# Reaper Jsfx Midi

> MIDI processing patterns for JSFX plugins. Use when building MIDI utilities, arpeggiators, chord generators, note filters, or CC processors. Triggers on requests for MIDI effects, arpeggiator, chord, transpose, or MIDI routing in JSFX.

- Skill: `mthines/reaper-jsfx-midi` (Agent Skill, multi-file: 6 files)
- Install (CLI): `npx skillmds@latest add mthines/reaper-jsfx-midi`
- Raw SKILL.md: https://api.skillmd.com/api/skills/mthines/reaper-jsfx-midi/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: MIT
- Author: mthines (https://skillmd.com/u/mthines)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/mthines/reaper-jsfx-midi

---


# REAPER JSFX MIDI Processing

Patterns for MIDI utility plugins including note processing, arpeggiators, chord generators, and CC manipulation.

**Requires:** [reaper-jsfx-core](../reaper-jsfx-core/SKILL.md) for language fundamentals.

## Rules

| Rule | Description |
|------|-------------|
| [midi-basics](./rules/midi-basics.md) | MIDI message structure, receive/send, SysEx handling |
| [midi-transform](./rules/midi-transform.md) | Note transposition, velocity scaling, CC mapping |
| [midi-generate](./rules/midi-generate.md) | Arpeggiators, sequencers, chord generators |
| [midi-routing](./rules/midi-routing.md) | Multi-bus routing, channel filtering, splitting |

## Official Documentation

- [JSFX MIDI Functions](https://www.reaper.fm/sdk/js/midi.php)

## Key Principles

### 1. MIDI-Only Plugin Structure

```
desc:MIDI Utility
tags:midi

in_pin:none
out_pin:none

@block
while(midirecv(offset, msg1, msg2, msg3)) (
  // Process and forward
  midisend(offset, msg1, msg2, msg3);
);
```

### 2. Always Forward or Handle All Messages

```
@block
while(midirecv(offset, msg1, msg2, msg3)) (
  status = msg1 & $xF0;

  status == $x90 || status == $x80 ? (
    // Process notes
    process_note(offset, msg1, msg2, msg3);
  ) : (
    // Pass through everything else unchanged
    midisend(offset, msg1, msg2, msg3);
  );
);
```

### 3. Preserve Timing

```
@block
// Use original offset for timing-critical events
while(midirecv(offset, msg1, msg2, msg3)) (
  // Transform but keep original offset
  midisend(offset, new_msg1, new_msg2, new_msg3);
);
```

## Quick Reference

| Use Case | Rule |
|----------|------|
| Understanding MIDI | [midi-basics](./rules/midi-basics.md) |
| Transpose / Velocity | [midi-transform](./rules/midi-transform.md) |
| Arpeggiator | [midi-generate](./rules/midi-generate.md) |
| Channel Filter | [midi-routing](./rules/midi-routing.md) |