-only instead of shipping a silently broken binary.
+            $cudaArchFlag = $null
             if ($CudaArch) {
-                # Validate nvcc actually supports this architecture
                 if (Test-NvccArchSupport -NvccExe $NvccPath -Arch $CudaArch) {
-                    $CmakeArgs += "-DCMAKE_CUDA_ARCHITECTURES=$CudaArch"
+                    $cudaArchFlag = "-DCMAKE_CUDA_ARCHITECTURES=$CudaArch"
                 } else {
-                    # GPU arch too new for this toolkit -- fall back to highest supported.
-                    # PTX forward-compatibility will JIT-compile for the actual GPU at runtime.
+                    # GPU arch too new for this toolkit -- fall back to highest
+                    # supported. PTX forward-compat will JIT for the real GPU.
                     $maxArch = Get-NvccMaxArch -NvccExe $NvccPath
                     if ($maxArch) {
-                        $CmakeArgs += "-DCMAKE_CUDA_ARCHITECTURES=$maxArch"
+                        $cudaArchFlag = "-DCMAKE_CUDA_ARCHITECTURES=$maxArch"
                         substep "GPU is sm_$CudaArch but nvcc only supports up to sm_$maxArch" "Yellow"
                         substep "Building with sm_$maxArch (PTX will JIT for your GPU at runtime)" "Yellow"
                     }
-                    # else: omit flag entirely, let cmake pick defaults
                 }
             }
+            if ($cudaArchFlag) {
+                $CmakeArgs += '-DGGML_CUDA=ON'
+                # Accept a host MSVC newer than nvcc's whitelist; a fresh toolkit
+                # (e.g. CUDA 13.3) otherwise aborts with "#error -- unsupported
+                # Microsoft Visual Studio version!". Via env (covers the configure
+                # probe + build), after Refresh-Environment, idempotent.
+                $nvccAllowFlag = '-allow-unsupported-compiler'
+                if ([string]::IsNullOrEmpty($env:NVCC_PREPEND_FLAGS)) {
+                    $env:NVCC_PREPEND_FLAGS = $nvccAllowFlag
+                } elseif ($env:NVCC_PREPEND_FLAGS -notlike "*$nvccAllowFlag*") {
+                    $env:NVCC_PREPEND_FLAGS = "$($env:NVCC_PREPEND_FLAGS) $nvccAllowFlag"
+                }
+                substep "NVCC_PREPEND_FLAGS = $env:NVCC_PREPEND_FLAGS"
+                $CmakeArgs += "-DCUDAToolkit_ROOT=$CudaToolkitRoot"
+                $CmakeArgs += "-DCUDA_TOOLKIT_ROOT_DIR=$CudaToolkitRoot"
+                $CmakeArgs += "-DCMAKE_CUDA_COMPILER=$NvccPath"
+                $CmakeArgs += $cudaArchFlag
+                $LlamaCudaBuild = $true
+            } else {
+                substep "Could not resolve a supported CUDA architecture for this GPU/toolkit; building CPU-only to avoid a PTX-only binary that fails at runtime (#5854)" "Yellow"
+                $CmakeArgs += '-DGGML_CUDA=OFF'
+            }
         } else {
             $CmakeArgs += '-DGGML_CUDA=OFF'
         }
@@ -3025,6 +3057,63 @@ if (-not $NeedLlamaSourceBuild) {
         }
     }
 
+    # -- Step C.5: GPU smoke test + CUDA->CPU fallback (#5807 / #5854) --
+    # A CUDA build whose runtime backend fails to initialize still links and
+    # serves HTTP 200, but only from CPU; and setup.ps1 previously had no CPU
+    # fallback when a CUDA build failed at all. Both gaps are closed here.
+    if ($LlamaCudaBuild -and $BuildOk) {
+        $builtServer = Join-Path $BuildDir "bin\Release\llama-server.exe"
+        if (-not (Test-Path -LiteralPath $builtServer)) {
+            $builtServer = Join-Path $BuildDir "bin\llama-server.exe"
+        }
+        if (Test-Path -LiteralPath $builtServer) {
+            Write-Host ""
+            Write-Host "--- GPU smoke test ---" -ForegroundColor Cyan
+            # $LlamaCudaBuild gates this block, so the build is CUDA; pass the
+            # explicit kind so the installer's own probe cannot resolve a CPU
+            # kind and skip the offload gate.
+            & python "$PSScriptRoot\install_llama_prebuilt.py" --smoke-test "$builtServer" --install-dir "$LlamaCppDir" --install-kind "windows-cuda" 2>&1 | Out-String | Write-Host
+            $smokeExit = $LASTEXITCODE
+            if ($smokeExit -eq 2) {
+                substep "GPU build runs on CPU only (GPU backend failed to initialize)" "Yellow"
+                $BuildOk = $false
+                $FailedStep = "GPU smoke test (ran on CPU)"
+            } elseif ($smokeExit -ne 0) {
+                substep "GPU smoke test inconclusive (exit $smokeExit); keeping GPU build" "Yellow"
+            }
+        }
+    }
+
+    # If a CUDA build was attempted and configure/build/smoke-test left it
+    # unusable, retry once with CUDA disabled so the user still gets a working
+    # (if slower) CPU llama-server instead of nothing.
+    if ($LlamaCudaBuild -and -not $BuildOk) {
+        substep "CUDA build unusable at: $FailedStep; retrying CPU-only build..." "Yellow"
+        $CpuCmakeArgs = @($CmakeArgs | Where-Object {
+            $_ -ne '-DGGML_CUDA=ON' -and
+            $_ -notlike '-DCMAKE_CUDA_ARCHITECTURES=*' -and
+            $_ -notlike '-DCUDAToolkit_ROOT=*' -and
+            $_ -notlike '-DCUDA_TOOLKIT_ROOT_DIR=*' -and
+            $_ -notlike '-DCMAKE_CUDA_COMPILER=*'
+        })
+        $CpuCmakeArgs += '-DGGML_CUDA=OFF'
+        if (Test-Path -LiteralPath $BuildDir) { Remove-Item -LiteralPath $BuildDir -Recurse -Force }
+        $cpuConfigure = cmake @CpuCmakeArgs 2>&1 | Out-String
+        if ($LASTEXITCODE -eq 0) {
+            $cpuBuild = cmake --build $BuildDir --config Release --target llama-server -j $NumCpu 2>&1 | Out-String
+            if ($LASTEXITCODE -eq 0) {
+                $BuildOk = $true
+                $LlamaCudaBuild = $false
+                $FailedStep = $null
+                substep "CPU-only llama.cpp build succeeded" "Green"
+            } else {
+                Write-LlamaFailureLog -Output $cpuBuild
+            }
+        } else {
+            Write-LlamaFailureLog -Output $cpuConfigure
+        }
+    }
+
     # -- Step D: Build llama-quantize (optional, best-effort) --
     if ($BuildOk) {
         Write-Host ""
@@ -3083,6 +3172,35 @@ if (-not $NeedLlamaSourceBuild) {
     }
 }
 
+# ─────────────────────────────────────────────
+# Windows GPU: CPU prebuilt as a last resort
+# ─────────────────────────────────────────────
+# A GPU host reaches the source build only when no GPU prebuilt offloads (the
+# CPU prebuilt is deliberately not offered to a GPU host so it does not short
+# circuit the source build, #5807). If that build produced no binary (no CUDA
+# toolkit, compile failure), install the CPU prebuilt via --cpu-fallback so the
+# host still gets a working (if slower) llama-server instead of nothing.
+if ($script:LlamaCppDegraded -and ($HasNvidiaSmi -or $HasROCm)) {
+    substep "GPU build unavailable; trying CPU prebuilt as a last resort..." "Yellow"
+    $lastResortArgs = @(
+        "$PSScriptRoot\install_llama_prebuilt.py",
+        "--install-dir", $OriginalLlamaCppDir,
+        "--llama-tag", $RequestedLlamaTag,
+        "--published-repo", $HelperReleaseRepo,
+        "--simple-policy",
+        "--cpu-fallback"
+    )
+    $prevEAPLast = $ErrorActionPreference
+    $ErrorActionPreference = "Continue"
+    & python @lastResortArgs 2>&1 | Out-String | Write-Host
+    $lastResortExit = $LASTEXITCODE
+    $ErrorActionPreference = $prevEAPLast
+    if ($lastResortExit -eq 0) {
+        step "llama.cpp" "CPU prebuilt installed (GPU unavailable; inference will run on CPU)" "Yellow"
+        $script:LlamaCppDegraded = $false
+    }
+}
+
 # ─────────────────────────────────────────────
 # Footer
 # ─────────────────────────────────────────────

