megastep iOS Concurrency Router
You MUST use this skill for ANY concurrency, async/await, threading, or Swift 6 concurrency work.
When to Use
Use this router when:
- Writing async/await code
- Seeing concurrency errors (data races, actor isolation)
- Working with @MainActor
- Dealing with Sendable conformance
- Optimizing Swift performance
- Migrating to Swift 6 concurrency
- App freezes during loading (likely main thread blocking)
Conflict Resolution
ios-concurrency vs ios-performance: When app freezes or feels slow:
- Try ios-concurrency FIRST — Main thread blocking is the #1 cause of UI freezes. Check for synchronous work on @MainActor before profiling.
- Only use ios-performance if concurrency fixes don't help — Profile after ruling out obvious blocking.
ios-concurrency vs ios-build: When seeing Swift 6 concurrency errors:
- Use ios-concurrency, NOT ios-build — Concurrency errors are CODE issues, not environment issues
- ios-build is for "No such module", simulator issues, build failures unrelated to Swift language errors
ios-concurrency vs ios-data: When concurrency errors involve Core Data or SwiftData:
- Core Data threading (NSManagedObjectContext thread confinement, performBackgroundTask) → use ios-data first — Core Data has its own threading model distinct from Swift concurrency
- SwiftData + @MainActor ModelContext → use ios-concurrency — This is Swift concurrency isolation
- General "background saves losing data" → use ios-data first — Framework-specific threading rules take priority
Rationale: A 2-second freeze during data loading is almost always await on main thread or missing background dispatch. Domain knowledge solves this faster than Time Profiler. Core Data threading violations need Core Data-specific fixes, not generic concurrency patterns.
Routing Logic
Swift Concurrency Issues
Swift 6 concurrency patterns → $axiom-swift-concurrency
- async/await patterns
- @MainActor usage
- Actor isolation
- Sendable conformance
- Data race prevention
- Swift 6 migration
Swift performance → $axiom-swift-performance
- Value vs reference types
- Copy-on-write optimization
- ARC overhead
- Generic specialization
- Collection performance
Synchronous actor access → $axiom-assume-isolated
- MainActor.assumeIsolated
- @preconcurrency protocol conformances
- Legacy delegate callbacks
- Testing MainActor code synchronously
Thread-safe primitives → $axiom-synchronization
- Mutex (iOS 18+)
- OSAllocatedUnfairLock (iOS 16+)
- Atomic types
- Lock vs actor decision
Parameter ownership → $axiom-ownership-conventions
- borrowing/consuming modifiers
- Noncopyable types (~Copyable)
- ARC traffic reduction
- consume operator
Concurrency profiling → $axiom-concurrency-profiling
- Swift Concurrency Instruments template
- Actor contention diagnosis
- Thread pool exhaustion
- Task visualization
Automated Scanning
Concurrency audit → Launch concurrency-auditor agent or /axiom:audit concurrency (Swift 6 strict concurrency violations, unsafe Task captures, missing @MainActor, Sendable violations, actor isolation problems)
Decision Tree
- Data races / actor isolation / @MainActor / Sendable? → swift-concurrency
- Writing async/await code? → swift-concurrency
- Swift 6 migration? → swift-concurrency
- assumeIsolated / @preconcurrency? → assume-isolated
- Mutex / lock / synchronization? → synchronization
- borrowing / consuming / ~Copyable? → ownership-conventions
- Profile async performance / actor contention? → concurrency-profiling
- Value type / ARC / generic optimization? → swift-performance
- Want automated concurrency scan? → concurrency-auditor (Agent)
Anti-Rationalization
| Thought | Reality |
|---|---|
| "Just add @MainActor and it'll work" | @MainActor has isolation inheritance rules. swift-concurrency covers all patterns. |
| "I'll use nonisolated(unsafe) to silence the warning" | Silencing warnings hides data races. swift-concurrency shows the safe pattern. |
| "It's just one async call" | Even single async calls have cancellation and isolation implications. swift-concurrency covers them. |
| "I know how actors work" | Actor reentrancy and isolation rules changed in Swift 6.2. swift-concurrency is current. |
| "I'll fix the Sendable warnings later" | Sendable violations cause runtime crashes. swift-concurrency fixes them correctly now. |
Critical Patterns
Swift 6 Concurrency (swift-concurrency):
- Progressive journey: single-threaded → async → concurrent → actors
- @concurrent attribute for forced background execution
- Isolated conformances
- Main actor mode for approachable concurrency
- 11 copy-paste patterns
Swift Performance (swift-performance):
- ~Copyable for non-copyable types
- Copy-on-write (COW) patterns
- Value vs reference type decisions
- ARC overhead reduction
- Generic specialization
Example Invocations
User: "I'm getting 'data race' errors in Swift 6"
→ Invoke: $axiom-swift-concurrency
User: "How do I use @MainActor correctly?"
→ Invoke: $axiom-swift-concurrency
User: "My app is slow due to unnecessary copying"
→ Invoke: $axiom-swift-performance
User: "Should I use async/await for this network call?"
→ Invoke: $axiom-swift-concurrency
User: "How do I use assumeIsolated?"
→ Invoke: $axiom-assume-isolated
User: "My delegate callback runs on main thread, how do I access MainActor state?"
→ Invoke: $axiom-assume-isolated
User: "Should I use Mutex or actor?"
→ Invoke: $axiom-synchronization
User: "What's the difference between os_unfair_lock and OSAllocatedUnfairLock?"
→ Invoke: $axiom-synchronization
User: "What does borrowing do in Swift?"
→ Invoke: $axiom-ownership-conventions
User: "How do I use ~Copyable types?"
→ Invoke: $axiom-ownership-conventions
User: "My async code is slow, how do I profile it?"
→ Invoke: $axiom-concurrency-profiling
User: "I think I have actor contention, how do I diagnose it?"
→ Invoke: $axiom-concurrency-profiling
User: "My Core Data saves lose data from background tasks"
→ Route to: ios-data router (Core Data threading is framework-specific)
User: "Check my code for Swift 6 concurrency issues"
→ Invoke: concurrency-auditor agent