I am new to Xcode, so my apologies if there's a simple answer I've missed. I converted an existing project, built using Swift and Objective C from Swift 3 to Swift 5 under Xcode 11.6 . When I run under the debugger, I am seeing a series of assert fails: Cannot be called with asCopy = NO on non-main thread. I understand that this means that UI calls are being made from background threads, but I don't know where the asserts are coming from. Everything I can find online implies that Xcode should stop on an assert fail, but it's not doing so. I am building for Run with Build Configuration Debug. Debug executable is checked. I have created an Exception breakpoint which shows as All Objective-C Exceptions. I have turned on Main Thread Checker. Yet, I see the asserts logged, but Xcode doesn't stop. Is there a way to make Xcode break on these assert fails? If not, what is the best method to find where they are happening. I had thought that Main Thread Checker should have tripped on them as well. Thanks in advance