studio/setup.sh

@@ -919,12 +919,41 @@ if [ "$_NEED_LLAMA_SOURCE_BUILD" = true ] && \
    [ -z "$_LLAMA_PR" ] && \
    [ -x "$LLAMA_CPP_DIR/build/bin/llama-server" ] && \
    [ -x "$LLAMA_CPP_DIR/build/bin/llama-quantize" ]; then
-    step "llama.cpp" "existing source build found; skipping rebuild"
-    ln -sf build/bin/llama-quantize "$LLAMA_CPP_DIR/llama-quantize"
-    if [ "$_STUDIO_HOME_IS_CUSTOM" = true ]; then
-        : > "$LLAMA_CPP_DIR/$_STUDIO_OWNED_MARKER" 2>/dev/null || true
+    _REUSE_SOURCE=true
+    # On a GPU host, smoke-test the existing source binary first so a stale
+    # CPU-only build (e.g. an earlier no-toolkit fallback) is rebuilt instead of
+    # silently reused on a GPU host (#5807). Exit 2 = ran on CPU -> rebuild;
+    # anything else keeps the build (never downgrade on uncertain evidence).
+    _REUSE_KIND=""
+    if [ "$_HOST_SYSTEM" = "Darwin" ] && { [ "$_HOST_MACHINE" = "arm64" ] || [ "$_HOST_MACHINE" = "aarch64" ]; }; then
+        _REUSE_KIND="macos-arm64"
+    elif command -v nvidia-smi >/dev/null 2>&1; then
+        _REUSE_KIND="linux-cuda"
+    elif [ "$_LINUX_HAS_GPU" = true ]; then
+        _REUSE_KIND="linux-rocm"
+    fi
+    if [ -n "$_REUSE_KIND" ]; then
+        if python "$SCRIPT_DIR/install_llama_prebuilt.py" \
+                --smoke-test "$LLAMA_CPP_DIR/build/bin/llama-server" \
+                --install-dir "$LLAMA_CPP_DIR" \
+                --install-kind "$_REUSE_KIND" > "$LLAMA_CPP_DIR/gpu-smoke-existing.log" 2>&1; then
+            _REUSE_RC=0
+        else
+            _REUSE_RC=$?
+        fi
+        if [ "$_REUSE_RC" -eq 2 ]; then
+            substep "existing source build runs on CPU only; rebuilding for GPU..." "$C_WARN"
+            _REUSE_SOURCE=false
+        fi
+    fi
+    if [ "$_REUSE_SOURCE" = true ]; then
+        step "llama.cpp" "existing source build found; skipping rebuild"
+        ln -sf build/bin/llama-quantize "$LLAMA_CPP_DIR/llama-quantize"
+        if [ "$_STUDIO_HOME_IS_CUSTOM" = true ]; then
+            : > "$LLAMA_CPP_DIR/$_STUDIO_OWNED_MARKER" 2>/dev/null || true
+        fi
+        _NEED_LLAMA_SOURCE_BUILD=false
     fi
-    _NEED_LLAMA_SOURCE_BUILD=false
 fi
 
 # ── 8. WSL: pre-install GGUF build dependencies for fallback source builds ──
@@ -1340,6 +1369,68 @@ else
             fi
         fi
 
+        # Map an install_llama_prebuilt.py --smoke-test exit code to a decision
+        # token. 2 (EXIT_FALLBACK) = definitively GPU-intended but CPU-only ->
+        # rebuild CPU; 0 = offload confirmed; anything else (1/EXIT_ERROR,
+        # signals) = inconclusive -> keep the GPU build rather than downgrade on
+        # uncertain evidence. Tiny + side-effect free so
+        # tests/sh/test_llama_gpu_smoke.sh can exercise it.
+        _classify_smoke_exit() {
+            case "$1" in
+                0) echo "ok" ;;
+                2) echo "cpu_only" ;;
+                *) echo "inconclusive" ;;
+            esac
+        }
+
+        # Post-build GPU smoke test (#5807 / #5854): a GPU build whose runtime
+        # backend fails to initialize still links and serves HTTP 200, but only
+        # from CPU. Confirm the fresh binary actually offloads to the GPU; if it
+        # ran on CPU only, retry a CPU build so the user gets a working (if
+        # slower) llama-server instead of a silently CPU-only "GPU" build.
+        # _gpu_fallback_label is empty for a pure CPU build (nothing to verify).
+        if [ "$BUILD_OK" = true ]; then
+            _SMOKE_LABEL="$(_gpu_fallback_label)"
+            # Pass the GPU kind setup resolved, not the installer's own probe:
+            # on amd-smi-only / name-inferred ROCm hosts the child probe can miss
+            # the GPU and resolve a CPU kind, which would skip the offload gate.
+            _SMOKE_KIND=""
+            if [ "$_TRY_METAL_CPU_FALLBACK" = true ]; then
+                _SMOKE_KIND="macos-arm64"
+            elif [ "$GPU_BACKEND" = "cuda" ]; then
+                _SMOKE_KIND="linux-cuda"
+            elif [ "$GPU_BACKEND" = "rocm" ]; then
+                _SMOKE_KIND="linux-rocm"
+            fi
+            if [ -n "$_SMOKE_LABEL" ] && [ -n "$_SMOKE_KIND" ] && [ -f "$_BUILD_TMP/build/bin/llama-server" ]; then
+                # if/else keeps set -e from aborting before we read the code.
+                if python "$SCRIPT_DIR/install_llama_prebuilt.py" \
+                        --smoke-test "$_BUILD_TMP/build/bin/llama-server" \
+                        --install-dir "$_BUILD_TMP" \
+                        --install-kind "$_SMOKE_KIND" > "$_BUILD_TMP/gpu-smoke.log" 2>&1; then
+                    _SMOKE_RC=0
+                else
+                    _SMOKE_RC=$?
+                fi
+                case "$(_classify_smoke_exit "$_SMOKE_RC")" in
+                    cpu_only)
+                        substep "$_SMOKE_LABEL build runs on CPU only; retrying CPU build..." "$C_WARN"
+                        rm -rf "$_BUILD_TMP/build"
+                        if run_quiet_no_exit "cmake llama.cpp (cpu fallback)" cmake $CMAKE_GENERATOR_ARGS -S "$_BUILD_TMP" -B "$_BUILD_TMP/build" $CPU_FALLBACK_CMAKE_ARGS; then
+                            _BUILD_DESC="building (CPU fallback after $_SMOKE_LABEL smoke test ran on CPU)"
+                            GPU_BACKEND=""
+                            run_quiet_no_exit "build llama-server (cpu fallback)" cmake --build "$_BUILD_TMP/build" --config Release --target llama-server -j"$NCPU" || BUILD_OK=false
+                        else
+                            BUILD_OK=false
+                        fi
+                        ;;
+                    inconclusive)
+                        substep "GPU smoke test inconclusive (exit $_SMOKE_RC); keeping $_SMOKE_LABEL build" "$C_WARN"
+                        ;;
+                esac
+            fi
+        fi
+
         if [ "$BUILD_OK" = true ]; then
             run_quiet_no_exit "build llama-quantize" cmake --build "$_BUILD_TMP/build" --config Release --target llama-quantize -j"$NCPU" || true
         fi
@@ -1373,27 +1464,36 @@ else
 }
 fi  # end _SKIP_GGUF_BUILD check
 
-# ── arm64 Linux GPU: CPU prebuilt as a last resort ──
-# arm64 Linux with a GPU has no CUDA prebuilt anywhere (the unslothai fork is
-# x64 only; ggml-org ships no Linux CUDA build), so it source-builds for the
-# GPU above. If that produced no binary, install ggml-org's arm64 CPU prebuilt
-# instead of leaving the host without llama.cpp.
+# ── Linux GPU: CPU prebuilt as a last resort ──
+# A Linux GPU host reaches a source build when no GPU prebuilt offloads (the
+# CPU prebuilt is deliberately not offered to a GPU host so it does not short
+# circuit the source build -- #5807). If that build produced no binary (no
+# toolkit, compile failure), install a CPU prebuilt instead of leaving the host
+# without llama.cpp. arm64 has no CUDA prebuilt anywhere, so it always lands
+# here on a degraded GPU build; x86_64 only when its source build also failed.
+# Repo: ggml-org ships the arm64 CPU tarball; x86_64 uses the same published
+# repo as the primary path (the unslothai fork carries linux-x64-cpu).
 if [ "$_LLAMA_CPP_DEGRADED" = true ] \
         && [ "$_HOST_SYSTEM" = "Linux" ] \
-        && { [ "$_HOST_MACHINE" = "aarch64" ] || [ "$_HOST_MACHINE" = "arm64" ]; }; then
-    substep "GPU source build unavailable; trying ggml-org arm64 CPU prebuilt..."
-    _ARM64_CPU_CMD=(
+        && [ "$_LINUX_HAS_GPU" = true ]; then
+    if [ "$_HOST_MACHINE" = "aarch64" ] || [ "$_HOST_MACHINE" = "arm64" ]; then
+        _LASTRESORT_CPU_REPO="ggml-org/llama.cpp"
+    else
+        _LASTRESORT_CPU_REPO="$_HELPER_RELEASE_REPO"
+    fi
+    substep "GPU build unavailable; trying $_LASTRESORT_CPU_REPO CPU prebuilt as a last resort..."
+    _LASTRESORT_CPU_CMD=(
         python "$SCRIPT_DIR/install_llama_prebuilt.py"
         --install-dir "$LLAMA_CPP_DIR"
         --llama-tag "$_REQUESTED_LLAMA_TAG"
-        --published-repo "ggml-org/llama.cpp"
+        --published-repo "$_LASTRESORT_CPU_REPO"
         --simple-policy
         --cpu-fallback
     )
     # Trust the installer's exit code: it validates the server before exiting 0,
     # the same signal the primary prebuilt path above relies on.
-    if run_quiet_no_exit "arm64 CPU prebuilt" "${_ARM64_CPU_CMD[@]}"; then
-        step "llama.cpp" "arm64 CPU prebuilt installed (GPU build unavailable)" "$C_WARN"
+    if run_quiet_no_exit "CPU prebuilt (last resort)" "${_LASTRESORT_CPU_CMD[@]}"; then
+        step "llama.cpp" "CPU prebuilt installed (GPU unavailable; inference will run on CPU)" "$C_WARN"
         _LLAMA_CPP_DEGRADED=false
         print_installed_llama_prebuilt_release "$LLAMA_CPP_DIR"
     fi

