Open-source simless ditches iOS Simulator: 5 parallel coding agents use 150 MB instead of 11 GB, checks 10x faster

Low-Future-9387 · reddit · 2026-10-11

A developer open-sourced simless (Apache-2.0, by Abacus AI), which runs the real app target headlessly on Apple Silicon so coding agents can verify SwiftUI UI changes without iOS Simulator.

Key numbers (production app, 280 Swift files, M2 Pro/32GB):

Edit → verified screen: 2.2s vs 21.9s for one agent; stays 2.2s with 5 parallel agents (vs 37.4s)
Memory for 5 agents: 0.15 GB / 6 processes vs 11.2 GB / 1,071 processes (75x less)
Peak load average on 5 cold starts: 82 vs 1,010; unit tests 5.6s vs 13.1s

How it works:

Incrementally compiles edited files and hot-patches them into the running host (2s per loop)
Returns a 150-token accessibility tree instead of screenshots, saving vision tokens
Auto-flags issues: unlabeled controls, small tap targets, overlapping/off-screen elements
--matrix checks light/dark × device sizes in one call; --png when pixels are needed

Integration: one simless skill install for Claude Code, or a 5-line AGENTS.md snippet for Codex/Cursor.

Caveats: not pixel-identical to iOS (8 of 14 demo renders within 2pt), final checks still need Simulator/device; benchmark is one run on one app; requires Apple Silicon, Xcode 26+, a dev certificate and an app-hosted test target.

Repo: https://github.com/abacusai/simless

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Open-source simless ditches iOS Simulator: 5 parallel coding agents use 150 MB instead of 11 GB, checks 10x faster

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