nucleus-apple-mcp: Skill for Codex

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

// 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" }
}