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.swift platforms: 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.swift platforms: 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.