tests/run_all.sh

@@ -9,6 +9,7 @@ sh "$TESTS_DIR/sh/test_get_torch_index_url.sh"
 sh "$TESTS_DIR/sh/test_mac_intel_compat.sh"
 sh "$TESTS_DIR/sh/test_torch_constraint.sh"
 sh "$TESTS_DIR/sh/test_nvcc_meets_llama_minimum.sh"
+sh "$TESTS_DIR/sh/test_llama_gpu_smoke.sh"
 
 echo ""
 echo "=== Python tests ==="

tests/sh/test_llama_gpu_smoke.sh

@@ -0,0 +1,59 @@
+#!/bin/bash
+# Unit tests for _classify_smoke_exit() from studio/setup.sh.
+#
+# After a llama.cpp source build, setup.sh runs install_llama_prebuilt.py
+# --smoke-test against the fresh binary and maps its exit code to a decision:
+#   2 (EXIT_FALLBACK) -> "cpu_only"     : GPU was requested but the model ran
+#                                         on CPU -> rebuild CPU (#5807 / #5854).
+#   0                 -> "ok"           : GPU offload confirmed -> keep build.
+#   1 / signals / etc -> "inconclusive" : could not validate -> keep GPU build
+#                                         (never downgrade on uncertainty).
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+SETUP_SH="$SCRIPT_DIR/../../studio/setup.sh"
+PASS=0
+FAIL=0
+
+# Extract just the helper (same approach as test_nvcc_meets_llama_minimum.sh).
+# The function and its closing brace sit at 8-space indent inside setup.sh.
+_FUNC_FILE=$(mktemp)
+sed -n '/        _classify_smoke_exit() {/,/^        }/p' "$SETUP_SH" > "$_FUNC_FILE"
+
+assert_eq() {
+    _label="$1"; _expected="$2"; _actual="$3"
+    if [ "$_actual" = "$_expected" ]; then
+        echo "  PASS: $_label"
+        PASS=$((PASS + 1))
+    else
+        echo "  FAIL: $_label (expected '$_expected', got '$_actual')"
+        FAIL=$((FAIL + 1))
+    fi
+}
+
+run_classify() {
+    bash -c ". '$_FUNC_FILE'; _classify_smoke_exit '$1'"
+}
+
+echo "=== test_llama_gpu_smoke (_classify_smoke_exit) ==="
+
+assert_eq "exit 0 -> ok"            "ok"           "$(run_classify 0)"
+assert_eq "exit 2 -> cpu_only"      "cpu_only"     "$(run_classify 2)"
+assert_eq "exit 1 -> inconclusive"  "inconclusive" "$(run_classify 1)"
+assert_eq "exit 3 -> inconclusive"  "inconclusive" "$(run_classify 3)"
+assert_eq "exit 137 -> inconclusive" "inconclusive" "$(run_classify 137)"
+
+# Sanity: the extracted function is non-empty and well-formed.
+if [ -s "$_FUNC_FILE" ] && grep -q 'cpu_only' "$_FUNC_FILE"; then
+    echo "  PASS: _classify_smoke_exit extracted from setup.sh"
+    PASS=$((PASS + 1))
+else
+    echo "  FAIL: _classify_smoke_exit could not be extracted from setup.sh"
+    FAIL=$((FAIL + 1))
+fi
+
+rm -f "$_FUNC_FILE"
+
+echo ""
+echo "Results: $PASS passed, $FAIL failed"
+[ "$FAIL" -eq 0 ] || exit 1

tests/studio/install/fake_llama_server.py

@@ -0,0 +1,106 @@
+#!/usr/bin/env python3
+"""A fake llama-server for GPU-offload validation tests (no GPU required).
+
+It accepts the arguments install_llama_prebuilt.py's validate_server passes
+(``-m`` / ``--host`` / ``--port`` / ``--n-gpu-layers`` / ...), prints a canned
+llama.cpp startup log chosen by the ``FAKE_LLAMA_MODE`` env var, then serves
+HTTP 200 from ``/completion`` until killed. This lets CI exercise the real
+validate_server subprocess + HTTP + log-classifier path on GPU-less Windows /
+macOS / Linux runners: the same binary "starts and serves 200" while its log
+says CPU-only or GPU, which is exactly the #5807 / #5830 situation.
+
+FAKE_LLAMA_MODE (default "cuda"):
+  cuda           device_info enumerates CUDA0 (GPU offload confirmed)
+  cuda_buffer    older "CUDA0 model buffer size" + offloaded 33/33 lines
+  cpu            device_info enumerates only CPU (the silent CPU fallback)
+  offloaded_zero "offloaded 0/33 layers to GPU" (definite CPU-only)
+  no_signal      a log with no offload evidence (validator must not reject)
+"""
+
+import os
+import sys
+from http.server import BaseHTTPRequestHandler, HTTPServer
+
+
+LOGS = {
+    "cuda": (
+        "0.00 I device_info:\n"
+        "0.01 I   - CUDA0 : NVIDIA GeForce RTX 5070 (12282 MiB, 11000 MiB free)\n"
+        "0.01 I   - CPU : Generic CPU (32000 MiB free)\n"
+        "0.01 I system_info: n_threads = 8 | CUDA : ARCHS = 1200 | CPU : AVX2 = 1\n"
+        "0.02 I srv llama_server: model loaded\n"
+    ),
+    "cuda_buffer": (
+        "load_tensors: offloaded 33/33 layers to GPU\n"
+        "load_tensors:        CUDA0 model buffer size = 21000.0 MiB\n"
+        "load_tensors:   CPU_Mapped model buffer size =     0.6 MiB\n"
+        "srv llama_server: model loaded\n"
+    ),
+    "cpu": (
+        "0.00 I device_info:\n"
+        "0.00 I   - CPU : Generic CPU (32000 MiB free)\n"
+        "0.00 I system_info: n_threads = 8 | CPU : AVX2 = 1\n"
+        "0.01 I srv llama_server: model loaded\n"
+    ),
+    "offloaded_zero": (
+        "load_tensors: offloaded 0/33 layers to GPU\n"
+        "load_tensors:   CPU_Mapped model buffer size = 21000.0 MiB\n"
+        "srv llama_server: model loaded\n"
+    ),
+    "no_signal": (
+        "INFO [main] starting server\n"
+        "load_tensors: file format = GGUF V3\n"
+        "srv llama_server: model loaded\n"
+    ),
+}
+
+
+def _arg(name, default = None):
+    argv = sys.argv
+    for i, token in enumerate(argv):
+        if token == name and i + 1 < len(argv):
+            return argv[i + 1]
+        if token.startswith(name + "="):
+            return token.split("=", 1)[1]
+    return default
+
+
+class _Handler(BaseHTTPRequestHandler):
+    def _ok(self):
+        body = b'{"content": "x", "tokens_predicted": 1}'
+        self.send_response(200)
+        self.send_header("Content-Type", "application/json")
+        self.send_header("Content-Length", str(len(body)))
+        self.end_headers()
+        self.wfile.write(body)
+
+    def do_POST(self):
+        length = int(self.headers.get("Content-Length", 0) or 0)
+        if length:
+            self.rfile.read(length)
+        self._ok()
+
+    def do_GET(self):
+        self._ok()
+
+    def log_message(self, *args):
+        pass  # keep stdout clean for the classifier
+
+
+def main() -> int:
+    mode = os.environ.get("FAKE_LLAMA_MODE", "cuda")
+    sys.stdout.write(LOGS.get(mode, LOGS["cuda"]))
+    sys.stdout.flush()
+
+    host = _arg("--host", "127.0.0.1")
+    port = int(_arg("--port", "8080"))
+    server = HTTPServer((host, port), _Handler)
+    try:
+        server.serve_forever()
+    except KeyboardInterrupt:
+        pass
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

