Default minimum deployment targets for Apple-platform Swift Apps and how to deviate up or down. Default is iOS 18 / macOS 15 with Xcode 16+ (first toolchain to formally support Swift 6 language mode). Invoke when starting a new Apple-platform project, writing Package.swift `platforms:` list, deciding whether to adopt Liquid Glass / latest-OS-only APIs, or when asked "what should the minimum iOS / macOS version be?".
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Default minimum deployment targets for Apple-platform Swift Apps and how to deviate up or down. Default is iOS 18 / macOS 15 with Xcode 16+ (first toolchain to formally support Swift 6 language mode). Invoke when starting a new Apple-platform project, writing Package.swift `platforms:` list, deciding whether to adopt Liquid Glass / latest-OS-only APIs, or when asked "what should the minimum iOS / macOS version be?".
Apple Platform Deployment Targets
When to invoke
Starting a new iOS / macOS App and setting the minimum deployment version.
Writing the platforms: block in Package.swift.
Deciding whether to adopt latest-OS-only APIs (Liquid Glass .glassEffect(), new Observation, new SwiftData behaviour, etc.).
User asks about minimum iOS / macOS version or whether to support the previous major version.
Default decisions
iOS 18 / macOS 15 as the default minimum.
Toolchain: Xcode 16+ (the first version with formal Swift 6 language mode support).
Do not auto-bump with each Xcode major — bumping requires an explicit decision recorded in foundations.md.
Keep Package.swiftplatforms:aligned with the App target deployment target; no skew.
Rationale
iOS 18 / macOS 15 aligns with the Swift 6 toolchain, unlocking Observation / SwiftData improvements and async sequence enhancements.
Locking out auto-bumps prevents blindly chasing each Xcode major and losing users who haven't upgraded.
For solo / small projects the user base is small, so the compatibility tax is low compared to the stability benefit.
Deviation considerations
Bump up to iOS 26 / macOS 26 (or newer)
Trigger: adopting Liquid Glass (.glassEffect() is iOS 26+) or other latest-OS-only APIs.
Conditions: the project has no backward-compat baggage (brand new App, no existing user base); or it's a personal / showcase project willing to cut off older versions.
Cost: lose users on older OS; TestFlight beta testers must upgrade.
How to record: explicitly note "deviating from apple-platform-targets default" in foundations.md with a reason.
Drop down to iOS 17- / macOS 14-
Trigger: the App needs a large pool of older-device users (education, enterprise intranet, low-end markets).
Cost: lose full Observation behaviour, SwiftData fixes, parts of Swift 6 mode checking.
Advice: explicitly record which APIs are off-limits and which behaviours need polyfills.
Verification checklist when locking targets
Package.swiftplatforms: matches every App target's IPHONEOS_DEPLOYMENT_TARGET / MACOSX_DEPLOYMENT_TARGET.
Xcode version is locked via .mise.toml or the project README.
CI (e.g. Xcode Cloud) Xcode version matches the local lock.
Related skills
swift6-concurrency: Xcode 16+ / Swift 6 mode is coupled to this skill's version choice.
xcode-cloud-single-track-ci: CI Xcode version lock.
mise-tool-management: local Xcode-select / toolchain version management.
