How to solve Xcode error “The i386 architecture is deprecated.”

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# How to solve Xcode error "The i386 architecture is deprecated."

## Summary
- Deprecated i386 architecture prevented macOS app release builds
- Project settings contained legacy 32-bit Intel (i386) architecture references
- Debug builds succeeded due to "Build Active Architectures Only" masking the issue
- Resolution required manual removal of i386 from build settings

## Root Cause
- Legacy project configured with explicit `ARCHS = i386 x86_64` override
- `$(ARCHS_STANDARD)` no longer includes i386 in modern Xcode versions
- Project.pbxproj file contained frozen ARCHS settings from its creation era
- Release builds build all architectures (`Build Active Architecture Only = NO`)

## Why This Happens in Real Systems
- Projects migrated across Xcode versions retain outdated build settings
- Old .xcodeproj files persist architecture settings indefinitely
- "Build Active Architecture Only = YES" delays discovery during debugging
- Cross-platform compatibility concerns led to historical i386 inclusion
- Documentation gaps between evolving ARCHS_STANDARD definitions

## Real-World Impact
- Blocks App Store submission due to build failures
- Delays modernization of business-critical legacy applications
- Creates developer frustration with opaque build system errors
- Prevents ARM64 release builds from being generated
- Risks prolonged use of deprecated debug-only builds in production

## Example or Code
In project.pbxproj (located in `YourProject.xcodeproj`):

```ruby
/* Problematic setting */
ARCHS = i386; /* Remove this line */
VALID_ARCHS = "i386 x86_64"; /* Change to x86_64 */

In Xcode Build Settings:

// Before (Explicitly defined)
ARCHS = i386 x86_64

// After (Modern configuration)
ARCHS = x86_64 arm64

How Senior Engineers Fix It

  1. Audit build settings via terminal using: 
    xcodebuild -project YourProject.xcodeproj -showBuildSettings | grep ARCH
  2. Remove explicit i386 references:
    • Xcode UI: Build Settings → Architectures → Delete i386
    • Text editor: Remove from project.pbxproj (ARCHS = i386)
  3. Verify architecture support matrix:
    • Keep arm64 (Apple Silicon) + x86_64 (Intel 64-bit)
  4. Update VALID_ARCHS to x86_64 arm64
  5. Set “Build Active Architecture Only” to NO universally during final release
  6. Confirm using lipo -archs on output binary: 
    lipo -archs AppName.app/Contents/MacOS/AppName

Why Juniors Miss It

  • Over-reliance on Xcode UI without understanding .pbxproj internals
  • Misinterpreting $(ARCHS_STANDARD) as evergreen without validation
  • Not auditing build settings through CLI tools
  • Focusing only on scheme configurations
  • Assuming debug settings apply universally
  • Unfamiliarity with macOS architecture evolution