Concurrency

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

Bundled tools on Claude Code: if command -v <tool> finds nothing (hooks off or --bare), use <plugin-root>/tools/<tool> in place of the bare name; <plugin-root> is two levels above this skill's Base directory for this skill: path.

<!-- AXIOM_AUDITOR_INLINE_BEGIN — rewritten for Codex by scripts/build-codex.ts; do not hand-edit -->

Auditors are skills here. Where this router says "Launch some-auditor agent", invoke the matching Codex skill instead — same procedure, no Claude Code agent required.

Available: axiom-audit-concurrency.

The ones that shell out — builds, tests, simulators, crash symbolication — need shell access to run.

<!-- AXIOM_AUDITOR_INLINE_END -->

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
Warning-free build crashes with _dispatch_assert_queue_fail	See skills/isolation-inheritance-diag.md
Crash signature _swift_task_checkIsolatedSwift	See skills/isolation-inheritance-diag.md
Core Data context.perform runtime crash inside @MainActor class	See skills/isolation-inheritance-diag.md
Combine .map/.sink crash from receive(on:) placement	See skills/isolation-inheritance-diag.md
Delegate method crash from isolation inheritance (CLLocationManager, NSDocument, AVAudioPlayerDelegate, WKNavigationDelegate, PHPhotoLibraryChangeObserver)	See skills/isolation-inheritance-diag.md
PhotoKit performChanges block traps on PhotoKit's queue	See skills/isolation-inheritance-diag.md
Actor reentrancy / stale state across await	See skills/isolation-inheritance-diag.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

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/isolation-inheritance-diag.md" [label="warning-free build crashes\n_dispatch_assert_queue_fail\n_swift_task_checkIsolatedSwift"];
    what -> "skills/concurrency-profiling.md" [label="profile async perf,\nactor contention"];
}

1. 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 1b. NotificationCenter observer, userInfo casting, or Sendable errors unpacking a notification? → skills/swift-concurrency-ref.md (Typed Notifications — MainActorMessage/AsyncMessage, iOS 26+)
2. Writing async/await code? → skills/swift-concurrency.md
3. assumeIsolated / @preconcurrency? → skills/assume-isolated.md 3a. Warning-free Swift 6 build that crashes in production with _dispatch_assert_queue_fail or _swift_task_checkIsolatedSwift? → skills/isolation-inheritance-diag.md
4. Mutex / lock / synchronization? → skills/synchronization.md
5. Profile async performance / actor contention? → skills/concurrency-profiling.md
6. Value type / ARC / generic optimization? → See axiom-performance (skills/swift-performance.md)
7. borrowing / consuming / ~Copyable? → See axiom-swift (skills/ownership-conventions.md)
8. Combine / @Published / AnyCancellable / reactive streams? → See axiom-uikit (skills/combine-patterns.md)
9. Want automated concurrency scan? → concurrency-auditor (Agent)

Concurrency in practice

• HealthKit queries with Swift Concurrency (canonical bridging example) → See axiom-health (skills/queries.md)

Conflict Resolution

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

1. Try concurrency FIRST — Main thread blocking is the #1 cause of UI freezes. Check for synchronous work on @MainActor before profiling.
2. Only use axiom-performance if concurrency fixes don't help — Profile after ruling out obvious blocking.
3. To pin a specific freeze to app code, see the Hang Window Workflow in axiom-performance (skills/hang-diagnostics.md) — re-scope xcprof analyze to the hang window with --start-ms/--end-ms --user-binary to surface the app-owned frame on the main thread.

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
• GRDB Sendable patterns (struct records, databaseSelection as computed property, Swift 6 conformance) → See axiom-data (skills/grdb-performance.md) §8

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

Runtime Isolation Crashes (skills/isolation-inheritance-diag.md):

• _dispatch_assert_queue_fail and _swift_task_checkIsolatedSwift signatures
• Closure isolation inheritance (Core Data perform, Combine .map, NotificationCenter .sink)
• Delegate method isolation inheritance (CLLocationManager, NSDocument, AVAudioPlayerDelegate, WKNavigationDelegate)
• MainActor.assumeIsolated misuse
• Actor reentrancy state staleness

Automated Scanning

Concurrency audit → axiom-audit-concurrency (5-phase semantic audit: maps isolation architecture, detects 8 anti-patterns, reasons about missing concurrency patterns, correlates compound risks, scores Swift 6.4 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.
"My Swift 6 build has zero warnings, so isolation is correct"	Static checking can't see SDK callbacks. Runtime checks crash anyway. skills/isolation-inheritance-diag.md.
"I'll wrap the crash in MainActor.assumeIsolated"	assumeIsolated is a runtime trap, not a silencer. Wrong assumption = crash.
"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"	Check its isolation, don't assume. A @MainActor async function runs entirely on the main actor, and nonisolated async runs on the caller's actor under NonisolatedNonsendingByDefault (Swift 6.2+; new Xcode projects set SWIFT_APPROACHABLE_CONCURRENCY = YES, the setting's own default is NO, and SwiftPM does not enable it) — those are the main-thread freezes. Without that setting nonisolated async hops to the global executor. @concurrent forces off-actor work either way.

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 warning-free Swift 6 build crashes in production with _dispatch_assert_queue_fail" → Read: skills/isolation-inheritance-diag.md

User: "Core Data context.perform crashes inside an @MainActor view model" → Read: skills/isolation-inheritance-diag.md

User: "CLLocationManager delegate method is crashing with _swift_task_checkIsolatedSwift" → Read: skills/isolation-inheritance-diag.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