---
name: guide-swift-concurrency
description: 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.
type: guide
origin: https://github.com/twostraws/Swift-Concurrency-Agent-Skill
license: MIT
---

> **Guide Skill** — This is an expert workflow/pattern guide, not API reference documentation.
> Originally from [twostraws/Swift-Concurrency-Agent-Skill](https://github.com/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

| Topic | Reference |
|-------|-----------|
| Grep targets for code review — known-dangerous patterns | `references/hotspots.md` |
| Swift 6.2: default isolation, `@concurrent`, `Task.immediate`, isolated deinit | `references/new-features.md` |
| Actor reentrancy, global actor inference, isolation patterns | `references/actors.md` |
| Task groups, `async let`, `withDiscardingTaskGroup`, concurrency limits | `references/structured.md` |
| `Task` vs `Task.detached`, when `Task {}` is a code smell | `references/unstructured.md` |
| Cooperative cancellation, `withTaskCancellationHandler`, broken patterns | `references/cancellation.md` |
| `AsyncStream.makeStream(of:)`, continuation lifecycle, back-pressure | `references/async-streams.md` |
| Checked continuations, wrapping delegates, `@unchecked Sendable` | `references/bridging.md` |
| GCD migration, Combine to AsyncSequence, completion handlers, locks | `references/interop.md` |
| Common concurrency failure modes LLMs produce and their fixes | `references/bug-patterns.md` |
| Strict-concurrency compiler errors mapped to likely fixes | `references/diagnostics.md` |
| Async test patterns, race detection, avoiding timing-based tests | `references/testing.md` |


---
Source attribution: vabole
Original project: https://github.com/vabole/apple-skills
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Mega AI Hub.

## Upstream license notice

MIT License

Copyright (c) 2026 Ilia Abolhasani

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Mega AI Hub resource metadata

Creator / publisher: vabole
Original project: https://github.com/vabole/apple-skills
Attribution basis: Original project named by vendored-folder README

Fields of work: Development & Engineering, Product Management
Roles: Backend Developer, DevOps / SRE, Full-Stack Developer, Mobile Developer, Technical Product Manager
Specialties: Backend
Tools: Swift
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: bb277ea6f528bc715013b4c3b0bf846817967c7942ce8f81d0023214b9d62235
Metadata reviewed: 2026-10-27
This metadata supersedes earlier Mega AI Hub catalog attribution. Original instruction and supporting files are preserved.