Fix Studio silently exiting on Windows without error output (#4527)

* Fix Studio silently exiting on Windows without error output

On Windows, `unsloth studio` launches a child process via
subprocess.Popen to run the server in the studio venv. If the child
crashes (e.g. due to a missing package), the parent just calls
typer.Exit(rc) with no message -- the user sees "Launching Unsloth
Studio... Please wait..." and then the prompt returns with zero
feedback.

Root cause: `data_designer_unstructured_seed` is imported at the top
level in seed.py. If this package is not installed in the studio venv,
the entire import chain (seed.py -> routes/__init__.py -> main.py ->
run_server()) crashes with ModuleNotFoundError. Since run.py has no
try/except around run_server() and studio.py does not report nonzero
exit codes, the failure is completely silent.

Changes:
- run.py: wrap run_server() in try/except, print clear error with
  traceback to stderr. Also reconfigure stderr encoding on Windows so
  tracebacks with non-ASCII paths do not cause secondary failures.
- studio.py: print an error message when the child process exits with
  a nonzero code on Windows, so the user knows something went wrong.
- seed.py: make data_designer_unstructured_seed import optional with
  a try/except fallback. The server starts normally and only returns
  HTTP 500 if the unstructured seed endpoints are actually called.

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

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

* Skip Anaconda/Miniconda Python when creating Studio venv on Windows

Conda-bundled CPython ships modified DLL search paths that prevent
torch from loading c10.dll on Windows. The Studio server fails
silently at startup because the venv was created with conda's Python.

Standalone CPython (python.org, winget, uv) does not have this issue.

Both install.ps1 and setup.ps1 now skip any Python binary whose path
contains conda, miniconda, anaconda, miniforge, or mambaforge when
selecting the interpreter for the studio venv. If only conda Python
is available, the scripts print an error with instructions to install
standalone CPython.

* Fix multi-file preview crash and improve setup.ps1 Python discovery

Addresses review findings [10/10] and [8/10]:

1. seed.py: _read_preview_rows_from_multi_files() had a hard import
   of build_multi_file_preview_rows inside the function body, bypassing
   the optional-plugin guard. Moved it into the top-level try/except
   block and added a None guard matching the other functions.

2. setup.ps1: Python discovery now probes py.exe (Python Launcher)
   first, uses Get-Command -All to look past conda entries that shadow
   standalone CPython further down PATH, skips WindowsApps stubs, and
   resolves the actual executable path so venv creation does not
   re-resolve back to a conda interpreter.

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

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

* Check sys.base_prefix to catch venvs created from conda Python

A venv created from conda Python (e.g. C:\Users\danie\.venv) has a
path that does not contain "conda", but sys.base_prefix still points
to the conda install (e.g. C:\Users\danie\miniconda3). The previous
path-only check missed this case entirely.

Both install.ps1 and setup.ps1 now use a Test-IsConda helper that
checks both the executable path AND sys.base_prefix against the
conda/miniconda/anaconda/miniforge/mambaforge pattern. This catches:
- Direct conda Python executables
- Venvs created from conda Python (base_prefix reveals the origin)

* Fix install.ps1 passing version string to uv venv instead of resolved path

Find-CompatiblePython returned a bare version string (e.g. "3.13")
which was passed to `uv venv --python 3.13`. uv performs its own
interpreter discovery and can resolve that version string back to a
conda Python, defeating the entire conda-skip logic.

Now Find-CompatiblePython returns a hashtable with both .Version (for
display) and .Path (the resolved absolute executable path). The venv
is created with `uv venv --python <absolute-path>`, ensuring uv uses
the exact interpreter we validated.

