LykosAI/StabilityMatrix · error · InvalidOperationException

Failed to start Forge Classic install process

Error message

Failed to start Forge Classic install process

What it means

RunInstallScriptWithPromptHandling throws InvalidOperationException when venvRunner.RunDetached is called but venvRunner.Process is null, meaning the venv python process could not be spawned. This is a process-start failure, not a script failure.

Solutions

1. Recreate the venv (reinstall the package or delete the venv folder) so the python executable exists.
2. Verify the selected Python version is actually downloaded/available in Stability Matrix's python install directory.
3. Check antivirus/quarantine logs for blocked python.exe and whitelist the Stability Matrix directories.
4. Retry the install; transient file locks (Forge still running) can prevent process start.

var venvPython = Path.Combine(installLocation, "venv", OperatingSystem.IsWindows() ? "Scripts\\python.exe" : "bin/python");
if (!File.Exists(venvPython)) { /* recreate venv before installing */ }

try { await package.InstallPackage(...); }
catch (InvalidOperationException ex) when (ex.Message.Contains("Failed to start"))
{ RecreateVenvAndRetry(); }

When it happens

Trigger: InstallPackage -> RunInstallScriptWithPromptHandling calls RunDetached with launch args and the process fails to start (venv python executable missing, path too long, permissions, corrupted venv).

Common situations: Venv was deleted or corrupted mid-install; antivirus blocks python.exe; invalid python version chosen so the venv interpreter path doesn't exist.

Related errors

• Torch is not installed in the virtual environment.
• ComfyUI venv was not found at
• Windows ROCm bitsandbytes is only supported on Python 3.12.x
• pyvenv.cfg is UTF-16 encoded; expected UTF-8/ASCII
• pyvenv.cfg contains NUL bytes; expected UTF-8/ASCII

AI-assisted analysis of LykosAI/StabilityMatrix@af93d6ef57 (2026-09-12). Data as JSON: /api/errors/1b8de1ea49b44da5. Report an issue: GitHub.

Appendix: source

Thrown at StabilityMatrix.Core/Models/Packages/ForgeClassic.cs:342

                venvRunner.Process.StandardInput.WriteLine();
                enterSent = true;

                onConsoleOutput?.Invoke(
                    ProcessOutput.FromStdOutLine(
                        "[ForgeClassic] Detected legacy update prompt. Sent Enter automatically."
                    )
                );
            }
            catch (Exception e)
            {
                Logger.Warn(e, "Failed to auto-submit Enter for Forge Classic update prompt");
            }
        }

        venvRunner.RunDetached([.. launchArgs], HandleInstallOutput);
        var process =
            venvRunner.Process
            ?? throw new InvalidOperationException("Failed to start Forge Classic install process");
        await process.WaitForExitAsync(cancellationToken).ConfigureAwait(false);
        return process.ExitCode;
    }

    private void ResetVenvForPythonUpgrade(string installLocation, Action<ProcessOutput>? onConsoleOutput)
    {
        var venvPath = Path.Combine(installLocation, "venv");
        if (!Directory.Exists(venvPath))
            return;

        try
        {
            Directory.Delete(venvPath, recursive: true);
            onConsoleOutput?.Invoke(
                ProcessOutput.FromStdOutLine("[ForgeClassic] Removed existing venv before Python upgrade.")
            );
        }
        catch (Exception e)

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