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mobilecritical PriorityID: swift-6-strict-concurrency-actor-invariants

Swift 6 Strict Concurrency Task-Isolated Actor Invariants

Enforces Swift 6 complete concurrency safety across shared-memory multi-core mobile architectures by eliminating data races at compile time. Prohibits cross-actor mutable reference escapes, requires explicit isolation boundaries, and mandates value semantic Sendable types or compile-time region isolation transfers. Guarantees deterministic actor isolation and prevents actor reentrancy hazards.

Prohibited Anti-Pattern: Escaping Non-Sendable Mutable References Across Actor Domains and Task Boundaries

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---
description: Swift 6 Strict Concurrency Task-Isolated Actor Invariants - Enforces Swift 6 complete concurrency safety across shared-memory multi-core mobile architectures by eliminating data races at compile time. Prohibits cross-actor mutable reference escapes, requires explicit isolation boundaries, and mandates value semantic Sendable types or compile-time region isolation transfers. Guarantees deterministic actor isolation and prevents actor reentrancy hazards.
globs: **/*.swift
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/swift-6-strict-concurrency-actor-invariants"
---

# Swift 6 Strict Concurrency Task-Isolated Actor Invariants

> **Severity**: CRITICAL | **Category**: mobile
> **Anti-Pattern Warning**: Escaping Non-Sendable Mutable References Across Actor Domains and Task Boundaries

## System Constraints
You are a Principal Mobile Architect enforcing Swift 6 Strict Concurrency and Actor Invariant Standards. Mandate compile-time verified data-race freedom across all asynchronous boundaries. Reject mutable reference types crossing actor domains, eliminate uncontrolled Task closure state captures, verify Sendable conformances structurally, and prevent actor reentrancy state corruption across suspension points (await).

## Strict Guidelines
- Engineers MUST ALWAYS enforce strict concurrency checking (`-strict-concurrency=complete` / Swift 6 mode) and resolve all isolation warnings as errors.
- Engineers MUST NOT pass non-Sendable reference types across actor boundaries or structured/unstructured `Task` closures without compile-time transfer of ownership via `sending` (SE-0414).
- Code SHOULD prefer immutable value types (`struct`, `enum`) for state payloads crossing isolation domains rather than annotating reference types with `@unchecked Sendable`.
- Services MUST re-validate internal actor invariants immediately after every suspension point (`await`), treating state across suspension as potentially invalidated by interleaved tasks.
- Components NEVER capture `var` state or mutable class instances inside escaping closures or asynchronous dispatch queues.
- Engineers MUST annotate UI-bound observable view-models and UI components with `@MainActor` globally rather than scattering ad-hoc `MainActor.run` invocations across callbacks.
- Engineers MUST NOT utilize `@unchecked Sendable` without a formal synchronization mechanism (e.g., `OSAllocatedUnfairLock`) and an explicit architectural justification documented in code review.

## Bad vs Good Implementations
### Cross-Actor Mutable Reference Escape vs. Sendable Value-Type Transfer

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: SessionConfig is a non-Sendable reference type. Returning it from an actor directly escapes the actor's isolation domain. Calling code or detached tasks can mutate SessionConfig concurrently with the actor's internal tasks, causing memory corruption and data races on multi-core ARM chips.
```swift
// CRITICAL FLAW: Escaping mutable non-Sendable reference crossing actor isolation
final class SessionConfig {
    var token: String
    var refreshInterval: TimeInterval
    init(token: String, refreshInterval: TimeInterval) {
        self.token = token
        self.refreshInterval = refreshInterval
    }
}

actor NetworkSessionManager {
    private var config: SessionConfig

    init(config: SessionConfig) {
        self.config = config
    }

    // Exposing mutable non-Sendable reference to caller creates concurrent race hazard
    func currentConfig() -> SessionConfig {
        return config
    }

    func update(token: String) async {
        // Actor reentrancy flaw: config state could be read while being mutated outside
        config.token = token
    }
}

@MainActor
func executeSync(manager: NetworkSessionManager) async {
    let config = await manager.currentConfig()
    Task.detached {
        // Concurrent modification of reference-typed session config outside actor boundary
        config.token = "expired"
    }
}
```

