Swift has made big strides in how it handles asynchronous programming. When the concurrency model was introduced in Swift 5.5, tools like async/await, actors, and Sendable made it a lot easier—and safer—to write code that runs in parallel. Now with Swift 6, those tools are getting even more powerful. But along with that power comes a bit more strictness, as the compiler now enforces rules to help catch concurrency issues before they become real bugs. The compiler now enforces concurrency rules at a much deeper level, helping developers avoid dreaded data races. But with that power comes some migration work.
If your codebase already uses some concurrency features or you’ve been putting off the transition, now is the right time to look into migrating to Swift 6. This article will walk you through what to expect, how to start, and how to avoid being overwhelmed by the warning list that may pop up when you make the switch.
Why Should You Migrate?
The main reason to migrate to Swift 6 is simple: safety. Swift 6 introduces stricter checks that help catch concurrency issues—like data races—right when you build your project, instead of letting them slip into production where they’re much harder to find. This is especially helpful as your app grows and more code runs at the same time. The good news is, these new rules aren’t forced on you. Swift 6’s concurrency checks are opt-in, so nothing changes unless you choose to enable them.
Concurrency in Swift
Before jumping into migration, it helps to quickly go over what Swift’s concurrency model actually includes. You’ve probably seen features like async/await which simplify asynchronous code. Actors isolate mutable state, allowing only one task at a time to access it. Then there’s the concept of Sendable, which ensures certain values can be safely passed between threads or tasks. In Swift 5, these were mostly optional. But Swift 6 wants to be sure your code is safe, so it requires strict use of these concurrency rules—unless you specifically disable checks using compiler flags.
Don’t Panic: Warnings Are Normal
Here’s something you need to know up front: when you first enable concurrency checking, the compiler might throw a lot of warnings at you. Sometimes hundreds. That’s okay. It doesn’t mean your code is bad—it just means Swift is now pointing out places where concurrency rules aren’t guaranteed. Don’t let it scare you off. Many of these issues are repetitive and can be resolved by applying the same solution in multiple places. Once you fix a few, you’ll start to understand the patterns and move faster through the rest.
Start Small: Pick One Module
Instead of tackling your whole codebase at once, start by migrating just one module. Preferably choose one that sits at the top level and doesn’t have dependencies (or only a few). That way, any changes you make won’t have ripple effects across other parts of the app. This modular approach gives you more control and lets you track progress more easily.
Stay in Swift 5 Mode (For Now)
Rather than jumping straight into Swift 6 mode, you can begin the process while still using Swift 5. Swift provides upcoming feature flags that you can enable one at a time. These flags will issue warnings instead of errors, giving you room to fix problems gradually without breaking your build. For example, flags like DisableOutwardActorInference, GlobalConcurrency, and InferSendableFromCaptures can be enabled individually. That way, you can focus on understanding and fixing one kind of issue at a time.
How to Enable These Flags
To enable a feature flag, you just need to pass a couple of -Xfrontend parameters in your build settings. Here’s what it looks like
You can add these for each feature you want to test. Do it one at a time. As each flag exposes a new class of issues, your goal should be to understand the warning, fix the cause, and move on.
Fixing Warnings: Keep It Simple
When you start fixing warnings, keep this principle in mind: fix what’s broken now—refactor later. Don’t try to do major overhauls just to make your code “perfect.” If the compiler says a closure should be @Sendable, mark it as @Sendable. If a global variable causes concurrency warnings, consider moving it inside an actor or making it isolated. These small, targeted fixes can resolve dozens of issues without needing complex rewrites.
Work in Iterations
After you’ve cleaned up one module, move to the next. In many cases, a warning in one module is caused by a dependency that hasn’t yet been updated. Once you fix the underlying problem in the dependency, many warnings in the top-level module will disappear. You don’t need to fix everything before moving on—just aim to reduce the noise gradually. That’s the benefit of working iteratively.
Ready to Switch to Swift 6 Mode?
Once you’ve addressed the main set of warnings, you can take the final step: enable Swift 6 language mode for that module. In Xcode, set the SWIFT_VERSION to 6 in your build settings. Now the compiler will treat concurrency violations as errors rather than warnings, helping you lock in the safety guarantees. You can repeat this process module by module until your entire project runs safely in Swift 6 mode.
Common Issues You Might Run Into
When you start migrating to Swift 6, a few things will probably pop up. One is shared mutable state—like global variables used across threads. Wrapping them in an actor is usually the easiest fix.
You might also get warnings about your custom types not being Sendable. If they’re safe to use across threads, just conform them. If not, you can use @unchecked Sendable for now—but plan to clean it up later.
Another common one is capturing self in closures. Adding @Sendable usually clears it up, but check what the closure is doing.
Most of these fixes are quick. Just tackle them one by one—no need to rush.
Final Advice: Take It Slow and Steady
Don’t stress about doing everything at once. Start small, fix what you can, and keep going. Swift 6’s checks may be strict, but they’re here to help you write safe code. Once you get past the first few warnings, it gets easier – and your code will be better for it.
Useful Resources
Also Read: So… What Even Is an API? (A Non-Boring Guide for Beginners in 2025)
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