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open500

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Native add-ons for the Akai MPC500 (OS 1.31): in-browser patcher, HUB, Auto Chop

open500 is custom firmware for the Akai Professional MPC500, by @M45K. Open source under GPL-3.0, 14 stars on GitHub.

forAkai Professional MPC500·by@M45K

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Open500

Support Open500

New native pages for the Akai MPC500, loaded from a WAV file. No hardware mod, fully reversible.

Video: Open500 0.2 · first video

Website & installer: docs/index.html (GitHub Pages) — drop the official OS 1.31 file in the page and it adds the Open500 loader in your browser. Nothing is uploaded, and this repository contains no Akai code.

What's new in 0.2

  • No reflash: just copy the new WAVs (delete CHOPAGE.WAV, TRANSCHP.WAV, AUTOWARP.WAV: they are now in Chop Shop).
  • Up to 16 tools. Startup splash.
  • Chop Shop: BPM, Transient, Tap, Warp. Tap: chop live with the pads.
  • Grid Seq: add steps with the pads, playhead.
  • Pad Copy to many pads.
  • New: Scale Pads, Pad Mute/Solo, Seq Tools (double / half).
  • USB MIDI out (beta). Screen in your browser (beta).
  • CANCEL goes back one step.
  • Tool sources and a Claude skill for making tools.

Add-ons

Add-onFile
HUBHUB.WAVadd-on manager, load it first (up to 16 add-ons at the same time)
Chop ShopCHOPSHOP.WAVBPM (equal slices), Transient (slices on the hits), Tap (hit the pads while the sample plays), Warp (loop to the sequence tempo); pad preview, mono + one mute group
Master FXMASTERFX.WAV12 live SP-404-style effects on the whole mix, one per pad (held = on), slider = main parameter, tempo-synced
Grid SeqGRID.WAV16-step grid sequencer on the display, edits the current sequence while it plays; pads add/remove hits, playhead
Sample FXSAMPLEFX.WAVchain of up to 8 effects (fades, gain, bit crush, downsample, drive, vinyl crackle, silence trim) printed into a new sample; pad preview
Pad ToolsPADTOOLS.WAVswap / copy (to many pads) / clear pads (sample, whole pad, or pad + MIDI note); SHIFT+STOP
Scale PadsSCALEPAD.WAVone sound on the 12 pads of a bank, tuned to a scale and key
Pad MutePADMUTE.WAVmute / solo single pads; SHIFT+TRACK MUTE
Seq ToolsSEQTOOLS.WAVdouble or halve the current sequence
Input ThruINTHRU.WAVline input always open to the outputs (analog pass-through) with level
USB MIDI (beta)USBMIDI.WAVthe USB port as a class-compliant MIDI interface: pads and sequences to your computer
MIDI Screen (beta)MIDISCR.WAVdisplay, waveform, slices, pads and mixer in the browser (docs/screen.html, Chrome), over USB or MIDI

How it works

  • The loader (1.3 KB) is written into an unused, all-zero area of the MPC500 OS 1.31 and hooked into the OS main loop. It reserves the last 896 KB of sample memory: 384 KB for add-ons, 512 KB of effect buffers (Master FX).
  • Add-ons are WAV files with a signed, relocatable payload. After a LOAD, the HUB finds them in the sample list, installs them and adds their entry to MODE → OTHER. Their pages are real objects of the OS page framework.
  • The bootloader is never touched: flashing the original OS removes everything.

Make your own tool with Claude Code

Clone the repository, create your patched MPC500.bin with the website installer and put it in the repository folder, then open the folder in Claude Code. The open500-addon skill loads by itself: describe the tool you want ("a page that tunes the 12 pads to a chord"), and Claude designs the page, writes the add-on with the SDK, tests it in the emulator and gives you the WAV plus a checklist for the MPC. To use the skill in other folders, copy .claude/skills/open500-addon to ~/.claude/skills/. Without Claude: start from sdk/README.md.

Share your tool (beta)

Open an issue with the Submit a tool form and paste your .py (or open a pull request, see community/README.md). When the maintainer approves it, GitHub builds the WAV from your source, checks it and opens a pull request; once merged it appears in Community tools on the website.

Repository layout

  • docs/ — website, browser patcher (patcher.js), patch data (patch.js), add-on WAVs (docs/addons/), screen page (screen.html, o5link.js) and its protocol notes (PROTOCOL.md)
  • sdk/ — add-on SDK (Python → SH-3 code, emulator, testkit) and the source of every official add-on (sdk/addons/): start from sdk/README.md
  • .claude/skills/open500-addon/ — skill for Claude Code: helps you build and test a tool with the SDK
  • community/ — tools shared by users (source + tool.json); tools/community.py builds and checks them
  • tools/make_patch.py — builds docs/patch.js from the tested firmware and checks the base against the Akai zip
  • tools/test_patcher.js — node tools/test_patcher.js: original OS 1.31 + patch must equal the hardware-tested firmware byte for byte, and wrong files must be refused

License

Open500 is free software under the GNU GPL v3 (LICENSE): you can use, change and share it, and what you share must stay under the same license. Copyright (C) 2026 M45K and contributors. The Akai firmware is not part of this repository.

Unofficial project, not affiliated with Akai Professional or inMusic. Tested on one MPC500 with OS 1.31. Use at your own risk.