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Agent skill

Modern Swift

by johnrogers in johnrogers/claude-swift-engineering

A skill your agent uses when writing async/await code, enabling strict concurrency, fixing Sendable errors, migrating from completion handlers, managing shared state with actors, or using…

MITAuto-check passedMobile

Install Modern Swift

skills CLI
$ npx skills add johnrogers/claude-swift-engineering --skill modern-swift -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install johnrogers/claude-swift-engineering modern-swift --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/johnrogers/claude-swift-engineering.git skills-src && mkdir -p .claude/skills && cp -r skills-src/plugins/swift-engineering/skills/modern-swift .claude/skills/modern-swift && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
modern-swift
GitHub stars
231
Token cost
~1k tokens
SKILL.md length
447 words
Files
9 (incl. references)
Skills in repo
17
Repo updated
First seen
Licence
MIT

At a glance

A skill your agent uses when writing async/await code, enabling strict concurrency, fixing Sendable errors, migrating from completion handlers, managing shared state with actors, or using…

  • Works in 5 steps: Mark async functions with async, call… → Apply @MainActor to view models and… → Use actor instead of locks for shared… → …
  • Writing async/await code
  • SKILL.md covers Overview, Quick Reference, Core Workflow and Reference Loading Guide, plus 1 more section
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Modern Swift is an agent skill from johnrogers/claude-swift-engineering. Use when writing async/await code, enabling strict concurrency, fixing Sendable errors, migrating from completion handlers, managing shared state with actors, or using Task/TaskGroup for concurrency.

Its SKILL.md is about 1k tokens, which your agent loads only when the skill is triggered. The skill folder holds 9 other files, including reference files (for example `references/concurrency-essentials.md`, `references/macros.md` and `references/migration-patterns.md`).

It sits in Mobile, covering iOS development and Async programming. It works with Swift. The repository describes itself as: A collection of agents and skills to aid in the planning, implementation, documentation and testing of Swift/TCA code. The licence is MIT.

When your agent uses it

  • Writing async/await code
  • Enabling strict concurrency
  • Fixing Sendable errors
  • Migrating from completion handlers

Example prompts

  • “/modern-swift”

Workflow steps

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

  1. Mark async functions with async, call with await
  2. Apply @MainActor to view models and UI-updating code
  3. Use actor instead of locks for shared mutable state
  4. Check Task.isCancelled or call Task.checkCancellation() in loops
  5. Enable strict concurrency in Package.swift for compile-time safety

What it can do on your machine

Read from SKILL.md and the folder at commit 1dc2cf4. 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.

    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

Modern Swift loads about 1k tokens when it runs, and up to ~8.6k if it reads all its reference files. Until then it costs about 53 tokens; SKILL.md has 447 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~53
When it runs · the whole SKILL.md, loaded when a task matches
~1k
With references · SKILL.md plus every file in references/, read only if the agent opens them
~8.6k

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

Auto-check passed

The automated check found no risky patterns in SKILL.md.

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 johnrogers/claude-swift-engineering at commit 1dc2cf4, republished under its MIT licence (© johnrogers). 447 words, ~1,008 tokens.

Download SKILL.mdSave it as .claude/skills/modern-swift/SKILL.md (or your agent's skills folder). This skill also uses 8 other files; get the full folder from GitHub.
name
modern-swift
description
Use when writing async/await code, enabling strict concurrency, fixing Sendable errors, migrating from completion handlers, managing shared state with actors, or using Task/TaskGroup for concurrency.

Modern Swift (6.2+)

Swift 6.2 introduces strict compile-time concurrency checking with async/await, actors, and Sendable constraints that prevent data races at compile time instead of runtime. This is the foundation of safe concurrent Swift.

Overview

Modern Swift replaces older concurrency patterns (completion handlers, DispatchQueue, locks) with compiler-enforced safety. The core principle: if it compiles with strict concurrency enabled, it cannot have data races.

Quick Reference

NeedUseNOT
Async operationasync/awaitCompletion handlers
Main thread work@MainActorDispatchQueue.main
Shared mutable stateactorLocks, serial queues
Parallel tasksTaskGroupDispatchGroup
Thread safetySendable@unchecked everywhere

Core Workflow

When writing async Swift code:

  1. Mark async functions with async, call with await
  2. Apply @MainActor to view models and UI-updating code
  3. Use actor instead of locks for shared mutable state
  4. Check Task.isCancelled or call Task.checkCancellation() in loops
  5. Enable strict concurrency in Package.swift for compile-time safety

Reference Loading Guide

ALWAYS load reference files if there is even a small chance the content may be required. It's better to have the context than to miss a pattern or make a mistake.

ReferenceLoad When
Concurrency EssentialsWriting async code, converting completion handlers, using await
Swift 6 ConcurrencyUsing @concurrent, nonisolated(unsafe), or actor patterns
Task GroupsRunning multiple async operations in parallel
Task CancellationImplementing long-running or cancellable operations
Strict ConcurrencyEnabling Swift 6 strict mode or fixing Sendable errors
MacrosUsing or understanding Swift macros like @Observable
Modern AttributesMigrating legacy code or using @preconcurrency, @backDeployed
Migration PatternsModernizing delegate patterns or UIKit views
Show full SKILL.md (194 more words)Show less

Common Mistakes

  1. @unchecked Sendable as a quick fix — Using @unchecked Sendable to silence compiler errors means you've opted out of safety. If the error persists after @unchecked, your code has a potential data race. Fix the underlying issue instead.

