1---
2name: swift-concurrency
3description: Diagnose Swift Concurrency issues, refactor callback-based code to async/await, and guide Swift 6 migration when working with tasks, actors, @MainActor, Sendable, data races, thread safety, or concurrency-related compiler and linter warnings.
4---
5# Swift Concurrency
6 
7## Fast Path
8 
9Before proposing a fix:
10 
111. 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.
122. Capture the exact diagnostic and offending symbol.
133. Determine the isolation boundary: `@MainActor`, custom actor, actor instance isolation, or `nonisolated`.
144. 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.
15 
16Project settings that change concurrency behavior:
17 
18| Setting | SwiftPM (`Package.swift`) | Xcode (`.pbxproj`) |
19|---|---|---|
20| Language mode | `swiftLanguageVersions` or `-swift-version` (`// swift-tools-version:` is not a reliable proxy) | Swift Language Version |
21| Strict concurrency | `.enableExperimentalFeature("StrictConcurrency=targeted")` | `SWIFT_STRICT_CONCURRENCY` |
22| Default isolation | `.defaultIsolation(MainActor.self)` | `SWIFT_DEFAULT_ACTOR_ISOLATION` |
23| Upcoming features | `.enableUpcomingFeature("NonisolatedNonsendingByDefault")` | `SWIFT_UPCOMING_FEATURE_*` |
24| Approachable Concurrency | N/A (use individual upcoming features) | `SWIFT_APPROACHABLE_CONCURRENCY` |
25 
26> **Xcode 26 note**: New projects created in Xcode 26 will often start with `SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor` and `SWIFT_APPROACHABLE_CONCURRENCY = YES` enabled by default. Treat these as likely defaults for newly created projects, not as confirmed settings.
27 
28If any of these are unknown, ask the developer to confirm them before giving migration-sensitive guidance. Do not guess, even for new Xcode 26 projects.
29 
30Guardrails:
31 
32- Do not recommend `@MainActor` as a blanket fix. Justify why the code is truly UI-bound.
33- Prefer structured concurrency over unstructured tasks. Use `Task.detached` only with a clear reason.
34- If recommending `@preconcurrency`, `@unchecked Sendable`, or `nonisolated(unsafe)`, require a documented safety invariant and a follow-up removal plan.
35- Optimize for the smallest safe change. Do not refactor unrelated architecture during migration.
36- Course references are for deeper learning only. Use them sparingly and only when they clearly help answer the developer's question.
37 
38## Quick Fix Mode
39 
40Use Quick Fix Mode when all of these are true:
41 
42- The issue is localized to one file or one type.
43- The isolation boundary is clear.
44- The fix can be explained in 1-2 behavior-preserving steps.
45 
46Skip Quick Fix Mode when any of these are true:
47 
48- Build settings or default isolation are unknown.
49- The issue crosses module boundaries or changes public API behavior.
50- The likely fix depends on unsafe escape hatches.