tests/studio/install/run_smoke_spoof.py

@@ -0,0 +1,125 @@
+#!/usr/bin/env python3
+"""Cross-platform GPU-offload spoof driver for CI (no GPU required).
+
+Builds an OS-appropriate `llama-server` wrapper around fake_llama_server.py and
+drives install_llama_prebuilt.py --smoke-test against it, asserting the exit
+code contract the setup scripts depend on:
+
+  FAKE_LLAMA_MODE=cpu , GPU install_kind -> exit 2 (EXIT_FALLBACK)  rejected
+  FAKE_LLAMA_MODE=cuda, GPU install_kind -> exit 0 (EXIT_SUCCESS)   accepted
+  FAKE_LLAMA_MODE=cpu , CPU install_kind -> exit 0                  not gated
+
+Runs on windows-latest / macos-latest / ubuntu-latest. Exits non-zero on any
+mismatch so the CI job fails loudly.
+"""
+
+import os
+import stat
+import subprocess
+import sys
+import tempfile
+from pathlib import Path
+
+
+HERE = Path(__file__).resolve().parent
+REPO = HERE.parents[2]
+INSTALLER = REPO / "studio" / "install_llama_prebuilt.py"
+FAKE = HERE / "fake_llama_server.py"
+IS_WIN = sys.platform == "win32"
+
+# Offload-required GPU kind for the rejection contract. install_kind is passed
+# explicitly, so the classifier / validate_server logic is exercised
+# independent of the (GPU-less) runner OS. macOS Metal is intentionally NOT
+# offload-required, so use a CUDA kind even on macOS to drive the CUDA/ROCm
+# rejection path; Metal's accept-CPU behavior is covered by the macOS-only case.
+GPU_REQ_KIND = "windows-cuda" if IS_WIN else "linux-cuda"
+CPU_KIND = {
+    "win32": "windows-cpu",
+    "darwin": "macos-cpu",
+}.get(sys.platform, "linux-cpu")
+
+
+def make_wrapper(workdir: Path) -> Path:
+    if IS_WIN:
+        wrapper = workdir / "llama-server.bat"
+        wrapper.write_text(f'@"{sys.executable}" "{FAKE}" %*\r\n')
+        return wrapper
+    wrapper = workdir / "llama-server"
+    wrapper.write_text(f'#!/bin/sh\nexec "{sys.executable}" "{FAKE}" "$@"\n')
+    wrapper.chmod(wrapper.stat().st_mode | stat.S_IEXEC | stat.S_IXGRP | stat.S_IXOTH)
+    return wrapper
+
+
+def run_smoke(wrapper: Path, probe: Path, install_kind: str, mode: str) -> int:
+    env = dict(os.environ, FAKE_LLAMA_MODE = mode)
+    proc = subprocess.run(
+        [
+            sys.executable,
+            str(INSTALLER),
+            "--smoke-test",
+            str(wrapper),
+            "--probe",
+            str(probe),
+            "--install-kind",
+            install_kind,
+        ],
+        env = env,
+        capture_output = True,
+        text = True,
+    )
+    sys.stdout.write(proc.stdout)
+    sys.stderr.write(proc.stderr)
+    return proc.returncode
+
+
+def main() -> int:
+    failures = []
+    with tempfile.TemporaryDirectory() as tmp:
+        work = Path(tmp)
+        wrapper = make_wrapper(work)
+        probe = work / "probe.gguf"
+        probe.write_bytes(b"GGUF\x00fake")
+
+        cases = [
+            ("cpu", GPU_REQ_KIND, 2, "CPU-only binary tagged GPU is rejected"),
+            ("offloaded_zero", GPU_REQ_KIND, 2, "offloaded 0/N tagged GPU is rejected"),
+            ("cuda", GPU_REQ_KIND, 0, "GPU binary tagged GPU is accepted"),
+            ("cuda_buffer", GPU_REQ_KIND, 0, "GPU buffer-format binary is accepted"),
+            ("cpu", CPU_KIND, 0, "CPU binary tagged CPU is not gated"),
+            (
+                "no_signal",
+                GPU_REQ_KIND,
+                1,
+                "no-signal GPU log is inconclusive (exit 1)",
+            ),
+        ]
+        if sys.platform == "darwin":
+            # macOS Metal is not offload-required: a CPU-only Metal load is an
+            # unfixable environment limitation (headless / virtualized host), so
+            # the macos-arm64 prebuilt is accepted rather than rejected.
+            cases.append(
+                (
+                    "cpu",
+                    "macos-arm64",
+                    0,
+                    "macOS Metal CPU-only load is accepted (no rebuild remedy)",
+                )
+            )
+        for mode, kind, expected, label in cases:
+            rc = run_smoke(wrapper, probe, kind, mode)
+            ok = rc == expected
+            print(
+                f"[{'PASS' if ok else 'FAIL'}] {label}: mode={mode} kind={kind} exit={rc} (want {expected})"
+            )
+            if not ok:
+                failures.append(label)
+
+    if failures:
+        print(f"\n{len(failures)} spoof case(s) failed: {failures}")
+        return 1
+    print("\nAll GPU-offload spoof cases passed.")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())

tests/studio/install/test_gpu_offload_spoof.py

@@ -0,0 +1,176 @@
+"""End-to-end GPU-offload spoof: run the real validate_server (real subprocess
++ real HTTP + real log classifier) against a fake llama-server, no GPU needed.
+
+Unlike test_validate_server_gpu_offload.py (which mocks subprocess/urlopen),
+this launches an actual process that "starts and serves HTTP 200" while its log
+reports CPU-only or GPU offload, reproducing #5807 / #5830 end to end. POSIX
+only: validate_server execs the binary path directly, which needs a shebang
+wrapper; the Windows equivalent runs in the studio-gpu-offload-smoke workflow
+via a .bat shim.
+"""
+
+import importlib.util
+import os
+import stat
+import sys
+from pathlib import Path
+
+import pytest
+
+
+if sys.platform == "win32":
+    pytest.skip(
+        "POSIX-only (Windows covered by the spoof CI workflow)", allow_module_level = True
+    )
+
+PACKAGE_ROOT = Path(__file__).resolve().parents[3]
+MODULE_PATH = PACKAGE_ROOT / "studio" / "install_llama_prebuilt.py"
+FAKE_SERVER = Path(__file__).resolve().parent / "fake_llama_server.py"
+SPEC = importlib.util.spec_from_file_location(
+    "studio_install_llama_prebuilt_e2e", MODULE_PATH
+)
+M = importlib.util.module_from_spec(SPEC)
+sys.modules[SPEC.name] = M
+SPEC.loader.exec_module(M)
+
+HostInfo = M.HostInfo
+
+
+def linux_cuda_host(**overrides):
+    defaults = dict(
+        system = "Linux",
+        machine = "x86_64",
+        is_windows = False,
+        is_linux = True,
+        is_macos = False,
+        is_x86_64 = True,
+        is_arm64 = False,
+        nvidia_smi = "/usr/bin/nvidia-smi",
+        driver_cuda_version = (13, 0),
+        compute_caps = ["120"],
+        visible_cuda_devices = None,
+        has_physical_nvidia = True,
+        has_usable_nvidia = True,
+        has_rocm = False,
+    )
+    defaults.update(overrides)
+    return HostInfo(**defaults)
+
+
+@pytest.fixture
+def fake_server_binary(tmp_path):
+    """A `llama-server` that execs the fake server, so validate_server runs it
+    exactly as it would a real prebuilt binary."""
+    binary = tmp_path / "llama-server"
+    binary.write_text("#!/bin/sh\n" f'exec "{sys.executable}" "{FAKE_SERVER}" "$@"\n')
+    binary.chmod(binary.stat().st_mode | stat.S_IEXEC | stat.S_IXGRP | stat.S_IXOTH)
+    return binary
+
+
+def _validate(binary, tmp_path, host, install_kind, mode, monkeypatch):
+    monkeypatch.setenv("FAKE_LLAMA_MODE", mode)
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF\x00fake")
+    M.validate_server(binary, probe, host, tmp_path, install_kind = install_kind)
+
+
+def test_cpu_only_binary_tagged_cuda_is_rejected(
+    fake_server_binary, tmp_path, monkeypatch
+):
+    # The #5807 case: a binary that serves 200 but loaded the model on CPU.
+    with pytest.raises(M.GpuOffloadFailure):
+        _validate(
+            fake_server_binary,
+            tmp_path,
+            linux_cuda_host(),
+            "linux-cuda",
+            "cpu",
+            monkeypatch,
+        )
+
+
+def test_offloaded_zero_binary_tagged_cuda_is_rejected(
+    fake_server_binary, tmp_path, monkeypatch
+):
+    with pytest.raises(M.GpuOffloadFailure):
+        _validate(
+            fake_server_binary,
+            tmp_path,
+            linux_cuda_host(),
+            "linux-cuda",
+            "offloaded_zero",
+            monkeypatch,
+        )
+
+
+def test_gpu_binary_tagged_cuda_passes(fake_server_binary, tmp_path, monkeypatch):
+    _validate(
+        fake_server_binary,
+        tmp_path,
+        linux_cuda_host(),
+        "linux-cuda",
+        "cuda",
+        monkeypatch,
+    )
+
+
+def test_gpu_buffer_format_passes(fake_server_binary, tmp_path, monkeypatch):
+    _validate(
+        fake_server_binary,
+        tmp_path,
+        linux_cuda_host(),
+        "linux-cuda",
+        "cuda_buffer",
+        monkeypatch,
+    )
+
+
+def test_cpu_only_binary_tagged_cpu_is_accepted(
+    fake_server_binary, tmp_path, monkeypatch
+):
+    # A linux-cpu bundle is the intentional fallback; never GPU-gated.
+    _validate(
+        fake_server_binary, tmp_path, linux_cuda_host(), "linux-cpu", "cpu", monkeypatch
+    )
+
+
+def test_no_signal_binary_tagged_cuda_is_accepted(
+    fake_server_binary, tmp_path, monkeypatch
+):
+    # No offload evidence -> conservative: do not reject on no signal.
+    _validate(
+        fake_server_binary,
+        tmp_path,
+        linux_cuda_host(),
+        "linux-cuda",
+        "no_signal",
+        monkeypatch,
+    )
+
+
+def test_smoke_test_cli_exit_codes(fake_server_binary, tmp_path, monkeypatch):
+    # The contract setup.sh / setup.ps1 depend on, exercised end to end through
+    # the real --smoke-test CLI: CPU-only -> 2, GPU -> 0.
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF\x00fake")
+    monkeypatch.setattr(M, "detect_host", lambda: linux_cuda_host())
+
+    def run(mode):
+        monkeypatch.setenv("FAKE_LLAMA_MODE", mode)
+        monkeypatch.setattr(
+            sys,
+            "argv",
+            [
+                "install_llama_prebuilt.py",
+                "--smoke-test",
+                str(fake_server_binary),
+                "--probe",
+                str(probe),
+                "--install-kind",
+                "linux-cuda",
+            ],
+        )
+        return M.main()
+
+    assert run("cpu") == M.EXIT_FALLBACK
+    assert run("cuda") == M.EXIT_SUCCESS

