GRIM Reaper
Pricing Install

Talk to your REAPER session.

Graphically Rendered Intelligent Musician

GRIM Reaper docks a Claude-powered collaborator inside REAPER. Play a line, ask for a harmony, a fill, a key change or a master, and it writes the change into your project. Every change is one undo step.

For REAPER 7 on macOS 12 or later. You bring your own Anthropic API key.

Install on macOS See a session

It knows where you are.

Claude sees what you see in REAPER. When you say “here”, “these notes” or “what I just played”, it knows which part you mean. You don't have to name bars or tracks.

The bottom of the GRIM panel: a context line reading 13, 120 BPM 4/4, sel 9–13, Keys; a row of recently played phrases; and the message box.
The context line above the message box shows exactly what goes out with your next message.
  • Playhead and cursor“Put a drum fill right before here” becomes the end of bar 16, because the edit cursor sits on bar 17.
  • Your selectionTime selection, selected tracks and items, and the notes selected in the MIDI editor.
  • What you playedGRIM keeps listening to your MIDI keyboard, even with no track armed. Each phrase shows up as a chip. Tap one to attach it, or drop in a .mid file.
  • The whole projectTracks, items, tempo map, FX chains and plug-in parameters, read only when a request needs them.
GRIM Reaper
GRIM changing a song to the parallel minor at 128 BPM, then pointing out that the audio vocal stays in major and will clash at bar 17.
It also tells you what it didn't change. Audio can't be re-harmonized, so the vocal stays in major.

Every change is one Cmd+Z.

Each request lands in REAPER's undo history as a single named step, however many notes, tracks or plug-ins it touched. Nothing happens that you can't take back.

Undo HistoryNight Drive.rpp
You saidREAPER undo history
“Sketch a bassline under the verse”Claude: Sketch bassline under the verseBass: +28 notes, bars 1–8
“Double these notes an octave up”Claude: Double lead an octave upUndone. Too thick with the pads.
“Shift to the parallel minor and speed up to 128”Claude: Parallel minor + tempo 128Keys, Bass, Pads re-spelled; tempo 120 → 128
“Put a drum fill right before here”Claude: Drum fill into bar 17Drums: bar 16 beats 3–4, 6 notes out, 9 in
“Harmonize what I just played in thirds”Claude: Harmonize lead in thirds, bars 9–12Keys: +17 notes
“Master this for streaming, using this reference”Claude: Master chain for streamingMaster: ReaEQ, ReaXcomp, ReaLimit at −1.0 dBTP

Manual

Any change bigger than 16 notes waits for you, with a before-and-after piano roll. Apply it, reject it, or type what to do instead. You can set the threshold in Settings.

GRIM Reaper
A pending change card, Harmonize lead in thirds, bars 9–12, with Apply and Reject buttons and a before-and-after piano roll of the Keys track.
The harmony waits for Apply.

Auto

Changes land as soon as Claude makes them, so you can keep playing. Each one is still a single undo step, with an Undo button on its card.

GRIM Reaper
An applied change card, Drum fill into bar 17, showing the old and new drum notes and an Undo button.
The fill is already in. Undo is one click, or Cmd+Z.

Mastering, measured.

Ask it to master for streaming and GRIM renders your mix, measures it with ffmpeg, and compares it band by band with a reference track you attach. Then it sets up the master chain with the plug-ins you already have and measures again.

−16.8LUFS integrated
+0.4dBTP true peak
6.4LU loudness range
0.71stereo correlation
7tonal bands, sub to air
−14LUFS target, adjustable

Claude can't hear your mix. It works from these measurements, which are reliable for loudness, peaks and tonal balance. Feel and punch are still your call.

GRIM Reaper
The Analysis tab: integrated loudness −16.8 LUFS, true peak 0.4 dBTP, and a table comparing seven frequency bands of the mix with a reference track.
This mix is 2.8 LU quieter than the target, peaks over, and is 3 dB darker than the reference above 4 kHz.

Four agents, one project.

Open up to four GRIM panels, and each one runs its own agent. One can write a drum fill while another reworks the bass tone. They post what they're working on in the Town Hall tab.

Before an agent edits anything, it claims what it will touch, such as a track, a range of bars, an FX chain or the tempo map. Claims are exclusive, so two agents can't overwrite each other's work.

GRIM ReaperGRIM Reaper (2)GRIM Reaper (3)
The Town Hall tab listing three agents: Keys, harmonizing the lead in bars 9–12; Drummer, writing a fill into bar 17; and Low End, tightening the bass against the kick. Each shows its claims.
Three panels, three agents, no overlapping claims.

Install it.

  1. Build and install

    From the project folder, one script fetches the REAPER SDK, builds a universal extension and the panel UI, signs it, and copies it into REAPER's UserPlugins folder.

    scripts/install.sh

  2. Open the panel

    Start REAPER and run the action GRIM Reaper: Show/hide panel. Give it a shortcut and dock it wherever you like. Grab and Send are actions too, so you can map them to a pad or a footswitch.

  3. Add your API key

    In the panel, go to Settings, then API key, then Add. The key is stored in the macOS Keychain and never reaches the panel's web view. Requests go straight from REAPER to Anthropic.