Architecture

Claude Code’s MCP server subprocesses run inside a sandbox that blocks direct TCP connections to localhost. So a direct reapy connection from the MCP server isn’t possible. The solution is a bridge process that runs outside the sandbox:

Claude Code (MCP client)
    │
    ▼ file IPC via /tmp/reaper_rpc/
bridge.py  (runs in terminal, outside sandbox)
    │
    ▼ TCP localhost:2306
REAPER + reapy server

The MCP server writes tool call payloads to files in /tmp/reaper_rpc/. The bridge watches that directory, executes the calls via reapy, and writes responses back.

Setup

1. Start REAPER. The reapy server starts automatically on port 2306 once you’ve installed the reapy REAPER extension.

2. Start the bridge (keep this terminal open):

cd ~/claude/reaper-mcp
uv run python3 scripts/start_bridge.py

3. Reconnect MCP in Claude Code: /mcp

What it can do

With 58 tools exposed, you can do things like:

• Create, rename, delete, color tracks
• Set volume, pan, mute, solo
• Add FX and set parameters
• Create MIDI items and drum patterns
• Analyze loudness, dynamics, frequency spectrum
• Render stems or the full project
• Set tempo and time signature

Falling back to reapy directly

Some things aren’t exposed by the MCP tools (e.g. recording input assignment). For those, I drop down to reapy directly:

import reapy
reapy.connect()
p = reapy.Project()
t = p.tracks[0]
t.set_info_value('I_RECINPUT', 1034)  # ADAT 1/2

Track color encoding

One gotcha: set_info_value('I_CUSTOMCOLOR', ...) uses the wrong byte order on macOS if you encode manually. Use reapy’s native track.color = (r, g, b) property instead, which handles the platform-specific encoding. Then call reapy.update_arrange() to force a redraw.

Current session setup

9 tracks, each colored by instrument family:

Prerequisites

• REAPER is open
• reapy server is running inside REAPER (port 2306 — starts automatically if you have the reapy REAPER extension installed)
• Python environment: ~/claude/reaper-mcp with uv

Run all snippets from that directory:

cd ~/claude/reaper-mcp
uv run python3 -c "..."

Check track order first

Colors are applied by index (0-based). Always verify the order before running the color script — tracks may have been reordered since the session was created:

import reapy
reapy.connect()
p = reapy.Project()
for i, t in enumerate(p.tracks):
    print(i, t.name.strip())

Confirm the output matches the order you expect before proceeding.

Before and after

Without colors, all mixer channels look identical. At a glance you can’t tell which strip is which, especially once you’re deep in a session.

Before:

After:

Each track now has a color band at the top of its mixer strip and a colored number tab at the bottom. The TCP (track list on the left) gets the same color as a full background fill.

The sync problem

REAPER has two views that both display track colors:

• TCP — the track control panel (left side of the arrange view)
• MCP — the mixer control panel

They’re driven by the same underlying data (I_CUSTOMCOLOR) but don’t always redraw together when colors are set via the API. Calling update_arrange() only refreshes the arrange view. To get both in sync you need two extra calls:

rpr.TrackList_AdjustWindows(True)   # redraws TCP
rpr.ThemeLayout_RefreshAll()         # redraws MCP / mixer

Without ThemeLayout_RefreshAll(), the mixer strips stay grey even though the color is stored correctly.

The wrong encoding

The first attempt used manual bit-packing:

# WRONG on macOS — swaps R and B channels
t.set_info_value('I_CUSTOMCOLOR', r | (g << 8) | (b << 16) | 0x1000000)

On macOS, I_CUSTOMCOLOR expects (r << 16) | (g << 8) | b | 0x1000000. Getting this wrong produces wrong hues — teal becomes yellow, pink becomes purple.

The correct approach

Use SetTrackColor with ColorToNative to handle the platform encoding, then force both panels to redraw:

import reapy

reapy.connect()
p  = reapy.Project()
rpr = reapy.reascript_api

colors = [
    ( 76, 175,  80),  # Guitar      — green
    (255,  87,  34),  # Force       — deep orange
    (255, 152,   0),  # MPC         — amber
    ( 33, 150, 243),  # FL Studio   — blue
    (  0, 188, 212),  # DDJ 400     — teal
    (  0,   0,   0),  # Elektron    — black
    (233,  30,  99),  # SP404       — pink
    ( 80, 100, 230),  # Hydrasynth  — indigo
    (150, 150, 150),  # Sync        — grey
]

for idx, (r, g, b) in enumerate(colors):
    rpr.SetTrackColor(p.tracks[idx].id, rpr.ColorToNative(r, g, b))

rpr.TrackList_AdjustWindows(True)
rpr.ThemeLayout_RefreshAll()

ColorToNative(r, g, b) does the platform-specific conversion. SetTrackColor sets both the custom color and enables the custom color flag in one call — no need to manually OR in 0x1000000.

Color grouping logic

Colors are grouped by instrument family so related tracks cluster visually:

Samplers stay warm (orange → amber → pink), synths stay cool (indigo, black), and the utility track is neutral so it never draws the eye accidentally.