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guide-swift-concurrency

Swift concurrency patterns — actors, structured concurrency, cancellation, async streams, GCD migration, strict-concurrency diagnostics, common bug patterns. Use when writing, reviewing, or debugging concurrent Swift code.

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SKILL.md
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Guide Skill — This is an expert workflow/pattern guide, not API reference documentation. Originally from twostraws/Swift-Concurrency-Agent-Skill by Paul Hudson. MIT License.

Swift Concurrency Patterns

Review and write Swift concurrency code for correctness, modern API usage, and adherence to project conventions. Report only genuine problems — do not nitpick or invent issues.

Core Instructions

  • Target Swift 6.2 or later with strict concurrency checking.
  • Prefer structured concurrency (task groups) over unstructured (Task {}).
  • Prefer Swift concurrency over GCD for new code. GCD is still acceptable in low-level code, framework interop, or performance-critical synchronous work.
  • Do not suggest @unchecked Sendable to fix compiler errors — prefer actors, value types, or sending parameters.

Review Process

  1. Scan for known-dangerous patterns using references/hotspots.md.
  2. Check for Swift 6.2 concurrency behavior using references/new-features.md.
  3. Validate actor usage for reentrancy and isolation using references/actors.md.
  4. Ensure structured concurrency is preferred using references/structured.md.
  5. Check unstructured task usage using references/unstructured.md.
  6. Verify cancellation handling using references/cancellation.md.
  7. Validate async stream and continuation usage using references/async-streams.md.
  8. Check bridging code between sync and async using references/bridging.md.
  9. Review legacy concurrency migrations using references/interop.md.
  10. Cross-check against common failure modes using references/bug-patterns.md.
  11. If strict-concurrency errors exist, map diagnostics to fixes using references/diagnostics.md.
  12. If reviewing tests, check async test patterns using references/testing.md.

If doing partial work, load only the relevant reference files.

References

TopicReference
Grep targets for code review — known-dangerous patternsreferences/hotspots.md
Swift 6.2: default isolation, @concurrent, Task.immediate, isolated deinitreferences/new-features.md
Actor reentrancy, global actor inference, isolation patternsreferences/actors.md
Task groups, async let, withDiscardingTaskGroup, concurrency limitsreferences/structured.md
Task vs Task.detached, when Task {} is a code smellreferences/unstructured.md
Cooperative cancellation, withTaskCancellationHandler, broken patternsreferences/cancellation.md
AsyncStream.makeStream(of:), continuation lifecycle, back-pressurereferences/async-streams.md
Checked continuations, wrapping delegates, @unchecked Sendablereferences/bridging.md
GCD migration, Combine to AsyncSequence, completion handlers, locksreferences/interop.md
Common concurrency failure modes LLMs produce and their fixesreferences/bug-patterns.md
Strict-concurrency compiler errors mapped to likely fixesreferences/diagnostics.md
Async test patterns, race detection, avoiding timing-based testsreferences/testing.md
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skills/guide-swift-concurrency/SKILL.md

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