tests/studio/install/test_selection_logic.py

@@ -78,6 +78,7 @@ windows_cuda_upstream_asset_names = (
 )
 env_int = INSTALL_LLAMA_PREBUILT.env_int
 direct_upstream_release_plan = INSTALL_LLAMA_PREBUILT.direct_upstream_release_plan
+direct_linux_release_plan = INSTALL_LLAMA_PREBUILT.direct_linux_release_plan
 _pinned_windows_cuda_fallback = INSTALL_LLAMA_PREBUILT._pinned_windows_cuda_fallback
 CudaRuntimePreference = INSTALL_LLAMA_PREBUILT.CudaRuntimePreference
 published_windows_cuda_attempts = INSTALL_LLAMA_PREBUILT.published_windows_cuda_attempts
@@ -2297,11 +2298,14 @@ class TestDirectUpstreamBlackwellPin:
             self._release(), host, UPSTREAM_REPO, "latest"
         )
         order = [(a.tag, a.runtime_line or a.install_kind) for a in plan.attempts]
+        # No windows-cpu tail: a GPU host must fall through to the source build
+        # when every CUDA bundle fails the offload check, not silently install
+        # the CPU bundle (#5807). The pin + cuda12 are the only attempts.
         assert order == [
             ("b9360", "cuda13"),
             (self.TAG, "cuda12"),
-            (self.TAG, "windows-cpu"),
         ]
+        assert all(a.install_kind != "windows-cpu" for a in plan.attempts)
         assert plan.attempts[0].name == "llama-b9360-bin-win-cuda-13.1-x64.zip"
         # Direct/upstream path stays unverified-by-manifest (no approved hashes).
         assert plan.approved_checksums.artifacts == {}
@@ -2324,6 +2328,126 @@ class TestDirectUpstreamBlackwellPin:
         assert plan.attempts[0].name == f"llama-{self.TAG}-bin-win-cuda-13.3-x64.zip"
 
 
+# ===========================================================================
+# N.1c2. simple-policy plans: a GPU host never gets a CPU bundle appended
+# (#5807). All GPU bundles failing must raise PrebuiltFallback so setup builds
+# from source for the native arch, not silently install a CPU-only "GPU" build.
+# ===========================================================================
+
+
+class TestGpuHostNoSilentCpuFallback:
+    LTAG = "bTEST"
+
+    def _linux_release(self, *targets):
+        names = [f"app-{self.LTAG}-linux-x64-{t}.tar.gz" for t in targets]
+        return {
+            "tag_name": self.LTAG,
+            "assets": [
+                {"name": n, "browser_download_url": "https://x/" + n} for n in names
+            ],
+        }
+
+    def _win_release(self):
+        names = [
+            "llama-b9999-bin-win-cuda-13.3-x64.zip",
+            "cudart-llama-bin-win-cuda-13.3-x64.zip",
+            "llama-b9999-bin-win-cpu-x64.zip",
+        ]
+        return {
+            "tag_name": "b9999",
+            "assets": [
+                {"name": n, "browser_download_url": "https://x/" + n} for n in names
+            ],
+        }
+
+    def _no_torch(self, monkeypatch):
+        monkeypatch.setattr(
+            INSTALL_LLAMA_PREBUILT,
+            "detect_torch_cuda_runtime_preference",
+            lambda host: CudaRuntimePreference(runtime_line = None, selection_log = []),
+        )
+
+    # -- Linux --------------------------------------------------------------
+
+    def test_linux_nvidia_host_has_no_cpu_attempt(self, monkeypatch):
+        mock_linux_runtime(monkeypatch, ["cuda13", "cuda12"])
+        self._no_torch(monkeypatch)
+        host = make_host(compute_caps = ["120"], driver_cuda_version = (13, 0))
+        plan = direct_linux_release_plan(
+            self._linux_release("cuda13-newer", "cpu"),
+            host,
+            "unslothai/llama.cpp",
+            "latest",
+        )
+        kinds = [a.install_kind for a in plan.attempts]
+        assert "linux-cuda" in kinds
+        assert "linux-cpu" not in kinds
+
+    def test_linux_nvidia_unmatched_arch_raises_no_cpu(self, monkeypatch):
+        # GPU present but no CUDA bundle covers the arch: must raise (-> source
+        # build), not fall to the CPU bundle.
+        mock_linux_runtime(monkeypatch, ["cuda13", "cuda12"])
+        self._no_torch(monkeypatch)
+        host = make_host(compute_caps = ["50"], driver_cuda_version = (13, 0))
+        with pytest.raises(PrebuiltFallback):
+            direct_linux_release_plan(
+                self._linux_release("cuda13-newer", "cpu"),
+                host,
+                "unslothai/llama.cpp",
+                "latest",
+            )
+
+    def test_linux_cpu_host_keeps_cpu_attempt(self, monkeypatch):
+        mock_linux_runtime(monkeypatch, [])
+        host = make_host(
+            has_usable_nvidia = False,
+            has_physical_nvidia = False,
+            nvidia_smi = None,
+            compute_caps = [],
+        )
+        plan = direct_linux_release_plan(
+            self._linux_release("cuda13-newer", "cpu"),
+            host,
+            "unslothai/llama.cpp",
+            "latest",
+        )
+        assert [a.install_kind for a in plan.attempts] == ["linux-cpu"]
+
+    # -- Windows ------------------------------------------------------------
+
+    def test_windows_nvidia_host_has_no_cpu_attempt(self, monkeypatch):
+        mock_windows_runtime(monkeypatch, ["cuda13"])
+        self._no_torch(monkeypatch)
+        host = make_host(
+            system = "Windows",
+            machine = "AMD64",
+            driver_cuda_version = (13, 3),
+            compute_caps = ["120"],
+        )
+        plan = direct_upstream_release_plan(
+            self._win_release(), host, UPSTREAM_REPO, "latest"
+        )
+        kinds = [a.install_kind for a in plan.attempts]
+        assert "windows-cuda" in kinds
+        assert "windows-cpu" not in kinds
+
+    def test_windows_cpu_host_keeps_cpu_attempt(self, monkeypatch):
+        mock_windows_runtime(monkeypatch, [])
+        host = make_host(
+            system = "Windows",
+            machine = "AMD64",
+            has_usable_nvidia = False,
+            has_physical_nvidia = False,
+            nvidia_smi = None,
+            driver_cuda_version = None,
+            compute_caps = [],
+        )
+        plan = direct_upstream_release_plan(
+            self._win_release(), host, UPSTREAM_REPO, "latest"
+        )
+        assert [a.install_kind for a in plan.attempts] == ["windows-cpu"]
+
+
 # ===========================================================================
 # N.1d. published_windows_cuda_attempts -- version-dynamic ordering seed
 # ===========================================================================

