simslim

Run a lot more iOS simulators on one Mac by turning off the background daemons a simulator doesn't need.

A freshly booted iOS simulator starts around 180 background services: Siri, Spotlight indexing, photo analysis, News, wallpaper posters, iCloud sync, and so on. None of it matters when you're using the simulator for development, testing, or CI. simslim switches those services off, which cuts each simulator's memory roughly 4x. On the same laptop you go from a handful of simulators to a screenful.

https://github.com/user-attachments/assets/f4665e41-43b4-49cd-9388-3da533e9fd7b

12 slim simulators on a 16 GB M1 Pro, each driven by its own Claude Code agent through MobAI. Stock simulators start thrashing at around 5.

Numbers

One simulator, booted stock and then slimmed, same device and settle time (M1 Pro, 16 GB):

	Stock	Slim
Processes	258	70
Memory	4.0 GB	0.9 GB

Memory here is phys_footprint, the figure Activity Monitor shows, which counts compressed and swapped pages. That's what decides how many simulators fit before the machine starts swapping. Run simslim measure <udid> to see it for any booted simulator.

status and measure answer different questions. status counts only the allowlisted launchd labels SimSlim manages; it is not a live-process count. A slim boot still runs required core services plus system apps and extensions. measure walks every process under that simulator's launchd_sim and sums phys_footprint. A sum of ps RSS values is not comparable because RSS counts shared mappings once for every process that maps them.

Install

brew install mobai-app/tap/simslim

or

go install github.com/mobai-app/simslim/cmd/simslim@latest

macOS only, and you need Xcode with an iOS Simulator runtime, since simslim drives simulators through xcrun simctl.

macOS app

The SwiftUI app bundles the CLI and adds:

• Searchable simulator status, disk-size, and live RAM columns.
• Searchable service profiles with per-daemon controls and purpose summaries.
• Read-only disk analysis plus confirmed cleanup of allowlisted data.
• Clone, rename, erase, delete, and Finder shortcuts.

Build it locally with Go and Xcode:

make app
open build/SimSlim.app

Memory estimates are guidance rather than additive savings; see the measurement method. SimSlim recommends cloning before service or disk changes so the copy can serve as a backup.

Usage

simslim list             # simulators and their slim status (--booted to filter)
simslim profiles         # what a slim boot turns off
simslim profiles <id>    # the launchd labels in one category
simslim on <udid>        # slim a simulator and reboot it slim
simslim on <udid> --no-reboot   # slim the current boot session, no reboot
simslim off <udid>       # put it back to stock
simslim status <udid>    # managed launchd-label state (not a process count)
simslim verify <udid> --profile ci.json   # exact profile match; non-zero on drift
simslim doctor <udid> --requires push,storekit,universal-links
simslim measure <udid>   # a booted simulator's memory footprint
simslim top              # live fleet monitor; enter a sim for per-daemon RAM/CPU
simslim size <udid>      # total allocated simulator size
simslim disk-plan <udid> # measure reclaimable data; read-only
simslim disk-clean --categories caches,logs --confirm <udid>
simslim clone <udid> <name>
simslim repair-clone <source-udid> <clone-udid>
simslim rename <udid> <name>
simslim boot <udid>      # boot a simulator and wait for its services
simslim shutdown <udid>  # shut down a booted simulator
simslim erase <udid>     # erase apps, data, settings, and slimming overrides
simslim delete <udid>    # permanently delete a simulator

Read-only and simulator-management commands accept --json for integrations and the macOS app.