* Quote resolved Python path in uv venv call for paths with spaces

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

install.ps1

@@ -44,14 +44,45 @@ function Install-UnslothStudio {
     # Uses try-catch + stderr redirection so that App Execution Alias stubs
     # (WindowsApps) and other non-functional executables are probed safely
     # without triggering $ErrorActionPreference = "Stop".
+    #
+    # Skips Anaconda/Miniconda Python: conda-bundled CPython ships modified
+    # DLL search paths that break torch's c10.dll loading on Windows.
+    # Standalone CPython (python.org, winget, uv) does not have this issue.
+    #
+    # NOTE: A venv created from conda Python inherits conda's base_prefix
+    # even if the venv path does not contain "conda". We check both the
+    # executable path AND sys.base_prefix to catch this.
+    $script:CondaSkipPattern = '(?i)(conda|miniconda|anaconda|miniforge|mambaforge)'
+
+    function Test-IsCondaPython {
+        param([string]$Exe)
+        if ($Exe -match $script:CondaSkipPattern) { return $true }
+        try {
+            $basePrefix = (& $Exe -c "import sys; print(sys.base_prefix)" 2>$null | Out-String).Trim()
+            if ($basePrefix -match $script:CondaSkipPattern) { return $true }
+        } catch { }
+        return $false
+    }
+
+    # Returns @{ Version = "3.13"; Path = "C:\...\python.exe" } or $null.
+    # The resolved Path is passed to `uv venv --python` to prevent uv from
+    # re-resolving the version string back to a conda interpreter.
     function Find-CompatiblePython {
         # Try the Python Launcher first (most reliable on Windows)
+        # py.exe resolves to the standard CPython install, not conda.
         $pyLauncher = Get-Command py -CommandType Application -ErrorAction SilentlyContinue
-        if ($pyLauncher) {
+        if ($pyLauncher -and $pyLauncher.Source -notmatch $script:CondaSkipPattern) {
             foreach ($minor in @("3.13", "3.12", "3.11")) {
                 try {
                     $out = & $pyLauncher.Source "-$minor" --version 2>&1 | Out-String
-                    if ($out -match "Python (3\.1[1-3])\.\d+") { return $Matches[1] }
+                    if ($out -match "Python (3\.1[1-3])\.\d+") {
+                        $ver = $Matches[1]
+                        # Resolve the actual executable path and verify it is not conda-based
+                        $resolvedExe = (& $pyLauncher.Source "-$minor" -c "import sys; print(sys.executable)" 2>$null | Out-String).Trim()
+                        if ($resolvedExe -and (Test-Path $resolvedExe) -and -not (Test-IsCondaPython $resolvedExe)) {
+                            return @{ Version = $ver; Path = $resolvedExe }
+                        }
+                    }
                 } catch {}
             }
         }
@@ -61,25 +92,30 @@ function Install-UnslothStudio {
         # and can open the Microsoft Store as a side effect. Legitimate Store
         # Python is already detected via the py launcher above (Store packages
         # include py since Python 3.11).
+        # Skip Anaconda/Miniconda: check both path and sys.base_prefix.
         foreach ($name in @("python3", "python")) {
             foreach ($cmd in @(Get-Command $name -All -ErrorAction SilentlyContinue)) {
                 if (-not $cmd.Source) { continue }
                 if ($cmd.Source -like "*\WindowsApps\*") { continue }
+                if (Test-IsCondaPython $cmd.Source) { continue }
                 try {
                     $out = & $cmd.Source --version 2>&1 | Out-String
-                    if ($out -match "Python (3\.1[1-3])\.\d+") { return $Matches[1] }
+                    if ($out -match "Python (3\.1[1-3])\.\d+") {
+                        return @{ Version = $Matches[1]; Path = $cmd.Source }
+                    }
                 } catch {}
             }
         }
-        return ""
+        return $null
     }
 
     # ── Install Python if no compatible version (3.11-3.13) found ──
-    $DetectedPythonVersion = Find-CompatiblePython
-    if ($DetectedPythonVersion) {
-        Write-Host "==> Python already installed: Python $DetectedPythonVersion"
+    # Find-CompatiblePython returns @{ Version = "3.13"; Path = "C:\...\python.exe" } or $null.
+    $DetectedPython = Find-CompatiblePython
+    if ($DetectedPython) {
+        Write-Host "==> Python already installed: Python $($DetectedPython.Version)"
     }
-    if (-not $DetectedPythonVersion) {
+    if (-not $DetectedPython) {
         Write-Host "==> Installing Python ${PythonVersion}..."
         $pythonPackageId = "Python.Python.$PythonVersion"
         # Temporarily lower ErrorActionPreference so that winget stderr
@@ -95,9 +131,9 @@ function Install-UnslothStudio {
         Refresh-SessionPath
 