tests/studio/install/test_validate_server_gpu_offload.py

@@ -0,0 +1,648 @@
+"""Tests for the GPU-offload validation added to install_llama_prebuilt.py.
+
+Covers issue unslothai/unsloth#5807 (duplicates #5830 / #5106 / #5827): a
+llama-server whose GPU backend fails to initialize still serves HTTP 200 from
+CPU, so the old validate_server accepted it and Studio shipped a silently
+CPU-only install. The hardened validate_server rejects such a binary when GPU
+offload was requested, so the resolver / source build falls through to a
+GPU-capable bundle instead of a silently CPU-only one.
+
+Stdlib-only; the installer module is spec-loaded by absolute path (no
+PYTHONPATH / heavy deps), matching test_install_llama_prebuilt_logic.py.
+"""
+
+import importlib.util
+import sys
+from pathlib import Path
+
+import pytest
+
+
+PACKAGE_ROOT = Path(__file__).resolve().parents[3]
+MODULE_PATH = PACKAGE_ROOT / "studio" / "install_llama_prebuilt.py"
+SPEC = importlib.util.spec_from_file_location(
+    "studio_install_llama_prebuilt", MODULE_PATH
+)
+assert SPEC is not None and SPEC.loader is not None
+M = importlib.util.module_from_spec(SPEC)
+sys.modules[SPEC.name] = M
+SPEC.loader.exec_module(M)
+
+server_log_shows_gpu_offload = M.server_log_shows_gpu_offload
+resolve_smoke_test_install_kind = M.resolve_smoke_test_install_kind
+smoke_test_server_binary = M.smoke_test_server_binary
+validate_server = M.validate_server
+PrebuiltFallback = M.PrebuiltFallback
+HostInfo = M.HostInfo
+
+
+# -- HostInfo factories ------------------------------------------------------
+
+
+def nvidia_host(**overrides) -> HostInfo:
+    defaults = dict(
+        system = "Linux",
+        machine = "x86_64",
+        is_windows = False,
+        is_linux = True,
+        is_macos = False,
+        is_x86_64 = True,
+        is_arm64 = False,
+        nvidia_smi = "/usr/bin/nvidia-smi",
+        driver_cuda_version = (13, 0),
+        compute_caps = ["120"],
+        visible_cuda_devices = None,
+        has_physical_nvidia = True,
+        has_usable_nvidia = True,
+        has_rocm = False,
+    )
+    defaults.update(overrides)
+    return HostInfo(**defaults)
+
+
+def windows_nvidia_host(**overrides) -> HostInfo:
+    return nvidia_host(
+        system = "Windows",
+        is_windows = True,
+        is_linux = False,
+        nvidia_smi = "nvidia-smi",
+        **overrides,
+    )
+
+
+def rocm_host(**overrides) -> HostInfo:
+    return nvidia_host(
+        nvidia_smi = None,
+        driver_cuda_version = None,
+        compute_caps = [],
+        has_physical_nvidia = False,
+        has_usable_nvidia = False,
+        has_rocm = True,
+        **overrides,
+    )
+
+
+def macos_arm_host(**overrides) -> HostInfo:
+    defaults = dict(
+        system = "Darwin",
+        machine = "arm64",
+        is_windows = False,
+        is_linux = False,
+        is_macos = True,
+        is_x86_64 = False,
+        is_arm64 = True,
+        nvidia_smi = None,
+        driver_cuda_version = None,
+        compute_caps = [],
+        visible_cuda_devices = None,
+        has_physical_nvidia = False,
+        has_usable_nvidia = False,
+        has_rocm = False,
+    )
+    defaults.update(overrides)
+    return HostInfo(**defaults)
+
+
+def cpu_host(**overrides) -> HostInfo:
+    return nvidia_host(
+        nvidia_smi = None,
+        driver_cuda_version = None,
+        compute_caps = [],
+        has_physical_nvidia = False,
+        has_usable_nvidia = False,
+        has_rocm = False,
+        **overrides,
+    )
+
+
+# -- Canned llama-server logs ------------------------------------------------
+# Current llama.cpp (>= mid-2026) dropped the "model buffer size" lines and
+# enumerates a "device_info:" block instead; older builds still print the
+# buffer lines. Both formats must classify correctly.
+
+CUDA_DEVICE_INFO_LOG = (
+    "0.00.667 I log_info: verbosity = 3\n"
+    "0.00.667 I device_info:\n"
+    "0.01.101 I   - CUDA0   : NVIDIA GeForce RTX 5070 (12282 MiB, 11000 MiB free)\n"
+    "0.01.101 I   - CPU     : AMD Ryzen 7 9700X (32000 MiB free)\n"
+    "0.01.101 I system_info: n_threads = 8 | CUDA : ARCHS = 1200 | CPU : AVX2 = 1\n"
+    "0.01.174 I srv  llama_server: model loaded\n"
+)
+
+CPU_ONLY_DEVICE_INFO_LOG = (
+    "0.00.003 I log_info: verbosity = 3\n"
+    "0.00.003 I device_info:\n"
+    "0.00.003 I   - CPU     : AMD Ryzen 7 9700X (32000 MiB free)\n"
+    "0.00.003 I system_info: n_threads = 8 | CPU : AVX2 = 1\n"
+    "0.00.174 I srv  llama_server: model loaded\n"
+)
+
+CUDA_BUFFER_LOG = (
+    "load_tensors: offloaded 33/33 layers to GPU\n"
+    "load_tensors:        CUDA0 model buffer size = 21000.0 MiB\n"
+    "load_tensors:   CPU_Mapped model buffer size =     0.6 MiB\n"
+)
+
+CPU_BUFFER_LOG = (
+    "load_tensors:   CPU_Mapped model buffer size = 21000.0 MiB\n"
+    "load_tensors:          CPU model buffer size =     0.6 MiB\n"
+)
+
+ROCM_BUFFER_LOG = "load_tensors:        ROCm0 model buffer size = 21000.0 MiB\n"
+METAL_BUFFER_LOG = "load_tensors:       Metal model buffer size = 8000.0 MiB\n"
+VULKAN_DEVICE_INFO_LOG = (
+    "device_info:\n  - Vulkan0 : AMD Radeon (16000 MiB free)\n  - CPU : x86 (free)\n"
+)
+OPENCL_DEVICE_INFO_LOG = (
+    "device_info:\n  - OpenCL0 : Adreno (4000 MiB free)\n  - CPU : arm (free)\n"
+)
+OFFLOADED_ZERO_LOG = "load_tensors: offloaded 0/33 layers to GPU\n"
+NO_SIGNAL_LOG = "INFO [main] starting server\nload_tensors: file format = GGUF V3\n"
+# system_info advertises a *compiled* backend; not an available device.
+SYSTEM_INFO_ONLY_LOG = (
+    "system_info: n_threads = 8 | CUDA : ARCHS = 1200 | CPU : AVX2 = 1\n"
+)
+
+
+# -- 1. Pure classifier ------------------------------------------------------
+
+
+@pytest.mark.parametrize(
+    "log_text,expected",
+    [
+        (CUDA_DEVICE_INFO_LOG, True),
+        (CPU_ONLY_DEVICE_INFO_LOG, False),
+        (CUDA_BUFFER_LOG, True),
+        (CPU_BUFFER_LOG, False),
+        (ROCM_BUFFER_LOG, True),
+        (METAL_BUFFER_LOG, True),
+        (VULKAN_DEVICE_INFO_LOG, True),
+        (OPENCL_DEVICE_INFO_LOG, True),
+        (OFFLOADED_ZERO_LOG, False),
+        (NO_SIGNAL_LOG, None),
+        ("", None),
+        (SYSTEM_INFO_ONLY_LOG, None),
+    ],
+)
+def test_server_log_shows_gpu_offload(log_text, expected):
+    assert server_log_shows_gpu_offload(log_text) is expected
+
+
+def test_opencl_device_in_prefixes():
+    # Regression: OpenCL was matched by the device-row regex but missing from
+    # the GPU prefix list, so an OpenCL-only device_info wrongly classified as
+    # CPU-only and a working OpenCL build was rejected.
+    assert "opencl" in M._GPU_DEVICE_PREFIXES
+    assert server_log_shows_gpu_offload(OPENCL_DEVICE_INFO_LOG) is True
+
+
+def test_system_info_not_mistaken_for_device():
+    # A CUDA-compiled binary whose runtime failed still prints the system_info
+    # CUDA backend, but enumerates only CPU under device_info -> must be False.
+    log = (
+        "device_info:\n  - CPU : x86 (free)\n"
+        "system_info: | CUDA : ARCHS = 1200 | CPU : AVX2 = 1\n"
+    )
+    assert server_log_shows_gpu_offload(log) is False
+
+
+def test_signal2_offloaded_zero_beats_device_info():
+    # offloaded 0/33 (signal 2) is a definitive "nothing on GPU" and takes
+    # priority over a device_info CUDA0 row -> False.
+    log = (
+        "load_tensors: offloaded 0/33 layers to GPU\n"
+        "device_info:\n  - CUDA0 : NVIDIA (free)\n  - CPU : x (free)\n"
+    )
+    assert server_log_shows_gpu_offload(log) is False
+
+
+def test_kv_buffer_on_gpu_with_cpu_model_is_not_offload():
