Fix CUDA downloads on unsupported platforms (#770)

* Fix CUDA downloads on unsupported platforms

* fix: align CUDA status nullability

* fix: require CUDA download support flag

README.md

@@ -270,7 +270,8 @@ Use cases: agent dev loops (dictate a question, hear the answer in a cloned voic
 | Platform                 | Backend        | Notes                                          |
 | ------------------------ | -------------- | ---------------------------------------------- |
 | macOS (Apple Silicon)    | MLX (Metal)    | 4-5x faster via Neural Engine                  |
-| Windows / Linux (NVIDIA) | PyTorch (CUDA) | Auto-downloads CUDA binary from within the app |
+| Windows (NVIDIA)         | PyTorch (CUDA) | Auto-downloads CUDA binary from within the app |
+| Linux (NVIDIA)           | PyTorch (CUDA) | Use a local/remote Python backend with CUDA PyTorch |
 | Linux (AMD)              | PyTorch (ROCm) | Auto-configures HSA_OVERRIDE_GFX_VERSION       |
 | Windows (any GPU)        | DirectML       | Universal Windows GPU support                  |
 | Intel Arc                | IPEX/XPU       | Intel discrete GPU acceleration                |

app/src/lib/api/types.ts

@@ -287,7 +287,10 @@ export interface CudaDownloadProgress {
 export interface CudaStatus {
   available: boolean; // CUDA binary exists on disk
   active: boolean; // Currently running the CUDA binary
-  binary_path?: string;
+  binary_path: string | null;
+  cuda_libs_version: string | null;
+  download_supported: boolean; // Platform has a matching release asset
+  unsupported_reason: string | null;
   downloading: boolean; // Download in progress
   download_progress?: CudaDownloadProgress;
 }

backend/routes/cuda.py

@@ -26,6 +26,10 @@ async def download_cuda_backend():
     """Download the CUDA backend binary."""
     from ..services import cuda
 
+    unsupported_reason = cuda.get_cuda_download_unsupported_reason()
+    if unsupported_reason:
+        raise HTTPException(status_code=409, detail=unsupported_reason)
+
     if cuda.get_cuda_binary_path() is not None:
         raise HTTPException(status_code=409, detail="CUDA backend already downloaded")
 

backend/services/cuda.py

@@ -21,9 +21,9 @@ import tarfile
 from pathlib import Path
 from typing import Optional
 
+from .. import __version__
 from ..config import get_data_dir
 from ..utils.progress import get_progress_manager
-from .. import __version__
 
 logger = logging.getLogger(__name__)
 
@@ -31,6 +31,8 @@ GITHUB_RELEASES_URL = "https://github.com/jamiepine/voicebox/releases/download"
 
 PROGRESS_KEY = "cuda-backend"
 
+CUDA_DOWNLOAD_UNSUPPORTED_REASON = "Downloadable CUDA backend releases are currently only published for Windows."
+
 # The current expected CUDA libs version.  Bump this when we change the
 # CUDA toolkit version or torch's CUDA dependency changes (e.g. cu126 -> cu128).
 CUDA_LIBS_VERSION = "cu128-v1"
@@ -63,6 +65,25 @@ def get_cuda_exe_name() -> str:
     return "voicebox-server-cuda"
 
 
+def is_cuda_download_supported() -> bool:
+    """Return whether this platform has a matching CUDA release asset."""
+    return sys.platform == "win32"
+
+
+def get_cuda_download_unsupported_reason() -> str | None:
+    """Explain why this platform cannot use the release-download flow."""
+    if is_cuda_download_supported():
+        return None
+    return CUDA_DOWNLOAD_UNSUPPORTED_REASON
+
+
+def ensure_cuda_download_supported() -> None:
+    """Raise if downloading would fetch an asset built for another platform."""
+    reason = get_cuda_download_unsupported_reason()
+    if reason:
+        raise RuntimeError(reason)
+
+
 def get_cuda_binary_path() -> Optional[Path]:
     """Return path to the CUDA executable if it exists inside the onedir."""
     p = get_cuda_dir() / get_cuda_exe_name()
@@ -103,12 +124,15 @@ def get_cuda_status() -> dict:
     cuda_path = get_cuda_binary_path()
     progress = progress_manager.get_progress(PROGRESS_KEY)
     cuda_libs_version = get_installed_cuda_libs_version()
+    unsupported_reason = get_cuda_download_unsupported_reason()
 
     return {
         "available": cuda_path is not None,
         "active": is_cuda_active(),
         "binary_path": str(cuda_path) if cuda_path else None,
         "cuda_libs_version": cuda_libs_version,
+        "download_supported": unsupported_reason is None,
+        "unsupported_reason": unsupported_reason,
         "downloading": progress is not None and progress.get("status") == "downloading",
         "download_progress": progress,
     }
@@ -257,6 +281,8 @@ async def download_cuda_binary(version: Optional[str] = None):
 
 async def _download_cuda_binary_locked(version: Optional[str] = None):
     """Inner implementation of download_cuda_binary, called under _download_lock."""
+    ensure_cuda_download_supported()
+
     import httpx
 
     if version is None:
@@ -387,6 +413,11 @@ async def check_and_update_cuda_binary():
     if not cuda_path:
         return  # No CUDA binary installed, nothing to update
 