Keep a category you actually need, like Spotlight search:

simslim on <udid> --except search

Or keep one specific daemon, like push notifications:

simslim on <udid> --keep com.apple.apsd

All slimming categories

ID	Category	Turns off	~MB
widgets	Widgets & Wallpaper	Home and lock screen posters, widgets, and Live Activities.	675
siri	Siri & Intelligence	Siri, Apple Intelligence, speech, and on-device ML model services.	265
search	Spotlight & Search	On-device Spotlight and in-Settings search services.	50
icloud	iCloud & Apple Account	iCloud sync, Apple Account, keychain, and backup services.	100
store	App Store, Push & Media	App Store, push notification, StoreKit, and media services.	80
pim	Mail, Calendar & Contacts	Mail, Calendar, Contacts, Reminders, and related sync services.	80
web	Safari Sync & Web Services	Safari sync, web push, privacy, and universal-link services.	50
family	Family & Screen Time	Family Sharing, Screen Time, and usage tracking.	65
health	Health, Home & Fitness	HealthKit, HomeKit, and Fitness services.	135
photos	Photos & Media Analysis	Photos library, photo analysis, and media analysis services.	60
apps	News, Weather, Maps & Games	News, Weather, Maps, Tips, and game services.	90
messaging	Messaging & FaceTime	iMessage, FaceTime, call, and identity services.	60
connectivity	Sharing & Device Connectivity	AirDrop, Continuity, CarPlay, Watch, and Find My services.	65
telemetry	Ads, Diagnostics & Telemetry	DeviceCheck, ad privacy, analytics, diagnostics, and feedback services.	105
other	Other Background Services	Wallet, business services, assets, and miscellaneous background daemons.	195

Memory figures are iOS 26.5 clean-boot medians and are not additive; see the measurement method. Run simslim profiles <id> to see the daemons in a category.

Slow CI runners

simslim on boots the simulator, disables ~170 daemons one launchctl call at a time, then reboots — all under a single 10-minute deadline. Shared CI runners (like GitHub-hosted macOS runners) are slower and less predictable, and can blow that deadline mid-reconfigure with context deadline exceeded errors. Raise it with the global --boot-timeout flag or the SIMSLIM_BOOT_TIMEOUT environment variable:

simslim on <udid> --boot-timeout 15m
# or, for the whole job:
export SIMSLIM_BOOT_TIMEOUT=15m

Each individual launchctl transition is also bounded by its own 2-minute timeout, and the first transitions after a cold boot can exceed that on slow hosts (failed ones are retried automatically). Raise it with the global --spawn-timeout flag or the SIMSLIM_SPAWN_TIMEOUT environment variable:

simslim on <udid> --spawn-timeout 5m
# or, for the whole job:
export SIMSLIM_SPAWN_TIMEOUT=5m

Profile files

For a repeatable setup, commit a JSON profile alongside your project and apply it per run. A ci.json and a dev.json can slim differently for each purpose:

{
  "name": "ci",
  "description": "UI test runs",
  "except": ["search", "store"],
  "keep": ["com.apple.apsd"]
}

simslim on <udid> --profile ci.json

except and keep mirror the flags of the same name; name and description are for whoever reads the file. Unknown fields, unknown category IDs, and labels that no category disables are rejected, so a typo fails loudly. --profile is the single source of truth for its run and cannot be combined with --except or --keep.

To build one interactively, run simslim profile ci.json: name it, then use the arrow keys and space to tick whole features to keep enabled, or press → to open a feature and keep individual daemons within it. Point it at a directory to save <name>.json there, or omit the path to print to stdout.

Checking a simulator with doctor

Slimming a simulator turns features off on purpose, so a test suite that needs one of them wants a fast way to catch a mis-slimmed simulator before it runs. doctor checks a booted simulator against the features you name and exits non-zero if any of them are broken, which makes it a natural CI preflight:

simslim doctor <udid> --requires push,storekit,universal-links

<udid>: 2/3 required features OK
  ok     push
  ok     universal-links
  BROKEN storekit — com.apple.storekitd disabled

Feature IDs are finer-grained than the slimming categories: each maps to just the daemons that back one capability. Run simslim doctor --list to see them all. Both the check and the list support --json.