+    # A GPU KV/compute buffer while the model weights sit on CPU is still CPU
+    # inference; only a GPU *model* buffer counts as offload.
+    log = (
+        "load_tensors:   CUDA0 KV buffer size = 100.0 MiB\n"
+        "load_tensors:   CPU_Mapped model buffer size = 0.6 MiB\n"
+    )
+    assert server_log_shows_gpu_offload(log) is False
+
+
+def test_offloading_zero_repeating_does_not_mask_zero_total():
+    # Real CPU-only shape: a planning line says "offloading 0 repeating layers"
+    # before the definitive "offloaded 0/33". The count must decide -> False.
+    log = (
+        "llm_load_tensors: offloading 0 repeating layers to GPU\n"
+        "llm_load_tensors: offloaded 0/33 layers to GPU\n"
+        "llm_load_tensors: CPU model buffer size = 7338.64 MiB\n"
+    )
+    assert server_log_shows_gpu_offload(log) is False
+
+
+def test_uncounted_offloading_output_layer_is_gpu():
+    # The uncounted "offloading output layer to GPU" phrasing (no number, no
+    # later zero count) is a weak positive.
+    log = "llm_load_tensors: offloading output layer to GPU\n"
+    assert server_log_shows_gpu_offload(log) is True
+
+
+def test_hip_musa_cann_model_buffer_is_gpu():
+    for marker in ("HIP0", "MUSA0", "CANN0"):
+        log = f"load_tensors:   {marker} model buffer size = 21000.0 MiB\n"
+        assert server_log_shows_gpu_offload(log) is True, marker
+
+
+def test_device_row_case_insensitive():
+    log = "device_info:\n  - cuda0 : some gpu (free)\n  - CPU : x (free)\n"
+    assert server_log_shows_gpu_offload(log) is True
+
+
+def test_cuda_host_buffer_is_not_gpu_offload():
+    # CUDA_Host is host-pinned CPU RAM, not device memory. A binary that pins
+    # host memory but loads weights on CPU must not pass as GPU offload.
+    log = (
+        "load_tensors:   CUDA_Host model buffer size = 21000.0 MiB\n"
+        "load_tensors:   CPU_Mapped model buffer size =     0.6 MiB\n"
+    )
+    assert server_log_shows_gpu_offload(log) is False
+    # A real device buffer alongside a CUDA_Host line still reads as GPU.
+    log_ok = (
+        "load_tensors:   CUDA_Host model buffer size = 100.0 MiB\n"
+        "load_tensors:   CUDA0 model buffer size = 21000.0 MiB\n"
+    )
+    assert server_log_shows_gpu_offload(log_ok) is True
+
+
+def test_draft_offloaded_zero_before_main_offload_is_gpu():
+    # Speculative decoding: a draft model logs "offloaded 0/2" before the main
+    # model's "offloaded 33/33". The N>0 line must win.
+    log = (
+        "load_tensors: offloaded 0/2 layers to GPU\n"
+        "load_tensors: offloaded 33/33 layers to GPU\n"
+    )
+    assert server_log_shows_gpu_offload(log) is True
+
+
+def test_crlf_log_parses_identically():
+    # Windows logs use CRLF; classification must not change.
+    assert (
+        server_log_shows_gpu_offload(CUDA_DEVICE_INFO_LOG.replace("\n", "\r\n")) is True
+    )
+    assert (
+        server_log_shows_gpu_offload(CPU_ONLY_DEVICE_INFO_LOG.replace("\n", "\r\n"))
+        is False
+    )
+
+
+# -- 2. resolve_smoke_test_install_kind --------------------------------------
+
+
+@pytest.mark.parametrize(
+    "host,expected",
+    [
+        (nvidia_host(), "linux-cuda"),
+        (windows_nvidia_host(), "windows-cuda"),
+        (rocm_host(), "linux-rocm"),
+        (rocm_host(system = "Windows", is_windows = True, is_linux = False), "windows-hip"),
+        (macos_arm_host(), "macos-arm64"),
+        (cpu_host(), "linux-cpu"),
+        (cpu_host(system = "Windows", is_windows = True, is_linux = False), "windows-cpu"),
+    ],
+)
+def test_resolve_smoke_test_install_kind(host, expected):
+    assert resolve_smoke_test_install_kind(host) == expected
+    if expected.endswith("-cpu"):
+        assert expected not in M._GPU_INSTALL_KINDS
+    else:
+        assert expected in M._GPU_INSTALL_KINDS
+
+
+# -- 3. validate_server integration (mocked subprocess + HTTP) ---------------
+
+
+class _FakeResponse:
+    def __init__(self, status: int, body: bytes = b"{}"):
+        self.status = status
+        self._body = body
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        return False
+
+    def read(self):
+        return self._body
+
+
+class _FakeProc:
+    """Stands in for the llama-server subprocess: writes a canned startup log
+    to the handle it was given, then reports itself alive until terminated."""
+
+    def __init__(self, log_text: str):
+        self._log_text = log_text
+        self._alive = True
+
+    def __call__(self, command, *args, **kwargs):
+        handle = kwargs.get("stdout")
+        if handle is not None:
+            handle.write(self._log_text)
+            handle.flush()
+        return self
+
+    def poll(self):
+        return None if self._alive else 0
+
+    def wait(self, timeout = None):
+        self._alive = False
+        return 0
+
+    def terminate(self):
+        self._alive = False
+
+    def kill(self):
+        self._alive = False
+
+
+@pytest.fixture
+def patched_server(monkeypatch):
+    """Patch validate_server's external touchpoints; return a configure()
+    helper so each test picks the canned log it wants."""
+    monkeypatch.setattr(M, "free_local_port", lambda: 18000)
+    monkeypatch.setattr(M, "binary_env", lambda *a, **k: {})
+    monkeypatch.setattr(M.time, "sleep", lambda *a, **k: None)
+    monkeypatch.setattr(M.urllib.request, "urlopen", lambda *a, **k: _FakeResponse(200))
+
+    def configure(log_text):
+        monkeypatch.setattr(M.subprocess, "Popen", _FakeProc(log_text))
+
+    return configure
+
+
+def _run_validate(tmp_path, host, install_kind):
+    server = tmp_path / "llama-server"
+    server.write_text("#!/bin/sh\n")
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF")
+    validate_server(server, probe, host, tmp_path, install_kind = install_kind)
+
+
+def test_gpu_intent_cpu_only_rejected(patched_server, tmp_path):
+    # The #5807 bug: GPU requested, binary ran on CPU -> reject. The exception
+    # must be GpuOffloadFailure specifically (not just the PrebuiltFallback
+    # base) so the --smoke-test CLI can map it to EXIT_FALLBACK vs EXIT_ERROR.
+    patched_server(CPU_ONLY_DEVICE_INFO_LOG)
+    with pytest.raises(M.GpuOffloadFailure, match = "entirely on CPU"):
+        _run_validate(tmp_path, windows_nvidia_host(), "windows-cuda")
+    assert issubclass(M.GpuOffloadFailure, PrebuiltFallback)
+
+
+def test_gpu_intent_gpu_offload_accepted(patched_server, tmp_path):
+    patched_server(CUDA_DEVICE_INFO_LOG)
+    _run_validate(tmp_path, windows_nvidia_host(), "windows-cuda")  # no raise
+
+
+def test_cpu_kind_not_gpu_gated(patched_server, tmp_path):
+    # A windows-cpu bundle is the intentional fallback; never GPU-gated even
+    # if it (obviously) loads on CPU.
+    patched_server(CPU_ONLY_DEVICE_INFO_LOG)
+    _run_validate(tmp_path, windows_nvidia_host(), "windows-cpu")  # no raise
+
+
+def test_gpu_intent_no_signal_accepted(patched_server, tmp_path):
+    # No buffer/device signal -> conservative: do not reject on no evidence
+    # (plain install validation; require_gpu_signal defaults False).
+    patched_server(NO_SIGNAL_LOG)
+    _run_validate(tmp_path, nvidia_host(), "linux-cuda")  # no raise
+
+
+def test_smoke_test_no_signal_gpu_is_inconclusive(patched_server, tmp_path):
+    # The smoke-test CLI sets require_gpu_signal, so a no-signal GPU log is
+    # inconclusive (PrebuiltFallback -> EXIT_ERROR), not a silent pass.
+    patched_server(NO_SIGNAL_LOG)
+    server = tmp_path / "llama-server"
+    server.write_text("#!/bin/sh\n")
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF")
