PreviewsMCP

A standalone SwiftUI preview host for humans and AI agents.
Run, snapshot, and interact with #Preview blocks from the command line or over MCP — no Xcode process required.

Quickstart

git clone https://github.com/obj-p/PreviewsMCP.git
cd PreviewsMCP
swift run previewsmcp examples/spm/Sources/ToDo/ToDoView.swift

A live macOS preview window opens. Edit the source file and the window hot-reloads.

Why PreviewsMCP?

PreviewsMCP compiles your #Preview closure into a dylib and loads it into a real app process (macOS NSApplication or iOS simulator UIApplication) with hot-reload — driven entirely from the command line or over MCP. No Xcode process required.

That makes it a standalone, extensible preview workflow:

• CLI and MCP-native — preview, snapshot, and iterate from the terminal or let an AI agent drive the loop
• Hot-reload — edit a file, see changes immediately, with @State preserved across literal edits
• Trait and variant sweeps — render one preview across color schemes, dynamic type sizes, locales, and layout directions in a single call
• iOS interaction — walk the accessibility tree and inject taps/swipes through an in-simulator touch bridge
• Build system flexible — works with SPM, Xcode projects (.xcodeproj / .xcworkspace), and Bazel

Solving the Xcode preview sandbox problem

Xcode previews run your code inside Apple's preview agent — a real app process, but an opaque one. You can't hook into its lifecycle, run your own initialization, or extend it. FirebaseApp.configure(), custom font registration, auth setup, and DI containers have nowhere to run. The ecosystem answer is "mock everything," and at scale teams maintain micro apps — standalone app targets that render a single feature with controlled dependencies. Airbnb's dev apps drive over 50% of local iOS builds. Point-Free's isowords has 9 preview apps. Every team pays the maintenance tax: separate targets, schemes, and mock setups that drift.

Because PreviewsMCP hosts your preview in its own app process, you can extend that process. The setup plugin provides the hook: a PreviewSetup protocol where setUp() runs once per session (SDK init, auth, font registration, DI container) and wrap() surrounds every preview render (themes, environment values). It's the micro app's dependency layer extracted into a reusable framework — without maintaining a separate app target.

Installation

Homebrew

brew tap obj-p/tap
brew install previewsmcp

From source

git clone https://github.com/obj-p/PreviewsMCP.git
cd PreviewsMCP
swift build -c release

The binary is at .build/release/previewsmcp.

Requirements

• macOS 14+
• Xcode 16+ (for iOS simulator support)
• Apple Silicon

Capabilities

• Live previews — hot-reload SwiftUI on macOS or a real iOS simulator, preserving @State where it can.
• Variant & trait sweeps — render one preview across many trait combinations (colorScheme, dynamicTypeSize, locale, layoutDirection, legibilityWeight) in a single call, with presets for light/dark, xSmall–accessibility5, rtl, ltr, and boldText.
• Multi-preview selection — #Preview macros and legacy PreviewProvider, with mid-session switching.
• iOS interaction — walk the accessibility tree and inject taps/swipes through an in-simulator touch bridge.
• Setup plugin — one-time SDK init, auth, and DI registration via setUp(), per-render theme/environment wrapping via wrap(). See the full integration guide.
• Project config — .previewsmcp.json for per-project defaults (platform, device, traits, quality, setup target).

Usage

CLI

Every CLI subcommand talks to a daemon process over a Unix socket. The daemon auto-starts on first use (ADB-style) and stays alive across invocations — no manual lifecycle management needed.

previewsmcp help                   # top-level overview
previewsmcp help <subcommand>      # full options for any command

Previewing

previewsmcp MyView.swift                           # live macOS preview window
previewsmcp MyView.swift --platform ios            # iOS simulator
previewsmcp run MyView.swift --detach              # start in background, print session ID

Snapshotting

previewsmcp snapshot MyView.swift -o preview.png   # one-shot screenshot
previewsmcp variants MyView.swift \
  --variant light --variant dark -o ./shots         # multi-trait sweep

If a session is already running for the target file, snapshot and variants reuse it (fast — no recompile) and fall back to an ephemeral session otherwise.

