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MobAI-App/simslim

A macOS CLI and app that turns off background daemons in iOS, tvOS, watchOS, and visionOS simulators so more of them fit in memory on one Mac.

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What is it?

simslim targets the memory cost of Apple simulators. A freshly booted iOS simulator starts roughly 180 background services such as Siri, Spotlight indexing, photo analysis, and iCloud sync. simslim disables the services that development, testing, and CI workflows typically don't need, and it does this per simulator, leaving the host Mac untouched.

The tool drives simulators through xcrun simctl and offers a CLI with commands for slimming, restoring, verifying, and measuring a simulator, plus a SwiftUI macOS app that wraps the same functions. Services are grouped into categories such as Siri, iCloud, Health, and Messaging, and individual daemons can be kept on with flags or profile files. It also includes doctor and verify commands for checking that required features still work, and a watch mode that slims parallel-testing clones as they boot.

Some features stop working when services are off, including Spotlight search, push notifications, StoreKit testing, and universal links, so the category choices matter. A Go package is also available for building custom tooling on top of the same functionality.

Key features

  • 01Turns off roughly 170 background daemons per simulator, grouped into selectable categories.
  • 02Supports iOS, tvOS, watchOS, and visionOS simulators with the same commands.
  • 03Lets you keep specific categories or individual daemons on with flags or JSON profile files.
  • 04Provides doctor and verify commands to check required features and detect drifted settings.
  • 05Includes a watch mode that slims parallel-testing clones as they boot.
  • 06Offers a read-only disk analysis and confirmed cleanup of allowlisted simulator data.
  • 07Ships a SwiftUI macOS app alongside the CLI.

Use cases

  • Running many iOS simulators in parallel for AI agents or UI automation on a single Mac.
  • Speeding up parallel xcodebuild test runs in CI by slimming clones as they boot.
  • Creating repeatable simulator setups per team with committed profile files.
  • Reducing memory pressure on a development laptop that runs several simulators.

Quick start

  1. 1

    Install simslim with Homebrew

    brew install mobai-app/tap/simslim
  2. 2

    Make sure Xcode with an iOS, tvOS, watchOS, or visionOS Simulator runtime is installed

  3. 3

    Slim a simulator by its UDID

    simslim on <udid>

Things to know

  • macOS only, and it requires Xcode with simulator runtimes.
  • Some runtimes, such as iOS 17.x and 18.3, cannot persist slimming, so it must be re-applied after every reboot.
  • Savings on Apple TV, Apple Watch, and Apple Vision Pro are unverified, since the memory figures were measured on iOS.

In collections

Open-source Mac appsTesting frameworks & test automation tools

Topics

#ios#simulator#xcode#macos#testing#ci#memory-optimization#apple-platforms

FAQ

What is simslim?
A macOS CLI and app that turns off background daemons in iOS, tvOS, watchOS, and visionOS simulators so more of them fit in memory on one Mac. simslim targets the memory cost of Apple simulators. A freshly booted iOS simulator starts roughly 180 background services such as Siri, Spotlight indexing, photo analysis, and iCloud sync. simslim disables the services that development, testing, and CI workflows typically don't need, and it does this per simulator, leaving the host Mac untouched.
Is simslim free to use? What license does it have?
simslim is open source under the MIT license. Check the license terms before using it in commercial products.
Is simslim still maintained?
Actively maintained. The last commit to simslim was on Nov 4, 2026.
What can I use simslim for?
Running many iOS simulators in parallel for AI agents or UI automation on a single Mac. Speeding up parallel xcodebuild test runs in CI by slimming clones as they boot. Creating repeatable simulator setups per team with committed profile files. Reducing memory pressure on a development laptop that runs several simulators.
How do I install simslim?
Install simslim with Homebrew: brew install mobai-app/tap/simslim Make sure Xcode with an iOS, tvOS, watchOS, or visionOS Simulator runtime is installed Slim a simulator by its UDID: simslim on <udid>
Where can I get simslim?
The source code and installation instructions are on GitHub: https://github.com/MobAI-App/simslim

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