Verifying a profile still holds

The disable overrides are per-simulator state that is easy to lose without noticing: a deleted-and-recreated device or a simulator from a new runtime comes up stock, and nothing else fails loudly when that happens — the simulator just quietly runs heavy again. verify compares a booted simulator's overrides against a profile — the same --profile/--except/--keep you passed to on — and exits non-zero listing the drift, so a script or CI step can catch it and re-run simslim on (idempotent: it only applies the missing delta) to repair:

simslim verify <udid> --profile ci.json || simslim on <udid> --profile ci.json

Where doctor answers "do the features my tests need still work?", verify answers "is this simulator in exactly the slim state I configured?". Supports --json.

Slimming without a reboot

simslim on --no-reboot slims the running boot session in place: it disables each daemon and then boots it out of launchd, so the process stops now and cannot respawn, with no shutdown/boot cycle. It takes the same --profile/--except/--keep selection as on.

simslim on <udid> --no-reboot --profile ci.json

On iOS 18.5 and newer this is simply the faster path; the disable overrides are stored as well, so the next boot comes up slim too. On iOS 17.x and 18.3, which cannot persist overrides, it is the only way to slim at all, and the state is gone at the next reboot: re-run it after every boot. status says so when it reads such a simulator, and status --json carries a persistent field.

Live slimming only moves toward more-disabled. Managed daemons already disabled beyond the profile are left alone, because starting a daemon again live would need a per-label bootstrap; simslim off restores them with a reboot.

Slimming parallel-testing clones

xcodebuild parallel testing creates a fresh clone per worker in its own device set, runs the tests, and deletes the clones when the run ends. The clones are created stock, and disable overrides do not survive simctl clone, so there is no device to slim ahead of time. simslim watch closes that gap: it polls the device sets and slims each simulator no-reboot the moment it boots, then leaves it alone until it boots again. A clone slimmed a few seconds into a multi-minute run keeps its freed memory for the rest of the run. A failed slim is retried on the next scan, and a device that shut down and booted again is slimmed again, because below iOS 18.5 a no-reboot slim ends with the boot session.

simslim watch --except web,store &
xcodebuild test -scheme App -parallel-testing-enabled YES -parallel-testing-worker-count 4 ...

It takes the same --profile/--except/--keep selection as on, and scans the default and testing sets plus any passed with --set. Point --set at a custom parallel-testing device set if your Xcode uses one. It runs until Ctrl-C.

Disk cleanup

Disk cleanup is permanent and separate from service slimming. disk-plan is read-only. disk-clean shuts down the exact simulator, clears only allowlisted per-device directories, and refuses to run without --confirm.

simslim disk-categories
simslim disk-plan <udid>
simslim disk-clean --categories caches,logs,temporary --confirm <udid>
# Optional: also remove on-demand language models
simslim disk-clean --categories linguistic-data --confirm <udid>

Built-in apps and core OS language resources are part of a signed iOS runtime shared by every simulator using that version, so simslim never modifies them. Required Siri assets are measured only because iOS restores them on launch; on-demand language data is opt-in and may download again when needed.

disk-plan also reports a read-only storage breakdown for installed app bundles, Documents, app data, and user media. Those durable rows are never eligible for cleanup. See the disk cleanup safety model for recovery behavior, safeguards, and Xcode 26.6 validation results.

Use as a Go library

The repo root is an importable package with no external dependencies, so you can drive simulators from your own tooling instead of shelling out to the CLI:

go get github.com/mobai-app/simslim

package main

import (
	"context"
	"fmt"

	"github.com/mobai-app/simslim"
)

func main() {
	ctx := context.Background()

	devices, err := simslim.ListDevices(ctx)
	if err != nil {
		panic(err)
	}

	// Slim every booted simulator, keeping Siri and search enabled.
	profile, err := simslim.BuildProfile("", "siri,search", "")
	if err != nil {
		panic(err)
	}
	for _, d := range devices {
		if d.State != "Booted" {
			continue
		}
		changed, err := simslim.EnableSlim(ctx, d.Set, d.UDID, profile, func(msg string) {
			fmt.Println(d.UDID, msg)
		})
		fmt.Println(d.UDID, "changed:", changed, "err:", err)
	}
}

The package never writes to stdout — it returns values and reports progress through the simslim.Reporter callback you pass in. simslim.Categories, simslim.Features, and simslim.SlimmableSet() expose the same allowlist the CLI uses. macOS only, since everything runs through xcrun simctl.

