Composer fails to start

The process exits within seconds of launch — Desktop never paints its main window, or Runtime exits before printing a render-loop banner.

First, check whichever log surface is available:

• Desktop: open <install-root>\Logs\ and look at the most recent .log and .json files. Anything with severity fatal (identifier 4000–4008) names the missing piece.
• Runtime: redirect stderr (./vindralcomposerruntime ... 2> stderr.log) and re-run; many startup failures land on stderr before the log subsystem is online.

See Log files and Log identifiers for the full layout.

Common causes:

• NVIDIA driver out of date — Composer R2 2026 onward needs driver 590 or newer on Windows (CUDA 13); R3 2025 needs 570.26+ (CUDA 12.8); R1 2024 onward needs 551.61+ (CUDA 12.4). Run nvidia-smi and check the Driver Version column. Update the driver and reboot.
• CUDA toolkit not installed — cuInit failure or log identifier 4000 in the fatal log. Install CUDA 12.4 (or newer) from NVIDIA Developer.
• FFmpeg not found — Composer locates FFmpeg through the COMPOSER_FFMPEG6p environment variable or its registry equivalent. Set the variable to the path of your FFmpeg bin/ folder (default: C:\Program Files\FFmpeg\6p\bin\) and re-run.
• Not running as Administrator (Windows) — Composer needs to write settings.xml and apikeys.json to a system-protected location on first start. Right-click the launcher and choose Run as administrator.
• Licence verification rejected — log identifier 4007 / 4008. The activation server can't validate the licence; see Licence or activation problems below.
• Port already bound — Composer's HTTP server can't open its configured port because another process owns it. Find the offender with netstat -ano | findstr <port> (Windows) or ss -lntp | grep <port> (Linux); kill it or change Composer's port through Settings.

For Windows-specific install troubleshooting (cuDNN, environment variables, FFmpeg-installer prompts), see Installing on Windows in Getting Started.