# Swift Concurrency

> Implement or audit Swift 6.2 concurrency, MainActor default isolation, @concurrent work, Sendable dependencies, task ownership, cancellation, clocks, and race-free UI updates.

- Skill: `rcarmo/swift-concurrency` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add rcarmo/swift-concurrency`
- Raw SKILL.md: https://api.skillmd.com/api/skills/rcarmo/swift-concurrency/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- License: MIT
- Author: rcarmo (https://skillmd.com/u/rcarmo)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/rcarmo/swift-concurrency

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# Swift concurrency

Read `Package.swift` and `references/concurrency-checklist.md`. Every target uses Swift 6 language mode with MainActor default isolation, `InferIsolatedConformances`, and `NonisolatedNonsendingByDefault`. Use this skill for every asynchronous service, observable model, stream, task group, actor, sendability, `nonisolated`, or `@concurrent` change.

## Workflow

1. Identify mutable state and assign one isolation domain.
2. Keep UI-facing state on `@MainActor`.
3. Make dependency protocols and cross-task values `Sendable`.
4. Prefer structured `async` functions and task groups to callbacks or detached work.
5. Define which scope owns each `Task` and when cancellation occurs.
6. Propagate cancellation; do not convert it into a user-facing failure accidentally.
7. Prevent stale responses from overwriting newer state.
8. Inject clocks or continuations for deterministic tests instead of sleeping.
9. Run `make test` and `make package-build` with Swift 6.2; fix diagnostics at the boundary rather than weakening package settings or using broad suppressions.

## Reject

- `DispatchQueue.main.async` and global queues in new Swift concurrency code.
- `Task.sleep(nanoseconds:)`; use `Task.sleep(for:)` or an injected clock.
- `Task.detached` without a documented isolation/priority reason.
- mutable shared state outside an actor/global actor/explicit synchronisation mechanism.
- broad `@unchecked Sendable`; a narrow suppression requires a written thread-safety proof.
- retaining unstructured tasks without cancellation or lifecycle ownership.
- swallowing errors or cancellation.

## Review output

State the isolation of each mutable object, the task owner, cancellation path, and any value crossing an actor boundary. Prioritise data races and stale writes over stylistic concerns.