xcodebuild "building for iOS Simulator, but linking ... built for iOS" arch slice in CI

On Apple silicon runners the iOS Simulator is arm64, the same architecture as a real device. A framework that ships only the device arm64 slice links into a simulator build with the wrong platform, and the linker rejects it.

What this error means

The simulator build fails with "building for iOS Simulator, but linking in object file (.../SomeKit) built for iOS" or "ld: in ..., building for iOS Simulator, but linking ... built for iOS".

ld: building for 'iOS-simulator', but linking in object file
(SomeKit.framework/SomeKit) built for 'iOS'
clang: error: linker command failed with exit code 1

Common causes

A device-only arm64 framework

The binary framework has an ios-arm64 slice but no ios-arm64-simulator slice, so on Apple silicon the simulator build links a device object.

A fat framework cannot separate the two arm64 platforms

Universal fat frameworks tag arm64 once and cannot distinguish device from simulator; only XCFrameworks carry both platform variants.

How to fix it

Adopt an XCFramework with a simulator slice

1. Replace the device-only framework with an XCFramework that includes ios-arm64-simulator.
2. Clean DerivedData so the new slice is used.
3. Rebuild the simulator target.

xcodebuild -create-xcframework \
  -framework device/SomeKit.framework \
  -framework simulator/SomeKit.framework \
  -output SomeKit.xcframework

Build the simulator target under x86_64 as a stopgap

If only a device/x86_64 framework exists, run the simulator build via Rosetta or exclude arm64 for the simulator SDK.

xcodebuild -scheme App \
  -destination 'platform=iOS Simulator,name=iPhone 16' \
  EXCLUDED_ARCHS=arm64 build

How to prevent it

• Distribute binary frameworks as XCFrameworks with device and simulator slices.
• On Apple silicon runners, ensure simulator slices are arm64.
• Avoid blanket EXCLUDED_ARCHS=arm64 as a permanent fix; prefer XCFrameworks.

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