Use when writing ANY async code, actors, threads, or seeing ANY concurrency error. Covers Swift 6 concurrency, @MainActor, Sendable, data races, async/await patterns.

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 — auto maintained by scripts/build inlined auditors.ts; do not hand edit Can't launch Axiom agents? Where this router sends you to an agent, follow its procedure inline. Over MCP, axiom get agent returns it; elsewhere, read its file below, which needs only file search and read. Available here: skills/concurrency auditor.md . For any other agent, over MCP axiom get agent returns it too; run its xclog, xcsym and xcprof steps with the axiom xclog , axiom xcsym and axiom xcprof tools. Elsewhere, search Axiom's skills for skills/<agent .md ; if there is none, do that work yourself with this suite's skills, using any Axiom tool on your PATH . <! 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 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 → 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 6.4 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

axiom-concurrency