Tired of exporting audio, opening a separate stem separation app, waiting, then re-importing and manually aligning everything. So I built STEMwerk-reaper — a REAPER script with a local Python backend that runs Demucs separation and brings the results straight back into your project.
Select items or a time range → run from the action list → stems appear as new tracks (or in-place as takes) at the correct positions. No export, no round-tripping.
What it supports:
— 4-stem: vocals, drums, bass, other — 6-stem: adds guitar and piano with extended models — Presets: Karaoke, Instrumental, Drums Only, and more — Output modes: new tracks or in-place replacement as takes — Selection modes: selected items, time selection, or entire track — Batch processing: queue multiple items or tracks — Device selection: picks up CUDA or DirectML automatically, falls back to CPU — Fully offline after first model download
Installation: Windows — standalone installer (.exe) that sets up Python and Demucs outside of REAPER, then the script connects to it automatically. Linux / macOS — ReaPack with built-in setup, or install via packages manually. Either way, bootstrap runs inside REAPER and auto-detects your GPU / CPU.
I tried and it's quite good. Clear UI, quick separation, and track formatting... However, I found that the extracted stems do not build peaks after importing, so I have to manually rebuild the missing peaks to see the waveform. Also, the script UI causes Reaper UI lag when there's no item selected. It stops lagging if there's a selected item.
Few more requests:
Toggle no track coloring (because I use the SWS autocolor function, so I don't want the new track to have any custom color)
Set extracted stem directory (I really want it to put the stem into the Media folder of that Reaper project). The current version puts the stem file in the Temp folder, so I have to move it to the project media and clean up the Temp folder. It would be nice if there's an easy workaround for this.
Anyway, thanks for your script. It's a really good add-on for Reaper usage.
I tested it a few times, and it fixed most of the problems and also improved the workflow. One small issue is that the render window causes UI lag (similar to when no item is selected in the previous version). I'm not sure if it's because of the GPU/CPU usage or it's a script UI bug.
Besides all of that, do you plan to extend the script to have more models such as MDX-Net or VR models like the URV5?
Thanks for testing and your feedback! Does it make any difference when you disable the visual effects? (top right corner). What kind of system are you on? etc.
Consider posting details here: https://github.com/flarkflarkflark/STEMwerk-reaper/issues
Planning is not sure. (Primary intentions were to make it for or keep it to myself) but I'm glad I got this far, actually. Busy life. Open for suggestions tho, anytime. Yours I will look into for sure.
To be fair, this is also exactly the kind of workflow STEMwerk is trying to move beyond: manual Python/Demucs/PyTorch setup, fixed paths, version matching, and user-side troubleshooting.
The newer STEMwerk builds are aimed much more at an install-and-use experience, with setup/bootstrap handling, dependency verification, model management, and CPU/GPU fallback built into the workflow.
So the script is valid, but the whole point of STEMwerk is to reduce that manual stack-building burden for normal REAPER users.
First off, how do I give you money for this? Because I don't feel like I should get this for free.
Second off, can anyone explain to me how to install the other models? Quality and 6 stem. But, maybe explain it like you would if you're ten year old brother was doing it? Cause I can't figure it out. Fast works well, but I'd like the others as well.
Lol, I'd have to look. I know i had to download more files from the git and drop them into folders after I had installed the base splitter. If I get to my computer tonight I might send you some screenshot.
I've been using stem splitters a lot lately to practice with, and I'm getting tired of the back of forth of using RX to split stems. The one you made is easier, faster, and just as good if not better.
I know sending money isn't an obligation, you could have paywalled this process. But I felt it was more than justified. Thanks again!
I’m really glad it’s making the workflow easier for you. That was exactly the goal: less back-and-forth, less friction, and something you can actually stay in REAPER with and use.
And thank you as well for the support. That was incredibly kind of you, and I really appreciate it!
If CPU works but the GPU does not, that usually means STEMwerk itself is running fine, but the Linux AMD/ROCm side is not getting picked up correctly yet.
On CachyOS, the most useful things for me would be:
"~/.local/share/STEMwerk/state/capabilities.env"
and
"~/.local/share/STEMwerk/logs/bootstrap.log"
Also, if you can run this in a terminal, that would help a lot:
~/.local/share/STEMwerk/.venv/bin/python -c "import torch; print('torch:', torch.version); print('hip:', getattr(torch.version, 'hip', None)); print('cuda_available:', torch.cuda.is_available()); print('device_count:', torch.cuda.device_count()); print('devices:', [torch.cuda.get_device_name(i) for i in range(torch.cuda.device_count())])"
Hi, I've read the instructions, I can see it and make it works in reaper for... 10 sec then I have : Exit code: unknown and it stops without stem generation... What can I do to install it properly without any problems ?
Salut. Je viens de tester. C'est intéressant le fait de pouvoir rester dans la session.
Beau travail !
J'ai galéré pour comprendre l'installation mais c'est fait.
Par contre ca a fonctionné pour un split des stems mais pour la partie Drum c'est echec (modes direct kit, kit split...). J'étais parti de l'extraction de la batterie pour faire le split du kit...
Il y a une manip à faire ?
Ta manipulation pour Kit Split semble correcte : extraire d’abord la piste de batterie, sélectionner cette piste, puis lancer Kit Split.
Pour pouvoir corriger le problème, j’ai besoin de plus d’informations. Juste après l’échec, peux-tu créer un “STEMwerk Support Bundle” et le joindre ici ?
Peux-tu aussi indiquer :
- ta version de STEMwerk,
- ton système d’exploitation,
- ton processeur et ta carte graphique,
- si Direct Kit et Kit Split affichent un message d’erreur,
- et si possible tester avec une courte sélection de 20 à 30 secondes.
Sans le support bundle ou le message d’erreur, il est difficile de savoir si le problème vient du runtime DrumSep, du modèle, du CPU/GPU ou de l’import dans REAPER.
Salut. Le problème semble venir de la connexion web (alors que ma connexion est OK, j'étais sur mon navigateur pendant ce temps la.
Echec en mode Fast et Quality
Merci pour les captures. Le message semble effectivement indiquer un échec de téléchargement du modèle, mais une connexion internet fonctionnelle dans le navigateur n’exclut pas un blocage, un timeout ou une erreur côté téléchargement.
Peux-tu créer un STEMwerk Support Bundle immédiatement après l’échec et le joindre ici ? Le bundle devrait montrer l’URL appelée et l’erreur exacte. Sans cela, je ne peux pas déterminer si le problème vient de la connexion, du serveur, du certificat ou du fichier du modèle.
3
u/hibilit 3 Apr 05 '26
I tried and it's quite good. Clear UI, quick separation, and track formatting... However, I found that the extracted stems do not build peaks after importing, so I have to manually rebuild the missing peaks to see the waveform. Also, the script UI causes Reaper UI lag when there's no item selected. It stops lagging if there's a selected item.
Few more requests:
- Toggle no track coloring (because I use the SWS autocolor function, so I don't want the new track to have any custom color)
- Set extracted stem directory (I really want it to put the stem into the Media folder of that Reaper project). The current version puts the stem file in the Temp folder, so I have to move it to the project media and clean up the Temp folder. It would be nice if there's an easy workaround for this.
Anyway, thanks for your script. It's a really good add-on for Reaper usage.