JSFX: a bundle of Reaper plugins and a Reapack catalogue for free synths

A plugin catalogue, not a single application

The repository description is one line: a bundle of JSFX and scripts for Reaper. The repository layout makes the scale obvious, with around thirty directories at the top level, each holding one plugin or a group of related ones, such as Yutani, Protosynth, FMFilter, partials, Ravager, Reflectosaurus, Ripple, Abyss, Amaranth, Nuker, Poprocks, SatanVerb, Swellotron, and a Basics directory, plus a bric-a-brac folder and a SpectrumAnalyzer. There is also an index.xml at the root, which is the file that makes the whole thing installable, a regen_compat.py script, and separate folders for the saike MIDI arpeggiator and the saikedrums sample instruments. The audience is Reaper users, and specifically the kind who enjoy the free plugin ecosystem that Cockos JSFX represents, where plugins are small text files written in a specific language rather than compiled binaries. The licence is MIT, and the author asks for a mention if you reuse code in your own work rather than requiring attribution.

Installation runs through Reapack, one index file

The install path has five steps, and the second one is the whole mechanism. The README says the easiest way to use these is through Reapack, the package manager for Reaper, and points you at its own installation instructions first. Then you add this repository to Reapack using the raw index.xml URL on the master branch, which is the catalogue Reapack reads to know what plugins exist and where to fetch them. From there the README walks through the ordinary package manager flow: browse what is on offer, install the ones you want, then refresh your FX list or restart Reaper, because the plugins do not appear until you do. The note that everything is free and that the author has a repository of his own with plugins worth trying are the two things worth remembering. The index file is regenerated by a script in the repository, which is why it lives alongside the plugin folders rather than being a hand written list.

Yutani, a paraphonic bass synth built on non-linear filters

Yutani is the flagship instrument in the bundle, a monophonic or four voice paraphonic bass synthesizer. Its oscillator section is anti-aliased and covers saw, square, triangle, fin, PWM, comb saw, comb square, supersaw and glottis waveforms. The filter bank is the headline feature: 26 multi-mode filters, of which 21 are non-linear analog modelled ones, each with its own character, and the filter can be driven by audio rate modulation. On top of that sit velocity, modulation wheel and LFO modulation options, a stereo widening effect, a feedback allpass smearing effect, a noise source, a distortion module, glide, and support for mod wheel, MIDI velocity and pitch bend. Wave table support is there too, with wave files dragged and dropped into the oscillator, and for waves shorter than 2048 samples you can drag two waves in sequentially and Yutani interpolates between them. One practical note in the README: you have to import the bass presets when you download the thing.

Protosynth and FM Filter 2 share a filter design

Protosynth is the polyphonic instrument in the collection, with 8 oscillators across 4 anti-aliased types, namely Harmonic DSF, SuperSaw, Formant DSF and Noise. Its distinctive feature is the set of signal mix nodes: additive, amplitude modulation, ring modulation, XOR, XOR-diff, STFT convolution, an audio rate frequency shifter, and 3 flavors of phase modulation with feedback. It also carries two nonlinear filter modules ranging from clean to dirty, a stereo widening effect, a basic reverb module, pitch noise, glide, and mod wheel, MIDI velocity and pitch bend support. FM Filter 2 is the companion that borrows the same filter bank for processing an existing audio source rather than generating one. It offers the same 26 multi-mode filters, described as ranging from well behaved to gnarly analog modelled, with the cutoff and the gate both controllable by audio, using peak or RMS detection, or by MIDI. It adds three LFOs, mod wheel and MIDI velocity support, stereo widening and a distortion module.

Partials does modal synthesis in two different domains

Partials is a modal resonator effect with 5 note polyphony, and the README is refreshingly clear that it can be played almost like an instrument or used as a reverb-like transformation. It is described as good for string-like and marimba-like sounds and as pairing well with a dash of reverb. Pitch control works two ways. In non-MIDI mode, the plugin takes audio input and uses its internal resonators to make it tonal, and a small keyboard at the bottom lets you set up to 5 fundamental pitches, each of which drives a large number of resonators based on that fundamental, with clicks toggling notes on and off. In MIDI mode, the fundamental resonator frequencies are set by the notes you play. The list of features includes 10 physical models to choose from, custom partial models you can draw yourself, a frequency domain mode described as efficient, and a time domain mode described as inefficient but able to allow feedback, with the warning that note instability can occur. There is also a round robin sample mode where you drag impulse samples onto one of the four pads, and the decay parameters can be set to respond to note release.

Running the plugins outside Reaper takes a forked host

The README spends a section on other DAWs, and the picture is a little rough. It points first at ysfx, described as a great project that provides a custom implementation of JSFX, and says the author has had good experiences with the VST3 it provides as a JSFX host for the plugins in this repository. The caveat follows: the original author is no longer active and the latest plugins do not work with it. The author's own fork, at JoepVanlier/ysfx, exists to update the host so the newer plugins run, with releases published separately. On Windows, the alternative named is ReaJS, the official Cockos implementation, which the README notes has not been updated in a while. Some plugins simply work in ReaJS, while others need their ReaJS compatibility version, published separately and named with a _compat suffix. Those compatibility builds also require the regular plugin to be installed, because they share the same dependencies. So the honest summary is that the plugins are portable in principle, but the porting work is uneven and lives in side projects rather than in the main repository.

Documentation is in the tooltips, and releases lag the commits

The Tips section is one line: many of the plugins have tooltips, and you can hover anywhere for more information. That is the whole documentation strategy, which is a genuine constraint for anyone evaluating a plugin before installing it, since you often have to install it to find out what the parameters do. There is a Reflectosaurus_Manual directory, and a Basics folder, so some explanation exists, but there is no unified manual. On the release side, the tagged versions are v0.2.0 from 2024-09-28, v0.1.0 from 2023-12-17, and v0.0.1 from 2023-05-21, while the last push to the repository was on 2026-10-17. That gap is the most important operational fact here. Anyone installing through Reapack may get a snapshot of the master branch that is far newer than any tag, which for a bundle of plugins means you are testing unreleased code. The project topics also list jesusonic, which points at the lineage of the Cockos JSFX language these plugins are written in.

Editorial conclusion

Adopt JSFX if you work in Reaper and want a catalogue of free JSFX synths and effects that Reapack can install and update, and if you are willing to read the tooltips the author relies on for documentation. Do not adopt it if you need a stable commercial plugin set, since the last tagged release was v0.2.0 on 2024-09-28 while the repository was pushed to on 2026-10-17, and the running docs are the plugin tooltips rather than a manual. Verify first that your Reaper or JSFX host runs the plugins you actually want, since the author notes the original ysfx project is no longer active and that some plugins need separate compatibility builds.

Frequently asked questions

Official sources

1. Issues
2. JoepVanlier/JSFX on GitHub
3. License: MIT
4. README
5. Releases