51 
52## Common Diagnostics
53 
54| Diagnostic | First check | Smallest safe fix | Escalate to |
55|---|---|---|---|
56| `Main actor-isolated ... cannot be used from a nonisolated context` | Is this truly UI-bound? | Isolate the caller to `@MainActor` or use `await MainActor.run { ... }` only when main-actor ownership is correct. | `references/actors.md`, `references/threading.md` |
57| `Actor-isolated type does not conform to protocol` | Must the requirement run on the actor? | Prefer isolated conformance (e.g., `extension Foo: @MainActor SomeProtocol`); use `nonisolated` only for truly nonisolated requirements. | `references/actors.md` |
58| `Sending value of non-Sendable type ... risks causing data races` | What isolation boundary is being crossed? | Keep access inside one actor, or convert the transferred value to an immutable/value type. | `references/sendable.md`, `references/threading.md` |
59| `SwiftLint async_without_await` | Is `async` actually required by protocol, override, or `@concurrent`? | Remove `async`, or use a narrow suppression with rationale. Never add fake awaits. | `references/linting.md` |
60| `wait(...) is unavailable from asynchronous contexts` | Is this legacy XCTest async waiting? | Replace with `await fulfillment(of:)` or Swift Testing equivalents. | `references/testing.md` |
61| Core Data concurrency warnings | Are `NSManagedObject` instances crossing contexts or actors? | Pass `NSManagedObjectID` or map to a Sendable value type. | `references/core-data.md` |
62| `Thread.current` unavailable from asynchronous contexts | Are you debugging by thread instead of isolation? | Reason in terms of isolation and use Instruments/debugger instead. | `references/threading.md` |
63| SwiftLint concurrency-related warnings | Which specific lint rule triggered? | Use `references/linting.md` for rule intent and preferred fixes; avoid dummy awaits. | `references/linting.md` |
64| `... cannot satisfy conformance requirement for a 'Sendable' type parameter` (`SendableMetatype`) | Does the conformance carry global-actor isolation? | Remove actor isolation from the conformance, or avoid passing the metatype across isolation boundaries. See `SendableMetatype` section in `references/actors.md`. | `references/actors.md` |
65 
66## When Quick Fixes Fail
67 
681. Gather project settings if not already confirmed.
692. Re-evaluate which isolation boundaries the type crosses.
703. Route to the matching reference file for a deeper fix.
714. If the fix may change behavior, document the invariant and add verification steps.
72 
73## Smallest Safe Fixes
74 
75Prefer changes that preserve behavior while satisfying data-race safety:
76 
77- **UI-bound state**: isolate the type or member to `@MainActor`.
78- **Shared mutable state**: move it behind an `actor`, or use `@MainActor` only if the state is UI-owned.
79- **Background work**: when work must hop off caller isolation, use an `async` API marked `@concurrent`; when work can safely inherit caller isolation, use `nonisolated` without `@concurrent`. When spawning a `Task`, match entry isolation to its synchronous prefix. If nothing before the first `await` needs the main actor, use `Task { @concurrent in ... }` and hop back via `await MainActor.run { ... }` for the UI update. If the prefix mixes a trivial non-main statement with main-actor work, keep the inherited `@MainActor` start—splitting the cheap line off-main is not worth an extra hop.
80- **Sendability issues**: prefer immutable values and explicit boundaries over `@unchecked Sendable`.
81 
82## Concurrency Tool Selection
83 
84| Need | Tool | Key Guidance |
85|---|---|---|
86| Single async operation | `async/await` | Default choice for sequential async work |
87| Fixed parallel operations | `async let` | Known count at compile time; auto-cancelled on throw |
88| Dynamic parallel operations | `withTaskGroup` | Unknown count; structured — cancels children on scope exit |
89| Sync → async bridge | `Task { }` | Inherits actor context; use `Task.detached` only with documented reason |
90| Shared mutable state | `actor` | Prefer over locks/queues; keep isolated sections small |
91| UI-bound state | `@MainActor` | Only for truly UI-related code; justify isolation |
92 
93### Common Scenarios
94 
95**Network request with UI update**
96```swift
97Task { @concurrent in
98    let data = try await fetchData()
99    await MainActor.run { self.updateUI(with: data) }
100}
101```
102 
103**Processing array items in parallel**
104```swift
105await withTaskGroup(of: ProcessedItem.self) { group in
106    for item in items {
107        group.addTask { await process(item) }
108    }
109    for await result in group {
110        results.append(result)
111    }
112}
113```
114 
115 
116## Task entry isolation
117 
118Match a `Task`'s entry isolation to its synchronous prefix (everything from `{` to the first `await`).
119 
120- If anything in that prefix needs `@MainActor`, keep the inherited `@MainActor` start.
121- If nothing in that prefix needs `@MainActor`, prefer `Task { @concurrent in ... }` and hop back only for UI-owned mutation.
