Swift Concurrency

What it does

When your agent uses it

• Working with async/await
• Data race compiler errors
• Sendable conformance
• Migrating Swift 5 code to Swift 6 strict concurrency

Example prompts

• “Sendable”
• “data race”
• “MainActor”
• “/swift-concurrency”

Workflow steps

10 steps, taken from the first numbered list in SKILL.md.

1. 1Never suppress concurrency errors with nonisolated(unsafe) without explicit user approval. Investigate and fix the isolation root cause…
2. 2In async contexts, actor is the default for shared mutable state. In synchronous contexts, prefer refactoring to async and using an actor…
3. 3Use @MainActor for all UI-bound state. Apply it to ObservableObject view models and @Observable types that drive the view hierarchy.
4. 4Prefer structured concurrency (async let, TaskGroup) over unstructured Task { }. Attach work to a parent task whenever possible.
5. 5Mark types Sendable explicitly when they cross isolation boundaries. Prefer value types (struct, enum) — they are implicitly Sendable when…
6. 6Propagate task cancellation. Check Task.isCancelled or call try Task.checkCancellation() inside loops or long-running operations.
7. 7Avoid @unchecked Sendable unless integrating a third-party type that is thread-safe but lacks annotation. Always add a comment explaining…
8. 8Use @preconcurrency import when adopting a framework not yet fully annotated for Swift 6, to suppress false-positive warnings at the…
9. 9No DispatchQueue in new Swift 6 code. Use actor for shared mutable state. Mutex is acceptable for short synchronous critical sections, but…
10. 10Never call MainActor.assumeIsolated outside of truly non-async contexts (e.g., UIKit delegate callbacks). In async code, just annotate or…

What it can do on your machine

Read from SKILL.md and the folder at commit f009d02. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  No scripts in the folder and no shell commands in SKILL.md (its code samples are swift).

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  No URLs in SKILL.md.

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Swift Concurrency loads about 1.7k tokens when it runs, and up to ~9k if it reads all its reference files. Until then it costs about 130 tokens; SKILL.md has 551 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~130

When it runs · the whole SKILL.md, loaded when a task matches

~1.7k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~9k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from nimblehq/ios-templates at commit f009d02, republished under its MIT licence (© nimblehq). 551 words, ~1,654 tokens.

name

swift-concurrency

description

Write, review, or fix Swift 6 concurrency code using actors, Sendable, structured concurrency, and the strict data-race-safety model. Use when working with async/await, actors, data race compiler errors, Sendable conformance, TaskGroup, AsyncSequence, or migrating Swift 5 code to Swift 6 strict concurrency. Triggers on: "actor", "Sendable", "data race", "MainActor", "async let", "TaskGroup", "AsyncStream", "concurrency warning", "strict concurrency", "Swift 6 concurrency", "nonisolated", "Mutex".

actor TokenStore {

    private var accessToken: String?
    private var refreshToken: String?

    func store(access: String, refresh: String) {
        accessToken = access
        refreshToken = refresh
    }

    func retrieveAccessToken() -> String? {
        accessToken
    }
}

@MainActor
final class HomeViewModel: ObservableObject {

    @Published private(set) var items: [HomeItem] = []
    @Published private(set) var isLoading = false

    private let repository: any HomeRepository

    init(repository: some HomeRepository) {
        self.repository = repository
    }

    func load() async {
        isLoading = true
        defer { isLoading = false }
        do {
            items = try await repository.fetchItems()
        } catch {
            items = []
        }
    }
}

func loadDashboard() async throws -> Dashboard {
    async let user = userRepository.fetchCurrentUser()
    async let orders = orderRepository.fetchRecentOrders()
    async let banners = bannerRepository.fetchActiveBanners()

    return try await Dashboard(
        user: user,
        orders: orders,
        banners: banners
    )
}

func fetchDetails(for ids: [String]) async throws -> [ItemDetail] {
    try await withThrowingTaskGroup(of: ItemDetail.self) { group in
        for id in ids {
            group.addTask {
                try await self.repository.fetchDetail(id: id)
            }
        }
        return try await group.reduce(into: []) { $0.append($1) }
    }
}

import Synchronization

final class RequestCounter: Sendable {

    private let _count = Mutex(0)

    func increment() {
        _count.withLock { $0 += 1 }
    }

    var count: Int {
        _count.withLock { $0 }
    }
}

© nimblehq, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 4 other files (references) in skills/swift-concurrency of nimblehq/ios-templates.

• SKILL.md
• references/actors.md
• references/async-sequences.md
• references/sendability.md
• references/structured-concurrency.md

Open the folder on GitHubat commit f009d02