Swift Concurrency

Overview

This skill provides expert guidance on Swift Concurrency, covering modern async/await patterns, actors, tasks, Sendable conformance, and migration to Swift 6. Use this skill to help developers write safe, performant concurrent code and navigate the complexities of Swift's structured concurrency model.

Agent Behavior Contract (Follow These Rules)

1. Analyze the project/package file to find out which Swift language mode (Swift 5.x vs Swift 6) and which Xcode/Swift toolchain is used when advice depends on it.
2. Before proposing fixes, identify the isolation boundary: @MainActor, custom actor, actor instance isolation, or nonisolated.
3. Do not recommend @MainActor as a blanket fix. Justify why main-actor isolation is correct for the code.
4. Prefer structured concurrency (child tasks, task groups) over unstructured tasks. Use Task.detached only with a clear reason.
5. If recommending @preconcurrency, @unchecked Sendable, or nonisolated(unsafe), require:
   • a documented safety invariant
   • a follow-up ticket to remove or migrate it
6. For migration work, optimize for minimal blast radius (small, reviewable changes) and add verification steps.
7. Course references are for deeper learning only. Use them sparingly and only when they clearly help answer the developer's question.

Recommended Tools for Analysis

When analyzing Swift projects for concurrency issues:

1. Project Settings Discovery
   • Use Read on Package.swift for SwiftPM settings (tools version, strict concurrency flags, upcoming features)
   • Use Grep for SWIFT_STRICT_CONCURRENCY or SWIFT_DEFAULT_ACTOR_ISOLATION in .pbxproj files
   • Use Grep for SWIFT_UPCOMING_FEATURE_ to find enabled upcoming features

Project Settings Intake (Evaluate Before Advising)

Concurrency behavior depends on build settings. Always try to determine:

• Default actor isolation (is the module default @MainActor or nonisolated?)
• Strict concurrency checking level (minimal/targeted/complete)
• Whether upcoming features are enabled (especially NonisolatedNonsendingByDefault)
• Swift language mode (Swift 5.x vs Swift 6) and SwiftPM tools version

Manual checks (no scripts)

• SwiftPM:
  • Check Package.swift for .defaultIsolation(MainActor.self).
  • Check Package.swift for .enableUpcomingFeature("NonisolatedNonsendingByDefault").
  • Check for strict concurrency flags: .enableExperimentalFeature("StrictConcurrency=targeted") (or similar).
  • Check tools version at the top: // swift-tools-version: ...
• Xcode projects:
  • Search project.pbxproj for:
    • SWIFT_DEFAULT_ACTOR_ISOLATION
    • SWIFT_STRICT_CONCURRENCY
    • SWIFT_UPCOMING_FEATURE_ (and/or SWIFT_ENABLE_EXPERIMENTAL_FEATURES)

If any of these are unknown, ask the developer to confirm them before giving migration-sensitive guidance.

Quick Decision Tree

When a developer needs concurrency guidance, follow this decision tree:

1. Starting fresh with async code?

   • Read references/async-await-basics.md for foundational patterns
   • For parallel operations → references/tasks.md (async let, task groups)

2. Protecting shared mutable state?

   • Need to protect class-based state → references/actors.md (actors, @MainActor)
   • Need thread-safe value passing → references/sendable.md (Sendable conformance)

3. Managing async operations?

   • Structured async work → references/tasks.md (Task, child tasks, cancellation)
   • Streaming data → references/async-sequences.md (AsyncSequence, AsyncStream)

4. Working with legacy frameworks?

   • Core Data integration → references/core-data.md
   • General migration → references/migration.md

5. Performance or debugging issues?

   • Slow async code → references/performance.md (profiling, suspension points)
   • Testing concerns → references/testing.md (XCTest, Swift Testing)

6. Understanding threading behavior?

   • Read references/threading.md for thread/task relationship and isolation

7. Memory issues with tasks?

   • Read references/memory-management.md for retain cycle prevention

Triage-First Playbook (Common Errors -> Next Best Move)

• SwiftLint concurrency-related warnings
  • Use references/linting.md for rule intent and preferred fixes; avoid dummy awaits as “fixes”.
• SwiftLint async_without_await warning
  • Remove async if not required; if required by protocol/override/@concurrent, prefer narrow suppression over adding fake awaits. See references/linting.md.
• "Sending value of non-Sendable type ... risks causing data races"
  • First: identify where the value crosses an isolation boundary
  • Then: use references/sendable.md and references/threading.md (especially Swift 6.2 behavior changes)
• "Main actor-isolated ... cannot be used from a nonisolated context"
  • First: decide if it truly belongs on @MainActor
  • Then: use references/actors.md (global actors, `nonisolat

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