122 
123```swift
124// ❌ Synchronous prefix is empty; first work hops away
125Task {
126    await hopToOtherIsolationDomain()
127}
128 
129// ❌ Synchronous prefix is only `print` (trivial, non-main); first await hops away
130Task {
131    print("Also not main-thread-bound")
132    await hopToOtherIsolationDomain()
133}
134 
135// ✅ Start off the main actor, hop back only for UI work
136Task { @concurrent in
137    await hopToOtherIsolationDomain()
138    await MainActor.run { updateUI() }
139}
140 
141// ✅ Synchronous prefix DOES contain main-actor work — keep inheritance
142Task {
143    print("debug")              // trivial, non-main — rides along
144    self.isLoading = true       // needs @MainActor, before any await
145    await fetchData()
146}
147```
148 
149## Swift 6 Migration Quick Guide
150 
151Key changes in Swift 6:
152- **Strict concurrency checking** enabled by default
153- **Complete data-race safety** at compile time
154- **Sendable requirements** enforced on boundaries
155- **Isolation checking** for all async boundaries
156 
157### Migration Validation Loop
158 
159Apply this cycle for each migration change:
160 
1611. **Build** — Run `swift build` or Xcode build to surface new diagnostics
1622. **Fix** — Address one category of error at a time (e.g., all Sendable issues first)
1633. **Rebuild** — Confirm the fix compiles cleanly before moving on
1644. **Test** — Run the test suite to catch regressions (`swift test` or Cmd+U)
1655. **Only proceed** to the next file/module when all diagnostics are resolved
166 
167If a fix introduces new warnings, resolve them before continuing. Never batch multiple unrelated fixes — keep commits small and reviewable.
168 
169For detailed migration steps, see `references/migration.md`.
170 
171## Reference Router
172 
173Open the smallest reference that matches the question:
174 
175- Foundations
176  - `references/async-await-basics.md` — async/await syntax, execution order, async let, URLSession patterns
177  - `references/tasks.md` — Task lifecycle, cancellation, priorities, task groups, structured vs unstructured
178  - `references/actors.md` — Actor isolation, @MainActor, global actors, reentrancy, custom executors, Mutex
179  - `references/sendable.md` — Sendable conformance, value/reference types, @unchecked, region isolation
180  - `references/threading.md` — Execution model, suspension points, Swift 6.2 isolation behavior
181- Streams
182  - `references/async-sequences.md` — AsyncSequence, AsyncStream, when to use vs regular async methods
183  - `references/async-algorithms.md` — Debounce, throttle, merge, combineLatest, channels, timers
184- Applied topics
185  - `references/testing.md` — Swift Testing first, XCTest fallback, leak checks
186  - `references/performance.md` — Profiling with Instruments, reducing suspension points, execution strategies
187  - `references/memory-management.md` — Retain cycles in tasks, memory safety patterns
188  - `references/core-data.md` — NSManagedObject sendability, custom executors, isolation conflicts
189- Migration and tooling
190  - `references/migration.md` — Swift 6 migration strategy, closure-to-async conversion, @preconcurrency, FRP migration
191  - `references/linting.md` — Concurrency-focused lint rules and SwiftLint `async_without_await`
192- Glossary
193  - `references/glossary.md` — Quick definitions of core concurrency terms
194 
195## Verification Checklist
196 
197When changing concurrency code:
198 
1991. Re-check build settings before interpreting diagnostics.
2002. Build and clear one category of errors before moving on. Do not batch unrelated fixes into the same change.
2013. Run tests, especially actor-, lifetime-, and cancellation-sensitive tests.
2024. Use Instruments for performance claims instead of guessing.
2035. Verify deallocation and cancellation behavior for long-lived tasks.
2046. Check `Task.isCancelled` in long-running operations.
2057. Never use semaphores or ad hoc locking in async contexts when actor isolation or `Mutex` would express ownership more safely.
206 
207---
208 
209**Note**: This skill is based on the comprehensive [Swift Concurrency Course](https://www.swiftconcurrencycourse.com?utm_source=github&utm_medium=agent-skill&utm_campaign=skill-footer) by Antoine van der Lee.
210