Inspecting and interacting (iOS)

previewsmcp elements                               # dump accessibility tree as JSON
previewsmcp touch 120 200                           # tap at (120, 200)
previewsmcp touch 40 300 --to-x 300 --to-y 300     # swipe

Session management

previewsmcp configure --color-scheme dark           # change traits on a live session
previewsmcp switch 1                                # switch to the 2nd #Preview block
previewsmcp stop                                    # close the sole running session
previewsmcp stop --all                              # close every session

Commands that target a session resolve it automatically: --session <uuid> > --file <path> > the sole running session.

Enumeration and diagnostics

previewsmcp list MyView.swift                       # enumerate #Preview blocks
previewsmcp simulators                              # list available iOS simulators
previewsmcp status                                  # daemon alive?
previewsmcp logs -f                                 # stream the daemon log (see Debugging)
previewsmcp kill-daemon                             # stop the daemon process

Structured output

Read-oriented commands support --json for scripts and agent consumption:

previewsmcp run MyView.swift --detach --json | jq .sessionID
previewsmcp simulators --json | jq '.simulators[] | select(.state == "Booted")'
previewsmcp list MyView.swift --json
previewsmcp snapshot MyView.swift -o out.png --json
previewsmcp variants MyView.swift --variant light --variant dark -o ./shots --json
previewsmcp status --json
previewsmcp elements --json

Project config

Drop a .previewsmcp.json at your project root to set defaults for every CLI command and MCP tool call (see examples/.previewsmcp.json for the canonical shape):

{
  "platform": "ios",
  "device": "iPhone 16 Pro",
  "traits": { "colorScheme": "dark", "locale": "en" }
}

Explicit CLI/MCP parameters override config values. The config is auto-discovered by walking up from the source file directory.

MCP server

Add to your agent's MCP config — same mcpServers shape whether it lands in .mcp.json (Claude Code), ~/.cursor/mcp.json (Cursor), .vscode/mcp.json (VS Code), or claude_desktop_config.json (Claude Desktop):

{
  "mcpServers": {
    "previews": {
      "command": "/path/to/previewsmcp",
      "args": ["serve"]
    }
  }
}

Once connected, ask your agent "what previews tools are available?" — it will describe them directly from the server's registered schemas, including snapshotting, variant capture, accessibility-tree inspection, and touch injection.

Daemon model

The CLI uses an auto-started background daemon that manages preview sessions. On first CLI invocation, previewsmcp serve --daemon launches in the background and listens on ~/.previewsmcp/serve.sock. Subsequent commands connect to the existing daemon — no cold start. The daemon stays alive until explicitly killed (previewsmcp kill-daemon) or the machine reboots.

• previewsmcp status — check if the daemon is running and its PID.
• previewsmcp kill-daemon — stop the daemon and clean up the socket.
• Sessions persist across CLI invocations. run --detach starts one, stop closes it, and configure / switch / snapshot / elements / touch operate on it.

Debugging

When a command appears stuck — most commonly run during an iOS host build — there are two places to look:

1. The CLI's own stderr. The daemon streams progress messages for each build phase (detecting project, compiling host app, booting simulator, …) back to the CLI as MCP log notifications, which the CLI forwards to stderr. Whichever phase was printed last is where it's stuck.

2. The daemon log. Daemon stderr is redirected to ~/.previewsmcp/serve.log on spawn, so startup failures and anything the daemon logs outside an active RPC land in this file. Stream it in a second terminal:

   previewsmcp logs -f                       # follow new lines
   previewsmcp logs -n 200                   # snapshot the last 200 lines
   # or read the file directly:
   tail -F ~/.previewsmcp/serve.log

Other levers:

• previewsmcp status --json — confirm the daemon is still alive (running / transitional / stopped) while a command is blocked.
• previewsmcp kill-daemon followed by re-running the command — gives a clean daemon and a fresh serve.log worth of context.
• PREVIEWSMCP_SOCKET_DIR=/tmp/previewsmcp-debug previewsmcp … — relocates the socket, PID, and log files so you can isolate a debug run from an existing daemon. Export the variable (or prefix both invocations) so previewsmcp logs targets the same isolated dir rather than falling back to the default ~/.previewsmcp/serve.log.
• Subprocess failures (xcodebuild, swiftc, codesign) surface their captured stderr in the error returned to the CLI; a hang, however, won't — so if the last phase logged is a build phase and the command isn't progressing, check for a live xcodebuild or swiftc process with ps -ef | grep -E 'xcodebuild|swiftc'.