swift-concurrency-expert – Coding skill for AI agents

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.

When to Use

• When the user asks to review Swift concurrency usage or fix compiler diagnostics.
• When you need guidance on actor isolation, Sendable, @MainActor, or async migration.

Workflow

1. Triage the issue

• Capture the exact compiler diagnostics and the offending symbol(s).
• Check project concurrency settings: compiler version (6.2+), Swift language mode (5 or 6), 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

// Before: mutable model lacks actor isolation; an actual diagnostic depends
// on how it crosses isolation boundaries and the target settings
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

// 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
extension Foo: @MainActor SomeProtocol {
    func protocolMethod() { /* safely accesses main-actor state */ }
}

Background work with @concurrent

// 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
// Prefer structured offloading with the @concurrent example below. A detached
// task is unstructured: it does not inherit actor isolation/task locals and parent
// cancellation is not automatically forwarded; retain/cancel its handle explicitly.

// 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.

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.

Verification boundaries

The compiler prevents many data races, not logical races, stale responses, deadlocks,

or incorrect business invariants across await. Revalidate mutable assumptions after

suspension. Sendable/thread safety must hold for every stored reference; do not silence

diagnostics with @unchecked Sendable or nonisolated(unsafe) as a routine fix.

The before/after snippets are alternatives with missing domain types/helpers, not

compiled evidence. Example: start request A then B, finish A last and verify A cannot

overwrite B’s selected UI state; cancellation alone may not prevent stale completion.