Swift Concurrency

Swift concurrency

Read Package.swift and references/concurrency-checklist.md. Every target uses Swift 6 language mode with MainActor default isolation, InferIsolatedConformances, and NonisolatedNonsendingByDefault. Use this skill for every asynchronous service, observable model, stream, task group, actor, sendability, nonisolated, or @concurrent change.

Workflow

1. Identify mutable state and assign one isolation domain.
2. Keep UI-facing state on @MainActor.
3. Make dependency protocols and cross-task values Sendable.
4. Prefer structured async functions and task groups to callbacks or detached work.
5. Define which scope owns each Task and when cancellation occurs.
6. Propagate cancellation; do not convert it into a user-facing failure accidentally.
7. Prevent stale responses from overwriting newer state.
8. Inject clocks or continuations for deterministic tests instead of sleeping.
9. Run make test and make package-build with Swift 6.2; fix diagnostics at the boundary rather than weakening package settings or using broad suppressions.

Reject

• DispatchQueue.main.async and global queues in new Swift concurrency code.
• Task.sleep(nanoseconds:); use Task.sleep(for:) or an injected clock.
• Task.detached without a documented isolation/priority reason.
• mutable shared state outside an actor/global actor/explicit synchronisation mechanism.
• broad @unchecked Sendable; a narrow suppression requires a written thread-safety proof.
• retaining unstructured tasks without cancellation or lifecycle ownership.
• swallowing errors or cancellation.

Review output

State the isolation of each mutable object, the task owner, cancellation path, and any value crossing an actor boundary. Prioritise data races and stale writes over stylistic concerns.