swift-concurrency-expert

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

Audit 0.4.1 · source sha 3c36442853dc · 29 Oct 2026Report an issue or request a re-scan

• Publisherdimillian GitHub userOwner of the repository this skill is read from
• Source folderdimillian/skills/swift-concurrency-expertThe exact folder we audited
• LicenceMITAs the repository states it
• Commit pinned for this verdict3c36442853dcA new audit runs when the source changes
• Repository statusNot archivedLast push 213 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: Swift Concurrency review and remediation for Swift 6.2+. Use when asked to review Swift Concurrency usage, improve concurrency compliance, or fix Swift concurrency compiler errors in a feature or file. Concrete actions…

---
name: swift-concurrency-expert
description: Swift Concurrency review and remediation for Swift 6.2+. Use when asked to review Swift Concurrency usage, improve concurrency compliance, or fix Swift concurrency compiler errors in a feature or file. Concrete actions include adding Sendable conformance, applying @MainActor annotations, resolving actor isolation warnings, fixing data race diagnostics, and migrating completion handlers to async/await.
---

# Swift Concurrency Expert

## Overview

Review and fix Swift Concurrency issues in Swift 6.2+ codebases by applying actor isolation, Sendable safety, and modern concurrency patterns with minimal behavior changes.

## Workflow

### 1. Triage the issue

- Capture the exact compiler diagnostics and the offending symbol(s).
- Check project concurrency settings: Swift language version (6.2+), strict concurrency level, and whether approachable concurrency (default actor isolation / main-actor-by-default) is enabled.
- Identify the current actor context (`@MainActor`, `actor`, `nonisolated`) and whether a default actor isolation mode is enabled.
- Confirm whether the code is UI-bound or intended to run off the main actor.

### 2. Apply the smallest safe fix

Prefer edits that preserve existing behavior while satisfying data-race safety.

Common fixes:
- **UI-bound types**: annotate the type or relevant members with `@MainActor`.
- **Protocol conformance on main actor types**: make the conformance isolated (e.g., `extension Foo: @MainActor SomeProtocol`).
- **Global/static state**: protect with `@MainActor` or move into an actor.
- **Background work**: move expensive work into a `@concurrent` async function on a `nonisolated` type or use an `actor` to guard mutable state.
- **Sendable errors**: prefer immutable/value types; add `Sendable` conformance only when correct; avoid `@unchecked Sendable` unless you can prove thread safety.

### 3. Verify the fix

- Rebuild and confirm all concurrency diagnostics are resolved with no new warnings introduced.
- Run the test suite to check for regressions — concurrency changes can introduce subtle runtime issues even when the build is clean.
- If the fix surfaces new warnings, treat each one as a fresh triage (return to step 1) and resolve iteratively until the build is clean and tests pass.

### Examples

**UI-bound type — adding `@MainActor`**

```swift
// Before: data-race warning because ViewModel is accessed from the main thread
// but has no actor isolation
class ViewModel: ObservableObject {
    @Published var title: String = ""
    func load() { title = "Loaded" }
}

// After: annotate the whole type so all stored state and methods are
// automatically isolated to the main actor
@MainActor
class ViewModel: ObservableObject {
    @Published var title: String = ""
    func load() { title = "Loaded" }
}
```

**Protocol conformance isolation**

```swift
// Before: compiler error — SomeProtocol method is nonisolated but the
// conforming type is @MainActor
@MainActor
class Foo: SomeProtocol {
    func protocolMethod() { /* accesses main-actor state */ }
}

// After: scope the conformance to @MainActor so the requirement is
// satisfied inside the correct isolation context
@MainActor
extension Foo: SomeProtocol {
    func protocolMethod() { /* safely accesses main-actor state */ }
}
```

**Background work with `@concurrent`**

```swift
// Before: expensive computation blocks the main actor
@MainActor
func processData(_ input: [Int]) -> [Int] {
    input.map { heavyTransform($0) }   // runs on main thread
}

// After: hop off the main actor for the heavy work, then return the result
// The caller awaits the result and stays on its own actor
nonisolated func processData(_ input: [Int]) async -> [Int] {
    await Task.detached(priority: .userInitiated) {
        input.map { heavyTransform($0) }
    }.value
}

// Or, using a @concurrent async function (Swift 6.2+):
@concurrent
func processData(_ input: [Int]) async -> [Int] {
    input.map { heavyTransform($0) }
}
```

## Reference material

- See `references/swift-6-2-concurrency.md` for Swift 6.2 changes, patterns, and examples.
- See `references/approachable-concurrency.md` when the project is opted into approachable concurrency mode.
- See `references/swiftui-concurrency-tour-wwdc.md` for SwiftUI-specific concurrency guidance.

Excerpt, first 4,340 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/dimillian/skills
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Works in

1. 2026-10-29
   ✓SAFE B 89 / 100

   first audit · source 3c36442853dc

Questions about swift-concurrency-expert

• github.com/dimillian/skills/tree/main/swift-concurrency-expert — the audited source at commit 3c36442853dc and every finding aboveObserved · 29 Oct 2026

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