swift-concurrency-pro

name	swift-concurrency-pro
description	Reviews Swift code for concurrency correctness, modern API usage, and common async/await pitfalls. Use when reading, writing, or reviewing Swift concurrency code.
license	MIT
metadata	{"author":"Paul Hudson","version":"1.0"}

Source: vendored from twostraws/Swift-Concurrency-Agent-Skill (Paul Hudson, MIT). Discovered via Paul Solt's roundup tweet: https://x.com/PaulSolt/article/2042716870512353294 — check there for updates and the other skill packs.

Review Swift concurrency code for correctness, modern API usage, and adherence to project conventions. Report only genuine problems - do not nitpick or invent issues.

Review process:

1. Scan for known-dangerous patterns using references/hotspots.md to prioritize what to inspect.
2. Check for recent Swift 6.2 concurrency behavior using references/new-features.md.
3. Validate actor usage for reentrancy and isolation correctness using references/actors.md.
4. Ensure structured concurrency is preferred over unstructured where appropriate using references/structured.md.
5. Check unstructured task usage for correctness using references/unstructured.md.
6. Verify cancellation is handled correctly using references/cancellation.md.
7. Validate async stream and continuation usage using references/async-streams.md.
8. Check bridging code between sync and async worlds using references/bridging.md.
9. Review any legacy concurrency migrations using references/interop.md.
10. Cross-check against common failure modes using references/bug-patterns.md.
11. If the project has strict-concurrency errors, map diagnostics to fixes using references/diagnostics.md.
12. If reviewing tests, check async test patterns using references/testing.md.

If doing a partial review, load only the relevant reference files.

Core Instructions

• Target Swift 6.2 or later with strict concurrency checking.
• If code spans multiple targets or packages, compare their concurrency build settings before assuming behavior should match.
• Prefer structured concurrency (task groups) over unstructured (Task {}).
• Prefer Swift concurrency over Grand Central Dispatch for new code. GCD is still acceptable in low-level code, framework interop, or performance-critical synchronous work where queues and locks are the right tool – don't flag these as errors.
• If an API offers both async/await and closure-based variants, always prefer async/await.
• Do not introduce third-party concurrency frameworks without asking first.
• Do not suggest @unchecked Sendable to fix compiler errors. It silences the diagnostic without fixing the underlying race. Prefer actors, value types, or sending parameters instead. The only legitimate use is for types with internal locking that are provably thread-safe.

Output Format

Organize findings by file. For each issue:

1. State the file and relevant line(s).
2. Name the rule being violated.
3. Show a brief before/after code fix.

Skip files with no issues. End with a prioritized summary of the most impactful changes to make first.

Example output:

DataLoader.swift

Line 18: Actor reentrancy – state may have changed across the await.

// Before
actor Cache {
    var items: [String: Data] = [:]

    func fetch(_ key: String) async throws -> Data {
        if items[key] == nil {
            items[key] = try await download(key)
        }
        return items[key]!
    }
}

// After
actor Cache {
    var items: [String: Data] = [:]

    func fetch(_ key: String) async throws -> Data {
        if let existing = items[key] { return existing }
        let data = try await download(key)
        items[key] = data
        return data
    }
}

Line 34: Use withTaskGroup instead of creating tasks in a loop.

// Before
for url in urls {
    Task { try await fetch(url) }
}

// After
try await withThrowingTaskGroup(of: Data.self) { group in
    for url in urls {
        group.addTask { try await fetch(url) }
    }

    for try await result in group {
        process(result)
    }
}

Summary

1. Correctness (high): Actor reentrancy bug on line 18 may cause duplicate downloads and a force-unwrap crash.
2. Structure (medium): Unstructured tasks in loop on line 34 lose cancellation propagation.

End of example.

References

• references/hotspots.md - Grep targets for code review: known-dangerous patterns and what to check for each.
• references/new-features.md - Swift 6.2 changes that alter review advice: default actor isolation, isolated conformances, caller-actor async behavior, @concurrent, Task.immediate, task naming, and priority escalation.
• references/actors.md - Actor reentrancy, shared-state annotations, global actor inference, and isolation patterns.
• references/structured.md - Task groups over loops, discarding task groups, concurrency limits.
• references/unstructured.md - Task vs Task.detached, when Task {} is a code smell.
• references/cancellation.md - Cancellation propagation, cooperative checking, broken cancellation patterns.
• references/async-streams.md - AsyncStream factory, continuation lifecycle, back-pressure.
• references/bridging.md - Checked continuations, wrapping legacy APIs, @unchecked Sendable.
• references/interop.md - Migrating from GCD, Mutex/locks, completion handlers, delegates, and Combine.
• references/bug-patterns.md - Common concurrency failure modes and their fixes.
• references/diagnostics.md - Strict-concurrency compiler errors, protocol conformance fixes, and likely remedies.
• references/testing.md - Async test strategy with Swift Testing, race detection, avoiding timing-based tests.