Four Common Swift 6 Concurrency Errors and How Developers Can Fix Them

Enabling Swift 6 strict concurrency mode on an existing project can surface dozens of errors, many of which represent genuine data races and actor isolation bugs that were silently present in production code. One of the most frequent mistakes is capturing non-Sendable reference types inside Task closures, which can be resolved by converting classes to structs or using actors for mutable shared state. Another common issue involves attempting to update MainActor-bound properties from within a Task that does not automatically inherit that isolation context, which requires explicit @MainActor annotations on the relevant class or function. Developers often suppress errors using nonisolated or @unchecked Sendable without understanding the underlying thread-safety implications, a practice that masks rather than resolves potential race conditions. A subtler pitfall is assuming that async helper functions called from a @MainActor class will automatically run on the main actor, when in fact isolation must be declared explicitly at the function level.