Concurrency & Async

Skills for Swift concurrency: async/await, actors, Sendable, and the compile-time data-race checking in Swift 6. This section also collects the Swift language skills that sit beside concurrency: performance, ownership, modern idioms, and Combine.

Solid arrows lead to skills in the axiom-concurrency suite. Dotted arrows lead to skills in other suites that the router points to; Modern Swift Idioms belongs to axiom-swift and is listed here because it covers the same code.

Skills

• Swift Concurrency – Swift 6 strict concurrency: async/await, @MainActor, Sendable, and actor isolation

  • "I'm getting 'Main actor-isolated property accessed from nonisolated context' errors everywhere."
  • "My code is throwing 'Type does not conform to Sendable' warnings when passing data between threads."
  • "I have a stored task causing memory leaks. How do I write it correctly with weak self?"

• Swift Concurrency Reference – API reference for actors, Sendable, Task and TaskGroup, AsyncStream, continuations, and migration patterns

  • "How do I create a TaskGroup?"
  • "What's the AsyncStream continuation API?"
  • "How do I convert completion handlers to async?"

• Runtime Isolation Crash Diagnostics – Production crashes in warning-free Swift 6 builds, with _dispatch_assert_queue_fail or _swift_task_checkIsolatedSwift in the crash report

  • "My Swift 6 build has zero warnings but crashes in production with _dispatch_assert_queue_fail."
  • "Why does my CLLocationManager delegate method crash with _swift_task_checkIsolatedSwift?"

• assumeIsolated – Synchronous actor access for tests, legacy callbacks, and performance-critical code

  • "How do I access MainActor state from a delegate callback that runs on the main thread?"
  • "What's the difference between Task { @MainActor in } and MainActor.assumeIsolated?"

• Synchronization – Thread-safe primitives: Mutex (iOS 18+), OSAllocatedUnfairLock, and atomic types

  • "Should I use Mutex or an actor for this hot path?"
  • "What's the difference between os_unfair_lock and OSAllocatedUnfairLock?"

• Concurrency Profiling – Instruments workflows for async/await performance

  • "My async code is slow. How do I profile it?"
  • "I think I have actor contention. How do I diagnose it?"

• Swift Performance – Faster Swift code: ARC overhead, unspecialized generics, collection inefficiencies, and the cost of actor isolation

  • "My Swift code is allocating too much memory."
  • "How do I reduce ARC overhead in hot loops?"

• Ownership Conventions – The borrowing and consuming modifiers, for performance and for noncopyable types

  • "What does borrowing do in Swift?"
  • "How do I use ~Copyable types?"

• Combine Patterns – Reactive programming with Combine: publisher lifecycle, operators, @Published traps, and bridging to async/await

  • "Should I use Combine or async/await for this?"
  • "My Combine pipeline silently stops producing values."

• Modern Swift Idioms – Replaces outdated Swift that AI assistants often write: legacy APIs, old syntax, and Foundation calls with modern equivalents

  • "Review my Swift code for outdated patterns."
  • "Should I use FormatStyle instead of DateFormatter?"

Agent and Command

• concurrency-auditor – Scans your code for concurrency anti-patterns and scores its Swift 6 readiness; run it with /axiom:audit concurrency