TL;DR

Error asyncLet_finish_after_task_completion is due to a regression in the Swift 6.1 compiler (swiftlang/swift#81771), which ships with Xcode 16.3 and later. Any execution of async let inside a try/catch block will fail.

For me, the workaround was pinning FirebaseFunctions to v12.4.0 (pre-Swift 6.1).

In ios/Podfile:

pod 'FirebaseFunctions', '~> 12.4.0'

As of the writing of this post, the issue #81771 is still open on GitHub.

After a recent update for macOS and Xcode, I was working on a Flutter app that had been running without issue. But today, it was crashing anytime I tried calling a Firebase cloud function. And it wasn't just crashing; the app died with no logs or anything.

👻 Chasing a Ghost

Obviously, I suspected that Firebase or cloud_functions was the culprit. I cleaned and re-installed, verified the cloud function was deployed and responding, and began stepping through the code. I asked Claude for help debugging and we pursued the Firebase issue for hours.

I could trace the issue down to the functions call, but could not, for the life of me, get a breakpoint to stick. Finally giving up on breakpoints and debugPrint statements in the code, I pulled up Xcode to view the device logs.

Thread 34 Crashed::  Dispatch queue: com.apple.root.user-initiated-qos.cooperative
0   libswift_Concurrency.dylib    swift::swift_Concurrency_fatalErrorv
...
6   libswift_Concurrency.dylib    swift::_swift_task_dealloc_specific
7   libswift_Concurrency.dylib    asyncLet_finish_after_task_completion
8   FirebaseFunctions             HTTPSCallable.call(_:)  (HTTPSCallable.swift:163)
9   FirebaseFunctions             closure #1 in HTTPSCallable.call(_:completion:)  (HTTPSCallable.swift:123)

To my tired brain, this just confirmed that the problem was, indeed, a Firebase issue. But this was the iOS plugin; I had no idea what to do.

💡 The breakthrough

Exhausted and defeated, I started trying out different workflows. Half investigating, half poking around and grumbling to myself, and... wait, image_picker is also triggering the crash?

That was our breakthrough. Claude immediately began searching the Swift issue tracker and quickly surfaced swiftlang/swift#81771: a regression in the Swift 6.1 compiler.

It matched my symptoms exactly!

I don't know how long it would have taken me to figure that out, if ever. I don't know enough about the Swift/Xcode-side of Flutter to even know what to look for.

🔧 The Fix

Soon, I had a proposed resolution: pin FirebaseFunctions (or whatever is using the try { async let ...} catch {...} pattern) to a version before Swift 6.1. For my case: 12.4.0.

In ios/Podfile:

pod 'FirebaseFunctions', '~> 12.4.0'

Then:

cd ios
rm -rf Pods Podfile.lock
pod install --repo-update
cd ..
flutter clean
flutter run

📌 Status as of April 2026

Issue #81771 is still open. A fix PR surfaced on the Swift forums in late February 2026, but as of this writing it hasn't shipped in a stable Xcode release. If you're reading this from the future and Xcode 16.5+ is out, try updating Xcode first - the compiler fix may finally be in, and you won't need the pod pin at all.