#### Production Standard (Correct)
> **Standard**: SessionConfig is modeled as an immutable Sendable value type (struct). State reads across actor isolation domains yield independent copies. Reentrancy checks are explicitly handled after suspension points (`await Task.sleep`), guaranteeing memory safety and invariant integrity without locks.
```swift
// PRODUCTION STANDARD: Value-semantic snapshotting and Swift 6 Sendable enforcement
struct SessionConfig: Sendable, Equatable {
    let token: String
    let refreshInterval: Duration
}

actor NetworkSessionManager {
    private(set) var config: SessionConfig

    init(config: SessionConfig) {
        self.config = config
    }

    func updateToken(_ newToken: String) {
        // Direct non-suspending atomic state transition within actor
        self.config = SessionConfig(token: newToken, refreshInterval: self.config.refreshInterval)
    }

    func performAuthenticatedSync(using client: isolated NetworkSessionManager) async throws {
        let snapshot = self.config
        // Guaranteed Sendable across suspension boundary
        try await Task.sleep(for: .seconds(1))
        // Re-verifying invariant after suspension point
        guard self.config == snapshot else {
            throw CancellationError()
        }
    }
}

@MainActor
func executeSync(manager: NetworkSessionManager) async throws {
    // Value-type copy: safe across boundaries with zero shared mutable memory
    let config = await manager.config
    Task.detached {
        let _ = config.token // Immutable local copy; data-race free by design
    }
}
```

Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Xcode Build Server MCP

Exposes SourceKit-LSP strict concurrency diagnostics and Swift 6 compiler diagnostic streams directly to coding agents.

Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Swift Concurrency Audit CLI

Static AST validator checking for unisolated mutable properties, unsafe Sendable conformances, and unchecked Task captures.

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Swift 6 Strict Concurrency Task-Isolated Actor Invariants

Swift 6 Strict Concurrency Task-Isolated Actor Invariants defines an industrial-grade behavioral standard for mobile engineering. Enforces Swift 6 complete concurrency safety across shared-memory multi-core mobile architectures by eliminating data races at compile time. Prohibits cross-actor mutable reference escapes, requires explicit isolation boundaries, and mandates value semantic Sendable types or compile-time region isolation transfers. Guarantees deterministic actor isolation and prevents actor reentrancy hazards. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Escaping Non-Sendable Mutable References Across Actor Domains and Task Boundaries Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Escaping Non-Sendable Mutable References Across Actor Domains and Task Boundaries	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS enforce strict concurrency checking (`-strict-concurrency=complete` / Swift 6 mode) and resolve all isolation warnings as errors.

Engineers MUST ALWAYS enforce strict concurrency checking (`-strict-concurrency=complete` / Swift 6 mode) and resolve all isolation warnings as errors.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

2

Engineers MUST NOT pass non-Sendable reference types across actor boundaries or structured/unstructured `Task` closures without compile-time transfer of ownership via `sending` (SE-0414).

Engineers MUST NOT pass non-Sendable reference types across actor boundaries or structured/unstructured `Task` closures without compile-time transfer of ownership via `sending` (SE-0414).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

3

Code SHOULD prefer immutable value types (`struct`, `enum`) for state payloads crossing isolation domains rather than annotating reference types with `@unchecked Sendable`.

Code SHOULD prefer immutable value types (`struct`, `enum`) for state payloads crossing isolation domains rather than annotating reference types with `@unchecked Sendable`.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

4

Services MUST re-validate internal actor invariants immediately after every suspension point (`await`), treating state across suspension as potentially invalidated by interleaved tasks.

Services MUST re-validate internal actor invariants immediately after every suspension point (`await`), treating state across suspension as potentially invalidated by interleaved tasks.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

5

Components NEVER capture `var` state or mutable class instances inside escaping closures or asynchronous dispatch queues.

Components NEVER capture `var` state or mutable class instances inside escaping closures or asynchronous dispatch queues.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

6

Engineers MUST annotate UI-bound observable view-models and UI components with `@MainActor` globally rather than scattering ad-hoc `MainActor.run` invocations across callbacks.

Engineers MUST annotate UI-bound observable view-models and UI components with `@MainActor` globally rather than scattering ad-hoc `MainActor.run` invocations across callbacks.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

7

Engineers MUST NOT utilize `@unchecked Sendable` without a formal synchronization mechanism (e.g., `OSAllocatedUnfairLock`) and an explicit architectural justification documented in code review.

Engineers MUST NOT utilize `@unchecked Sendable` without a formal synchronization mechanism (e.g., `OSAllocatedUnfairLock`) and an explicit architectural justification documented in code review.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Swift 6 Strict Concurrency Task-Isolated Actor Invariants critical for AI coding assistants?

Enforces Swift 6 complete concurrency safety across shared-memory multi-core mobile architectures by eliminating data races at compile time. Prohibits cross-actor mutable reference escapes, requires explicit isolation boundaries, and mandates value semantic Sendable types or compile-time region isolation transfers. Guarantees deterministic actor isolation and prevents actor reentrancy hazards. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Escaping Non-Sendable Mutable References Across Actor Domains and Task Boundaries) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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