         # Re-detect after install (PATH may have changed)
-        $DetectedPythonVersion = Find-CompatiblePython
+        $DetectedPython = Find-CompatiblePython
 
-        if (-not $DetectedPythonVersion) {
+        if (-not $DetectedPython) {
             # Python still not functional after winget -- force reinstall.
             # This handles both real failures AND "already installed" codes where
             # winget thinks Python is present but it's not actually on PATH
@@ -110,10 +146,10 @@ function Install-UnslothStudio {
             } catch { $wingetExit = 1 }
             $ErrorActionPreference = $prevEAP
             Refresh-SessionPath
-            $DetectedPythonVersion = Find-CompatiblePython
+            $DetectedPython = Find-CompatiblePython
         }
 
-        if (-not $DetectedPythonVersion) {
+        if (-not $DetectedPython) {
             Write-Host "[ERROR] Python installation failed (exit code $wingetExit)" -ForegroundColor Red
             Write-Host "        Please install Python $PythonVersion manually from https://www.python.org/downloads/" -ForegroundColor Yellow
             Write-Host "        Make sure to check 'Add Python to PATH' during installation." -ForegroundColor Yellow
@@ -145,11 +181,13 @@ function Install-UnslothStudio {
     }
 
     # ── Create venv (skip if it already exists and has a valid interpreter) ──
+    # Pass the resolved executable path to uv so it does not re-resolve
+    # a version string back to a conda interpreter.
     $VenvPython = Join-Path $VenvName "Scripts\python.exe"
     if (-not (Test-Path $VenvPython)) {
         if (Test-Path $VenvName) { Remove-Item -Recurse -Force $VenvName }
-        Write-Host "==> Creating Python ${DetectedPythonVersion} virtual environment (${VenvName})..."
-        uv venv $VenvName --python $DetectedPythonVersion
+        Write-Host "==> Creating Python $($DetectedPython.Version) virtual environment (${VenvName})..."
+        uv venv $VenvName --python "$($DetectedPython.Path)"
         if ($LASTEXITCODE -ne 0) {
             Write-Host "[ERROR] Failed to create virtual environment (exit code $LASTEXITCODE)" -ForegroundColor Red
             return

studio/backend/routes/data_recipe/seed.py

@@ -15,11 +15,19 @@ from typing import Any
 from uuid import uuid4
 
 from fastapi import APIRouter, HTTPException, UploadFile, File as FastAPIFile, Form
-from data_designer_unstructured_seed.chunking import (
-    build_unstructured_preview_rows,
-    normalize_unstructured_text,
-    resolve_chunking,
-)
+
+try:
+    from data_designer_unstructured_seed.chunking import (
+        build_multi_file_preview_rows,
+        build_unstructured_preview_rows,
+        normalize_unstructured_text,
+        resolve_chunking,
+    )
+except ImportError:
+    build_multi_file_preview_rows = None
+    build_unstructured_preview_rows = None
+    normalize_unstructured_text = None
+    resolve_chunking = None
 from core.data_recipe.jsonable import to_preview_jsonable
 from utils.paths import ensure_dir, seed_uploads_root, unstructured_uploads_root
 