  2. Missing await at call sites — Forgetting await when calling async functions is a compiler error, but checking Task.isCancelled in a loop without calling Task.checkCancellation() silently ignores cancellation.

  3. Capturing self in async blocks without weak — Holding a strong reference to self in a long-running async task prevents deinit. Always use [weak self] in closures or use .task which auto-manages the lifecycle.

  4. Not checking task cancellation — Long-running operations should regularly check Task.isCancelled or call Task.checkCancellation(), otherwise cancellation signals are ignored.

  5. Forgetting @MainActor on UI code and test suites — Main test struct and view models that update @Published properties need @MainActor. Forgetting it silently allows cross-thread mutations. Apply @MainActor to: view models, view structs, main test structs, and any type that touches UI.

  6. Actor re-entrancy surprises — await inside an actor method can release the lock temporarily. Another task may modify actor state. Design actor methods assuming state can change between await points.

© johnrogers, 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 8 other files (references) in plugins/swift-engineering/skills/modern-swift of johnrogers/claude-swift-engineering.

  • SKILL.md
  • references/concurrency-essentials.md
  • references/macros.md
  • references/migration-patterns.md
  • references/modern-attributes.md
  • references/strict-concurrency.md
  • references/swift6-concurrency.md
  • references/task-cancellation.md
  • references/task-groups.md

Open the folder on GitHubat commit 1dc2cf4

Compare with similar skills

Modern Swift next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Modern Swift compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Modern Swift this skilljohnrogers/claude-swift-engineering231—~1kAutomated safety check: PassMIT
Swift Concurrency Protwostraws/Swift-Concurrency-Agent-Skill566—~1.4kAutomated safety check: PassMIT
Swift ConcurrencyHoangNguyen0403/agent-skills-standard572—~545Automated safety check: PassMIT
Swift Concurrencyhenrypldev/react-native-nitro-mlx1003 repos~3.1kAutomated safety check: PassMIT
Swift Concurrencynimblehq/ios-templates110—~1.7kAutomated safety check: PassMIT
Swift Concurrencyjamesrochabrun/skills216—~1.9kAutomated safety check: PassMIT

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Works with

Categories

Questions about Modern Swift

What does Modern Swift do?

A skill your agent uses when writing async/await code, enabling strict concurrency, fixing Sendable errors, migrating from completion handlers, managing shared state with actors, or using…. Modern Swift is an agent skill from johnrogers/claude-swift-engineering. Use when writing async/await code, enabling strict concurrency, fixing Sendable errors, migrating from completion handlers, managing shared state with actors, or using Task/TaskGroup for concurrency.

When should I use Modern Swift?

Modern Swift fits situations like: writing async/await code; enabling strict concurrency; fixing Sendable errors; migrating from completion handlers.

How do I install Modern Swift in Claude Code?

Run `npx skills add johnrogers/claude-swift-engineering --skill modern-swift -a claude-code`. Or copy the skill folder (plugins/swift-engineering/skills/modern-swift in johnrogers/claude-swift-engineering) into .claude/skills/modern-swift in your project. Claude Code loads it when a task matches its description.

How do I install Modern Swift in Codex?

Run `npx skills add johnrogers/claude-swift-engineering --skill modern-swift -a codex`. Or copy the skill folder (plugins/swift-engineering/skills/modern-swift in johnrogers/claude-swift-engineering) into .agents/skills/modern-swift in your project. Codex loads it when a task matches its description.

Can I use Modern Swift in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add johnrogers/claude-swift-engineering --skill modern-swift -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/modern-swift, .gemini/skills/modern-swift, .github/skills/modern-swift and .opencode/skills/modern-swift in your project.

What does Modern Swift need to run?

SKILL.md names no scripts, command-line tools or credentials: Modern Swift is instructions for the agent only.

Does Modern Swift access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is Modern Swift safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Modern Swift use?

Modern Swift is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Modern Swift use?

About 1k tokens (SKILL.md is roughly 4k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 7.6k tokens, read only when the agent opens those files.

What are the alternatives to Modern Swift?

Skills that share tags, products or a category with Modern Swift: Swift Concurrency Pro (twostraws/Swift-Concurrency-Agent-Skill, 566 stars), Swift Concurrency (HoangNguyen0403/agent-skills-standard, 572 stars), Swift Concurrency (henrypldev/react-native-nitro-mlx, 100 stars) and Swift Concurrency (nimblehq/ios-templates, 110 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Modern Swift?

johnrogers (a GitHub user) maintains it in johnrogers/claude-swift-engineering, which has 231 GitHub stars. The repository holds 17 skills in this directory. The repository was last updated on January 18, 2026.

Source: johnrogers/claude-swift-engineering on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.