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.

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.

Quick Reference

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 → 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

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