@@ -232,6 +240,11 @@ def _read_preview_rows_from_unstructured_file(
     chunk_size: int | None,
     chunk_overlap: int | None,
 ) -> list[dict[str, Any]]:
+    if resolve_chunking is None or build_unstructured_preview_rows is None:
+        raise HTTPException(
+            500,
+            "Unstructured seed support not available (missing data_designer_unstructured_seed)",
+        )
     size, overlap = resolve_chunking(chunk_size, chunk_overlap)
     try:
         rows = build_unstructured_preview_rows(
@@ -256,7 +269,11 @@ def _read_preview_rows_from_multi_files(
     chunk_size: int | None,
     chunk_overlap: int | None,
 ) -> list[dict[str, str]]:
-    from data_designer_unstructured_seed.chunking import build_multi_file_preview_rows
+    if build_multi_file_preview_rows is None:
+        raise HTTPException(
+            500,
+            "Unstructured seed support not available (missing data_designer_unstructured_seed)",
+        )
 
     _validate_safe_id(block_id, "block_id")
     block_dir = UNSTRUCTURED_UPLOAD_ROOT / block_id
@@ -382,6 +399,8 @@ def _extract_text_from_file(file_path: Path, ext: str) -> str:
     else:
         raise ValueError(f"Unsupported file type: {ext}")
 
+    if normalize_unstructured_text is None:
+        return raw
     return normalize_unstructured_text(raw)
 
 

studio/backend/run.py

@@ -256,6 +256,14 @@ def run_server(
 if __name__ == "__main__":
     import argparse
     import signal
+    import traceback
+
+    # Ensure stderr can handle Unicode on Windows (tracebacks with non-ASCII paths)
+    if sys.platform == "win32" and hasattr(sys.stderr, "reconfigure"):
+        try:
+            sys.stderr.reconfigure(encoding = "utf-8", errors = "replace")
+        except Exception:
+            pass
 
     parser = argparse.ArgumentParser(description = "Run Unsloth UI Backend server")
     parser.add_argument("--host", default = "0.0.0.0", help = "Host to bind to")
@@ -273,7 +281,21 @@ if __name__ == "__main__":
     kwargs = dict(host = args.host, port = args.port, silent = args.silent)
     if args.frontend is not None:
         kwargs["frontend_path"] = Path(args.frontend)
-    run_server(**kwargs)
+
+    try:
+        run_server(**kwargs)
+    except Exception:
+        sys.stderr.write("\n")
+        sys.stderr.write("=" * 60 + "\n")
+        sys.stderr.write("ERROR: Unsloth Studio failed to start.\n")
+        sys.stderr.write("=" * 60 + "\n")
+        traceback.print_exc(file = sys.stderr)
+        sys.stderr.write("\n")
+        sys.stderr.write(
+            "If a package is missing, try re-running: unsloth studio setup\n"
+        )
+        sys.stderr.flush()
+        sys.exit(1)
 
     # ── Signal handler — ensures subprocess cleanup on Ctrl+C ────
     def _signal_handler(signum, frame):

studio/setup.ps1

@@ -946,23 +946,85 @@ if (Test-Path $OxcValidatorDir) {
 Write-Host ""
 Write-Host "Setting up Python environment..." -ForegroundColor Cyan
 
