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 → /skill axiom-swift-concurrency

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

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

axiom-ios-concurrency