Swift Concurrency

Fast Path

Before proposing a fix:

Analyze Package.swift or .pbxproj to determine Swift language mode, strict concurrency level, default isolation, and upcoming features. Do this always, not only for migration work.
Capture the exact diagnostic and offending symbol.
Determine the isolation boundary: @MainActor, custom actor, actor instance isolation, or nonisolated.
Confirm whether the code is UI-bound or intended to run off the main actor. When spawning unstructured tasks, inspect the synchronous prefix (everything before the first await): start on @MainActor only when that prefix truly needs main-actor access; otherwise use Task { @concurrent in ... } and hop back with MainActor.run only after the suspension. A trivial non-main line (for example, print) followed by main-actor work in the same prefix is not a reason to use @concurrent. For delayed retries, timers, and backoff tasks, separate the waiting from the UI mutation. The sleep often belongs off the main actor even when the final state update belongs on it.

Project settings that change concurrency behavior:

Setting	SwiftPM (`Package.swift`)	Xcode (`.pbxproj`)
Language mode	`swiftLanguageVersions` or `-swift-version` (`// swift-tools-version:` is not a reliable proxy)	Swift Language Version
Strict concurrency	`.enableExperimentalFeature("StrictConcurrency=targeted")`	`SWIFT_STRICT_CONCURRENCY`
Default isolation	`.defaultIsolation(MainActor.self)`	`SWIFT_DEFAULT_ACTOR_ISOLATION`
Upcoming features	`.enableUpcomingFeature("NonisolatedNonsendingByDefault")`	`SWIFT_UPCOMING_FEATURE_*`