-# Find Python
+# Find Python -- skip Anaconda/Miniconda distributions.
+# Conda-bundled CPython ships modified DLL search paths that break
+# torch's c10.dll loading on Windows. Standalone CPython (python.org,
+# winget, uv) does not have this issue.
+# Uses Get-Command -All to look past conda entries that shadow a valid
+# standalone Python further down PATH, and probes py.exe (the Python
+# Launcher) which reliably finds python.org installs.
+#
+# NOTE: A venv created from conda Python inherits conda's base_prefix
+# even though the venv path itself does not contain "conda". We check
+# both the executable path AND sys.base_prefix to catch this case.
+$CondaSkipPattern = '(?i)(conda|miniconda|anaconda|miniforge|mambaforge)'
 $PythonCmd = $null
-foreach ($candidate in @("python3.13", "python3.12", "python3.11", "python3", "python")) {
+
+# Helper: check if a Python executable is conda-based by inspecting
+# both the path and sys.base_prefix (catches venvs created from conda).
+function Test-IsConda {
+    param([string]$Exe)
+    if ($Exe -match $CondaSkipPattern) { return $true }
     try {
-        $ver = & $candidate --version 2>&1
-        if ($ver -match 'Python 3\.(\d+)') {
-            $minor = [int]$Matches[1]
-            if ($minor -ge 11 -and $minor -le 13) {
-                $PythonCmd = $candidate
-                break
-            }
-        }
+        $basePrefix = (& $Exe -c "import sys; print(sys.base_prefix)" 2>$null | Out-String).Trim()
+        if ($basePrefix -match $CondaSkipPattern) { return $true }
     } catch { }
+    return $false
+}
+
+# 1. Try the Python Launcher (py.exe) first -- most reliable on Windows.
+#    py.exe is installed by python.org and resolves to standalone CPython.
+$pyLauncher = Get-Command py -CommandType Application -ErrorAction SilentlyContinue
+if ($pyLauncher -and $pyLauncher.Source -notmatch $CondaSkipPattern) {
+    foreach ($minor in @("3.13", "3.12", "3.11")) {
+        try {
+            $out = & $pyLauncher.Source "-$minor" --version 2>&1 | Out-String
+            if ($out -match 'Python 3\.(\d+)') {
+                $pyMinor = [int]$Matches[1]
+                if ($pyMinor -ge 11 -and $pyMinor -le 13) {
+                    # Resolve the actual executable path so venv creation
+                    # does not re-resolve back to a conda interpreter.
+                    $resolvedExe = (& $pyLauncher.Source "-$minor" -c "import sys; print(sys.executable)" 2>$null | Out-String).Trim()
+                    if ($resolvedExe -and (Test-Path $resolvedExe) -and -not (Test-IsConda $resolvedExe)) {
+                        $PythonCmd = $resolvedExe
+                        break
+                    }
+                }
+            }
+        } catch { }
+    }
+}
+
+# 2. Fall back to scanning python3.x / python3 / python on PATH.
+#    Use Get-Command -All to look past conda entries.
+if (-not $PythonCmd) {
+    foreach ($candidate in @("python3.13", "python3.12", "python3.11", "python3", "python")) {
+        foreach ($cmdInfo in @(Get-Command $candidate -All -ErrorAction SilentlyContinue)) {
+            try {
+                if (-not $cmdInfo.Source) { continue }
+                if ($cmdInfo.Source -like "*\WindowsApps\*") { continue }
+                if (Test-IsConda $cmdInfo.Source) {
+                    Write-Host "   [SKIP] $($cmdInfo.Source) (conda Python breaks torch DLL loading)" -ForegroundColor Yellow
+                    continue
+                }
+                $ver = & $cmdInfo.Source --version 2>&1
+                if ($ver -match 'Python 3\.(\d+)') {
+                    $minor = [int]$Matches[1]
+                    if ($minor -ge 11 -and $minor -le 13) {
+                        $PythonCmd = $cmdInfo.Source
+                        break
+                    }
+                }
+            } catch { }
+        }
+        if ($PythonCmd) { break }
+    }
 }
 
 if (-not $PythonCmd) {
-    Write-Host "[ERROR] No Python 3.11-3.13 found." -ForegroundColor Red
+    Write-Host "[ERROR] No standalone Python 3.11-3.13 found (conda Python is not supported)." -ForegroundColor Red
+    Write-Host "        Install Python from https://python.org/downloads/ or via:" -ForegroundColor Yellow
+    Write-Host "        winget install -e --id Python.Python.3.12" -ForegroundColor Yellow
     exit 1
 }
 

unsloth_cli/commands/studio.py

@@ -118,6 +118,16 @@ def studio_default(
                 except KeyboardInterrupt:
                     # Child has its own signal handler — let it finish
                     rc = proc.wait()
+                if rc != 0:
+                    typer.echo(
+                        f"\nError: Studio server exited unexpectedly (code {rc}).",
+                        err = True,
+                    )
+                    typer.echo(
+                        "Check the error above. If a package is missing, "
+                        "re-run: unsloth studio setup",
+                        err = True,
+                    )
                 raise typer.Exit(rc)
             else:
                 os.execvp(str(studio_python), args)