+    unsupported_reason = get_cuda_download_unsupported_reason()
+    if unsupported_reason:
+        logger.info("Skipping CUDA backend auto-update: %s", unsupported_reason)
+        return
+
     need_server = _needs_server_download()
     need_libs = _needs_cuda_libs_download()
 

backend/tests/test_cuda_download.py

@@ -0,0 +1,32 @@
+import sys as py_sys
+import types
+
+import pytest
+
+from backend.services import cuda
+
+
+def test_cuda_status_reports_unsupported_linux_download(monkeypatch, tmp_path):
+    monkeypatch.setattr(cuda.sys, "platform", "linux")
+    monkeypatch.setattr(cuda, "get_data_dir", lambda: tmp_path)
+
+    status = cuda.get_cuda_status()
+
+    assert status["available"] is False
+    assert status["download_supported"] is False
+    assert status["unsupported_reason"] == cuda.CUDA_DOWNLOAD_UNSUPPORTED_REASON
+
+
+@pytest.mark.asyncio
+async def test_cuda_download_rejects_linux_before_network(monkeypatch, tmp_path):
+    monkeypatch.setattr(cuda.sys, "platform", "linux")
+    monkeypatch.setattr(cuda, "get_data_dir", lambda: tmp_path)
+
+    class UnexpectedClient:
+        def __init__(self, *args, **kwargs):
+            raise AssertionError("unsupported platforms should not start a release download")
+
+    monkeypatch.setitem(py_sys.modules, "httpx", types.SimpleNamespace(AsyncClient=UnexpectedClient))
+
+    with pytest.raises(RuntimeError, match="currently only published for Windows"):
+        await cuda._download_cuda_binary_locked("v0.5.0")

docs/content/docs/overview/gpu-acceleration.mdx

@@ -23,7 +23,7 @@ This page is for the cases where it doesn't:
 | **Windows + NVIDIA**        | PyTorch CUDA (cu128)      | Auto-downloads the CUDA backend binary on first use  |
 | **Windows + Intel Arc**     | PyTorch XPU (IPEX)        | New in 0.4 — works with Arc A-series and B-series    |
 | **Windows generic GPU**     | DirectML                  | Universal Windows GPU support; slower than CUDA      |
-| **Linux + NVIDIA**          | PyTorch CUDA (cu128)      | Same auto-download flow as Windows                   |
+| **Linux + NVIDIA**          | PyTorch CUDA (cu128)      | Use a local/remote Python backend with CUDA PyTorch  |
 | **Linux + AMD**             | PyTorch ROCm              | Auto-configures `HSA_OVERRIDE_GFX_VERSION`           |
 | **Linux + Intel Arc**       | PyTorch XPU (IPEX)        |                                                      |
 | **Any (no GPU)**            | PyTorch CPU               | Works everywhere; expect 5-50x slower than GPU       |
@@ -46,7 +46,7 @@ On M-series Macs, Voicebox ships an MLX-optimized backend that uses the Apple Ne
 
 The Whisper Turbo + MLX combo dropped transcription latency from ~20s to ~2-3s on M-series chips (see CHANGELOG entry for v0.1.10).
 
-## Windows / Linux + NVIDIA — The CUDA Backend Swap
+## Windows + NVIDIA — The CUDA Backend Swap
 
 Voicebox doesn't bundle CUDA into the main installer (it would balloon downloads to multi-gigabyte territory for users who don't have an NVIDIA GPU). Instead, when you first need it, the app downloads a separate **CUDA backend binary** that contains the PyTorch + CUDA runtime.
 

docs/content/docs/overview/introduction.mdx

@@ -75,7 +75,8 @@ No cloud fallback, no bring-your-own-API-key. Local is the product.
 | Platform | Backend | Notes |
 |----------|---------|-------|
 | macOS (Apple Silicon) | MLX (Metal) | 4-5x faster via Neural Engine |
-| Windows / Linux (NVIDIA) | PyTorch (CUDA) | Auto-downloads CUDA binary from within the app |
+| Windows (NVIDIA)         | PyTorch (CUDA) | Auto-downloads CUDA binary from within the app |
+| Linux (NVIDIA)           | PyTorch (CUDA) | Use a local/remote Python backend with CUDA PyTorch |
 | Linux (AMD) | PyTorch (ROCm) | Auto-configures HSA_OVERRIDE_GFX_VERSION |
 | Windows (any GPU) | DirectML | Universal Windows GPU support |
 | Intel Arc | IPEX/XPU | Intel discrete GPU acceleration |