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:

1. Try ios-concurrency FIRST — Main thread blocking is the #1 cause of UI freezes. Check for synchronous work on @MainActor before profiling.
2. 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 → /skill axiom-swift-concurrency

• async/await patterns
• @MainActor usage
• Actor isolation
• Sendable conformance
• Data race prevention
• Swift 6 migration

Swift concurrency API reference → /skill axiom-swift-concurrency-ref

• Actor definition, reentrancy, global actors
• Sendable patterns, @unchecked Sendable
• Task/TaskGroup/cancellation API
• AsyncStream, continuations
• DispatchQueue → actor migration

Swift performance → /skill axiom-swift-performance

• Value vs reference types
• Copy-on-write optimization
• ARC overhead
• Generic specialization
• Collection performance

Synchronous actor access → /skill axiom-assume-isolated

• MainActor.assumeIsolated
• @preconcurrency protocol conformances
• Legacy delegate callbacks
• Testing MainActor code synchronously

Thread-safe primitives → /skill axiom-synchronization

• Mutex (iOS 18+)
• OSAllocatedUnfairLock (iOS 16+)
• Atomic types
• Lock vs actor decision

Parameter ownership → /skill axiom-ownership-conventions

• borrowing/consuming modifiers
• Noncopyable types (~Copyable)
• ARC traffic reduction
• consume operator

Concurrency profiling → /skill axiom-concurrency-profiling

• Swift Concurrency Instruments template
• Actor contention diagnosis
• Thread pool exhaustion
• Task visualization

Combine reactive patterns → /skill axiom-combine-patterns

• Publisher/Subscriber lifecycle, AnyCancellable
• Combine vs async/await decision
• @Published + ObservableObject
• Operator patterns, bridging

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

1. Data races / actor isolation / @MainActor / Sendable? → swift-concurrency 1a. Need specific API syntax (actor definition, TaskGroup, AsyncStream, continuation)? → swift-concurrency-ref
2. Writing async/await code? → swift-concurrency
3. Swift 6 migration? → swift-concurrency
4. assumeIsolated / @preconcurrency? → assume-isolated
5. Mutex / lock / synchronization? → synchronization
6. borrowing / consuming / ~Copyable? → ownership-conventions
7. Profile async performance / actor contention? → concurrency-profiling
8. Value type / ARC / generic optimization? → swift-performance
9. Want automated concurrency scan? → concurrency-auditor (Agent)
10. Combine / @Published / AnyCancellable / reactive streams? → combine-patterns

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.
"Combine is dead, just use async/await"	Combine has no deprecation notice. Rewriting working pipelines wastes time and introduces bugs. combine-patterns covers incremental migration.

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: /skill axiom-swift-concurrency

User: "How do I use @MainActor correctly?" → Invoke: /skill axiom-swift-concurrency

User: "My app is slow due to unnecessary copying" → Invoke: /skill axiom-swift-performance

User: "Should I use async/await for this network call?" → Invoke: /skill axiom-swift-concurrency

User: "How do I use assumeIsolated?" → Invoke: /skill axiom-assume-isolated

User: "My delegate callback runs on main thread, how do I access MainActor state?" → Invoke: /skill axiom-assume-isolated

User: "Should I use Mutex or actor?" → Invoke: /skill axiom-synchronization

User: "What's the difference between os_unfair_lock and OSAllocatedUnfairLock?" → Invoke: /skill axiom-synchronization

User: "What does borrowing do in Swift?" → Invoke: /skill axiom-ownership-conventions

User: "How do I use ~Copyable types?" → Invoke: /skill axiom-ownership-conventions

User: "My async code is slow, how do I profile it?" → Invoke: /skill axiom-concurrency-profiling

User: "I think I have actor contention, how do I diagnose it?" → Invoke: /skill 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: "How do I create a TaskGroup?" → Invoke: /skill axiom-swift-concurrency-ref

User: "What's the AsyncStream API?" → Invoke: /skill axiom-swift-concurrency-ref

User: "How do I create a custom global actor?" → Invoke: /skill axiom-swift-concurrency-ref

User: "How do I convert a completion handler to async?" → Invoke: /skill axiom-swift-concurrency-ref

User: "What are the actor reentrancy rules?" → Invoke: /skill axiom-swift-concurrency-ref

User: "My Combine pipeline silently stopped producing values" → Invoke: /skill axiom-combine-patterns

User: "Should I use Combine or async/await for this data flow?" → Invoke: /skill axiom-combine-patterns

User: "How do I bridge a Combine publisher into async/await code?" → Invoke: /skill axiom-combine-patterns

User: "AnyCancellable is leaking memory" → Invoke: /skill axiom-combine-patterns

User: "Check my code for Swift 6 concurrency issues" → Invoke: concurrency-auditor agent