Concurrency

You MUST use this skill for ANY concurrency, async/await, threading, or Swift 6 concurrency work.

Quick Reference

Symptom / Task	Reference
async/await patterns, @MainActor, actors	See `skills/swift-concurrency.md`
Data race errors, Sendable conformance	See `skills/swift-concurrency.md`
Swift 6 migration, @concurrent attribute	See `skills/swift-concurrency.md`
Actor definition, reentrancy, global actors	See `skills/swift-concurrency-ref.md`
Task/TaskGroup/cancellation API	See `skills/swift-concurrency-ref.md`
AsyncStream, continuations	See `skills/swift-concurrency-ref.md`
DispatchQueue → actor migration	See `skills/swift-concurrency-ref.md`
Mutex (iOS 18+), OSAllocatedUnfairLock	See `skills/synchronization.md`
Atomic types, lock vs actor decision	See `skills/synchronization.md`
MainActor.assumeIsolated	See `skills/assume-isolated.md`
@preconcurrency protocol conformances	See `skills/assume-isolated.md`
Legacy delegate callbacks	See `skills/assume-isolated.md`
Swift Concurrency Instruments template	See `skills/concurrency-profiling.md`
Actor contention diagnosis	See `skills/concurrency-profiling.md`
Thread pool exhaustion	See `skills/concurrency-profiling.md`

Decision Tree

dot

digraph concurrency {
    start [label="Concurrency task" shape=ellipse];
    what [label="What do you need?" shape=diamond];

    start -> what;
    what -> "skills/swift-concurrency.md" [label="async/await, actors,\nSendable, data races,\nSwift 6 migration"];
    what -> "skills/swift-concurrency-ref.md" [label="API syntax lookup\n(TaskGroup, AsyncStream,\ncontinuations, migration)"];
    what -> "skills/synchronization.md" [label="Mutex, locks,\natomic types"];
    what -> "skills/assume-isolated.md" [label="assumeIsolated,\n@preconcurrency"];
    what -> "skills/concurrency-profiling.md" [label="profile async perf,\nactor contention"];
}

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

Conflict Resolution

concurrency vs axiom-performance: When app freezes or feels slow:

Try concurrency FIRST — Main thread blocking is the #1 cause of UI freezes. Check for synchronous work on @MainActor before profiling.
Only use axiom-performance if concurrency fixes don't help — Profile after ruling out obvious blocking.

concurrency vs axiom-build: When seeing Swift 6 concurrency errors:

Use concurrency, NOT axiom-build — Concurrency errors are CODE issues, not environment issues.

concurrency vs axiom-data: When concurrency errors involve Core Data or SwiftData:

Core Data threading (NSManagedObjectContext thread confinement) → use axiom-data first
SwiftData + @MainActor ModelContext → use concurrency
General "background saves losing data" → use axiom-data first

Critical Patterns

Swift Concurrency (

skills/swift-concurrency.md

Progressive journey: single-threaded → async → concurrent → actors
@concurrent attribute for forced background execution
Isolated conformances, main actor mode
12 copy-paste patterns including delegate value capture, weak self in Tasks
Comprehensive decision tree for 7 common error messages

API Reference (

skills/swift-concurrency-ref.md

Actor definition, reentrancy, global actors, nonisolated
Sendable patterns, @unchecked Sendable, sending parameter
Task/TaskGroup/cancellation, async let, withDiscardingTaskGroup
AsyncStream, continuations, buffering policies
Isolation patterns (#isolation, @preconcurrency, nonisolated(unsafe))
DispatchQueue/DispatchGroup/completion handler migration

Synchronization (

skills/synchronization.md

Mutex (iOS 18+), OSAllocatedUnfairLock (iOS 16+), Atomic types
Lock vs actor decision tree
Danger patterns: locks across await, semaphores in async context

Profiling (

skills/concurrency-profiling.md

Swift Concurrency Instruments template
Diagnosing main thread blocking, actor contention, thread pool exhaustion
Safe vs unsafe primitives for cooperative pool

Automated Scanning

Concurrency audit → Launch

concurrency-auditor

agent or

/axiom:audit concurrency

(5-phase semantic audit: maps isolation architecture, detects 8 anti-patterns, reasons about missing concurrency patterns, correlates compound risks, scores Swift 6.3 readiness)

Anti-Rationalization

Thought	Reality
"Just add @MainActor and it'll work"	@MainActor has isolation inheritance rules. `skills/swift-concurrency.md` covers all patterns.
"I'll use nonisolated(unsafe) to silence the warning"	Silencing warnings hides data races. `skills/swift-concurrency.md` shows the safe pattern.
"It's just one async call"	Even single async calls have cancellation and isolation implications.
"I know how actors work"	Actor reentrancy and isolation rules changed in Swift 6.2.
"I'll fix the Sendable warnings later"	Sendable violations cause runtime crashes. Fix them now.
"Combine is dead, just use async/await"	Combine has no deprecation notice. Rewriting working pipelines wastes time. See See axiom-uikit (skills/combine-patterns.md).
"I'll use @unchecked Sendable to silence this"	You're hiding a data race from the compiler. It will crash in production.
"This async function runs on a background thread"	`async` suspends without blocking but resumes on the same actor. Use `@concurrent` to force background.

Example Invocations

User: "I'm getting 'data race' errors in Swift 6" → Read:

skills/swift-concurrency.md

User: "How do I use @MainActor correctly?" → Read:

skills/swift-concurrency.md

User: "How do I create a TaskGroup?" → Read:

skills/swift-concurrency-ref.md

User: "What's the AsyncStream API?" → Read:

skills/swift-concurrency-ref.md

User: "How do I use assumeIsolated?" → Read:

skills/assume-isolated.md

User: "Should I use Mutex or actor?" → Read:

skills/synchronization.md

User: "My async code is slow, how do I profile it?" → Read:

skills/concurrency-profiling.md

User: "My app is slow due to unnecessary copying" → See axiom-performance (skills/swift-performance.md)

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

concurrency-auditor

agent

axiom-concurrency

NPX Install

Tags

SKILL.md Content