I ran into this exact issue on iOS 18.0–18.3. In my case it was triggered by how I implemented the AsyncSequence / AsyncIteratorProtocol. I had custom iterators that only implemented the new typed-throws overload: mutating func next(isolation actor: isolated (any Actor)?) async throws(Self.Failure) -> Self.Element? The fix for me was to also implement the original next() overload and forward it into the newer one: public mutating func next() async throws -> Element? { try await next(isolation: #isolation) } Once I added this overload, the app ran without crashing on all affected 18.x versions, and the iterator still correctly routed calls into the newer next(isolation:) implementation. Hope this helps someone else running into the same issue.