+    with pytest.raises(PrebuiltFallback):
+        smoke_test_server_binary(
+            str(server), nvidia_host(), install_dir = str(tmp_path), probe = str(probe)
+        )
+
+
+def test_rocm_cpu_only_rejected(patched_server, tmp_path):
+    patched_server(CPU_ONLY_DEVICE_INFO_LOG)
+    with pytest.raises(PrebuiltFallback):
+        _run_validate(tmp_path, rocm_host(), "linux-rocm")
+
+
+def test_macos_metal_not_in_offload_required_kinds():
+    # macos-arm64 ships a GPU backend (Metal) and is launched with
+    # --n-gpu-layers, but a CPU-only Metal load is an unfixable environment
+    # limitation, so it must NOT be in the offload-required (reject) set.
+    assert "macos-arm64" in M._GPU_INSTALL_KINDS
+    assert "macos-arm64" not in M._GPU_OFFLOAD_REQUIRED_KINDS
+
+
+def test_macos_metal_cpu_only_not_rejected(patched_server, tmp_path):
+    # The macOS regression in #5858 CI: GitHub macOS runners have no usable
+    # Metal, so the macos-arm64 prebuilt loads on CPU. It must be accepted, not
+    # rejected into a source build that also runs on CPU and breaks the install.
+    patched_server(CPU_ONLY_DEVICE_INFO_LOG)
+    _run_validate(tmp_path, macos_arm_host(), "macos-arm64")  # no raise
+
+
+def test_smoke_test_macos_metal_cpu_only_passes(patched_server, tmp_path):
+    # The smoke-test CLI must also accept a CPU-only macOS Metal load (exit 0),
+    # so setup.sh does not pointlessly retry a CPU source build on a Mac.
+    patched_server(CPU_ONLY_DEVICE_INFO_LOG)
+    server = tmp_path / "llama-server"
+    server.write_text("#!/bin/sh\n")
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF")
+    # No raise -> the CLI maps this to EXIT_SUCCESS.
+    smoke_test_server_binary(
+        str(server),
+        macos_arm_host(),
+        install_dir = str(tmp_path),
+        probe = str(probe),
+        install_kind = "macos-arm64",
+    )
+
+
+# -- 4. smoke_test_server_binary ---------------------------------------------
+
+
+def test_smoke_test_missing_binary(tmp_path):
+    with pytest.raises(PrebuiltFallback, match = "not found"):
+        smoke_test_server_binary(
+            str(tmp_path / "nope"), nvidia_host(), install_dir = str(tmp_path)
+        )
+
+
+def test_smoke_test_delegates_to_validate_server(monkeypatch, tmp_path):
+    server = tmp_path / "llama-server"
+    server.write_text("#!/bin/sh\n")
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF")
+    seen = {}
+
+    def fake_validate(server_path, probe_path, host, install_dir, **kw):
+        seen["install_kind"] = kw.get("install_kind")
+
+    monkeypatch.setattr(M, "validate_server", fake_validate)
+    kind = smoke_test_server_binary(
+        str(server), nvidia_host(), install_dir = str(tmp_path), probe = str(probe)
+    )
+    assert kind == "linux-cuda"
+    assert seen["install_kind"] == "linux-cuda"
+
+
+def test_smoke_test_propagates_failure(monkeypatch, tmp_path):
+    server = tmp_path / "llama-server"
+    server.write_text("#!/bin/sh\n")
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF")
+
+    def fake_validate(*a, **k):
+        raise PrebuiltFallback("loaded the model entirely on CPU")
+
+    monkeypatch.setattr(M, "validate_server", fake_validate)
+    with pytest.raises(PrebuiltFallback):
+        smoke_test_server_binary(
+            str(server),
+            nvidia_host(),
+            install_dir = str(tmp_path),
+            probe = str(probe),
+        )
+
+
+# -- 5. --smoke-test CLI exit-code contract (the load-bearing fix contract) --
+# setup.sh / setup.ps1 branch on: 2 = ran on CPU (rebuild CPU), 1 = inconclusive
+# (keep GPU build), 0 = offload confirmed.
+
+
+def _run_main_smoke(monkeypatch, smoke_impl):
+    monkeypatch.setattr(M, "detect_host", lambda: nvidia_host())
+    monkeypatch.setattr(M, "smoke_test_server_binary", smoke_impl)
+    monkeypatch.setattr(
+        sys, "argv", ["install_llama_prebuilt.py", "--smoke-test", "/x/llama-server"]
+    )
+    return M.main()
+
+
+def test_main_smoke_exit_success(monkeypatch):
+    rc = _run_main_smoke(monkeypatch, lambda *a, **k: "linux-cuda")
+    assert rc == M.EXIT_SUCCESS
+
+
+def test_main_smoke_exit_fallback_on_cpu_only(monkeypatch):
+    def impl(*a, **k):
+        raise M.GpuOffloadFailure("loaded the model entirely on CPU")
+
+    rc = _run_main_smoke(monkeypatch, impl)
+    assert rc == M.EXIT_FALLBACK  # 2 -> setup scripts rebuild CPU
+
+
+def test_main_smoke_exit_error_on_inconclusive(monkeypatch):
+    def impl(*a, **k):
+        raise PrebuiltFallback("llama-server exited during startup")
+
+    rc = _run_main_smoke(monkeypatch, impl)
+    assert rc == M.EXIT_ERROR  # 1 -> setup scripts keep the GPU build
+
+
+# -- 6. existing_gpu_install_offloads: re-validate a matching install (#5807) --
+
+
+def _gpu_plan(install_kind = "linux-cuda"):
+    choice = M.AssetChoice(
+        repo = "unslothai/llama.cpp",
+        tag = "b9001",
+        name = "app-b9001-linux-x64-cuda13-newer.tar.gz",
+        url = "https://example.com/x",
+        source_label = "published",
+        install_kind = install_kind,
+    )
+    return M.InstallReleasePlan(
+        requested_tag = "latest",
+        llama_tag = "b9001",
+        release_tag = "rel",
+        attempts = [choice],
+        approved_checksums = M.ApprovedReleaseChecksums(
+            repo = "unslothai/llama.cpp",
+            release_tag = "rel",
+            upstream_tag = "b9001",
+            artifacts = {},
+        ),
+    )
+
+
+def _with_server(tmp_path):
+    server = tmp_path / "llama-server"
+    server.write_text("#!/bin/sh\n")
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF")
+    return probe
+
+
+def test_existing_cpu_kind_install_is_kept(tmp_path):
+    # A non-GPU existing install is never offload-gated.
+    probe = _with_server(tmp_path)
+    assert (
+        M.existing_gpu_install_offloads(
+            tmp_path, nvidia_host(), _gpu_plan("linux-cpu"), probe
+        )
+        is True
+    )
+
+
+def test_existing_gpu_install_cpu_only_triggers_reinstall(monkeypatch, tmp_path):
+    probe = _with_server(tmp_path)
+
+    def fake_validate(*a, **k):
+        raise M.GpuOffloadFailure("loaded the model entirely on CPU")
+
+    monkeypatch.setattr(M, "validate_server", fake_validate)
+    assert (
+        M.existing_gpu_install_offloads(
+            tmp_path, nvidia_host(), _gpu_plan("linux-cuda"), probe
+        )
+        is False
+    )
+
+
+def test_existing_gpu_install_offloading_is_kept(monkeypatch, tmp_path):
+    probe = _with_server(tmp_path)
+    monkeypatch.setattr(M, "validate_server", lambda *a, **k: None)
+    assert (
+        M.existing_gpu_install_offloads(
+            tmp_path, nvidia_host(), _gpu_plan("linux-cuda"), probe
+        )
+        is True
+    )
+
+
+def test_existing_gpu_install_inconclusive_is_kept(monkeypatch, tmp_path):
+    probe = _with_server(tmp_path)
+
+    def fake_validate(*a, **k):
+        raise PrebuiltFallback("llama-server exited during startup")
+
+    monkeypatch.setattr(M, "validate_server", fake_validate)
+    assert (
+        M.existing_gpu_install_offloads(
+            tmp_path, nvidia_host(), _gpu_plan("linux-cuda"), probe
+        )
+        is True
+    )
+
+
+def test_existing_gpu_install_no_binary_is_kept(tmp_path):
+    # No llama-server present -> let the normal flow reinstall, don't crash.
+    probe = tmp_path / "probe.gguf"
+    probe.write_bytes(b"GGUF")
+    assert (
+        M.existing_gpu_install_offloads(
+            tmp_path, nvidia_host(), _gpu_plan("linux-cuda"), probe
+        )
+        is True
+    )