How it works

simslim on writes persistent launchctl disable entries for the chosen launchd labels into the simulator's own launchd database, then reboots it. The entries stick across reboots, so the simulator comes up slim in a single boot from then on. simslim off clears them and reboots back to stock. Your Mac is never touched, only the simulator you point it at, and only services that are safe to disable. Core workflow services such as sharingd, plus the handful that wedge a simulator when turned off, are left running.

Applying a profile normally costs no launchctl call at all: the overrides are written straight to the store launchd_sim reads when it starts, so the simulator simply boots slim the first time. On an iPhone 17 Pro / iOS 26.5 that turns a 2m14s first slim into 33s, nearly all of it the boot itself. A booted simulator takes the same route — an override only takes effect at the next boot, so the shutdown it already owed is spent up front, and slimming one measures 17s end to end. A profile that already matches costs a single read and no reboot at all. The booted simulator is still read back afterwards, and if a runtime ignores the store simslim falls back to running launchctl per daemon rather than report a slim simulator that isn't one. That fallback runs eight transitions at a time: 2m14s → 38s for 170 labels.

The earliest runtime with verified persistence is iOS 18.5. iOS 17.x and 18.3 accept each launchctl disable, but the simulator comes back stock after a reboot. simslim on rejects runtimes older than 18.5 before booting or changing launchd state and points at --no-reboot, which slims the current boot session only (see Slimming without a reboot). Supported runtimes are still read back after the reboot, and the command fails instead of claiming success if any requested override was lost.

This is per-simulator state, not a global setting. The daemon disables live in that one simulator's launchd database and nowhere else. simslim clone copies the source's exact SimSlim-managed profile along with its installed apps, app data, and settings. It also rebases simulator-local symlinks, regenerates app registration databases, and clears copied system caches and temporary files. Before finishing, it boots the clone and audits its running processes for open paths into the source. The clone finishes shutdown, and the source returns to the boot state it had before cloning.

Clones created by an older SimSlim build can be repaired in place without deleting their installed apps or app data:

simslim repair-clone <source-udid> <clone-udid>

The repair preserves the clone's current SimSlim profile and boot state. The source is kept shutdown while copied paths are rebuilt; if direct links or open files into that source remain, the clone stays shutdown instead of being restarted. CoreSimulator does not expose clone lineage, so pass the exact source UDID: SimSlim verifies references to the source you specify.

erase, delete and recreate, and "Erase All Content and Settings" still produce stock service state, so the simulator's memory climbs back until you rerun simslim on. A simulator created from a new or updated runtime also starts stock. The slim state persists across simulator shutdowns and reboots on the same Mac. Moving or restoring a simulator onto another Mac does not preserve the profile. Run simslim list to see current state, or simslim verify to check one against a specific profile.

What you lose

Turning services off is fine for most development, UI automation, and CI, but some features genuinely stop working. The ones worth knowing:

• Spotlight and in-Settings search return nothing (search).
• Push notifications need apsd, StoreKit testing needs storekitd (store).
• Universal links need swcd (web).
• The Contacts, Photos, and Calendar pickers can act up without their categories.

simslim profiles lists every category, so you can keep a category with --except or individual daemons with --keep.

Why

Testing is shifting. Once agents are writing apps, you want agents running them too, and the place an iOS app runs is a simulator. One agent, one simulator. So how much work you get through at once comes down to how many simulators a machine can hold, and stock simulators are heavy enough that a laptop fills up fast. Slimming them is the cheapest way to raise that ceiling: more simulators on the box means more agents working in parallel on it.

Built for MobAI to run more simulators on one machine.

License

MIT, copyright Interlap.