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mobilecritical PriorityID: kmp-swift6-strict-concurrency-sendable-arc-isolation

KMP to Swift 6 Strict Concurrency & ARC Actor Boundary Standard

Enforces thread-safe, deadlock-free interop between Kotlin Multiplatform (KMP) coroutines and Swift 6 Strict Concurrency. Eliminates ARC reference leaks, cross-runtime actor deadlocks, and Sendable violations across Kotlin/Native runtime boundaries and Swift cooperative thread pools.

Prohibited Anti-Pattern: Crossing Swift 6 Actor Boundaries with Raw Kotlin Reference Types and Unisolated Completion Closures

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---
description: KMP to Swift 6 Strict Concurrency & ARC Actor Boundary Standard - Enforces thread-safe, deadlock-free interop between Kotlin Multiplatform (KMP) coroutines and Swift 6 Strict Concurrency. Eliminates ARC reference leaks, cross-runtime actor deadlocks, and Sendable violations across Kotlin/Native runtime boundaries and Swift cooperative thread pools.
globs: iosApp/**/*.swift, shared/src/**/*.kt, **/*Bridge*.swift, **/*ViewModel*.swift
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/kmp-swift6-strict-concurrency-sendable-arc-isolation"
---

# KMP to Swift 6 Strict Concurrency & ARC Actor Boundary Standard

> **Severity**: CRITICAL | **Category**: mobile
> **Anti-Pattern Warning**: Crossing Swift 6 Actor Boundaries with Raw Kotlin Reference Types and Unisolated Completion Closures

## System Constraints
You are a Principal Mobile Architect enforcing zero-tolerance invariants for Swift 6 strict concurrency and Kotlin Multiplatform interop. Kotlin/Native objects bridged to Swift lack compile-time Sendable verification by default and participate in cross-runtime ARC management that will deadlock or corrupt memory if shared unsafely across Swift Tasks and Kotlin Coroutine dispatchers. You must mandate strict actor isolation, Sendable value-type boundary mapping, and explicit cooperative cancellation across all KMP-Swift boundaries.

## Strict Guidelines
- Engineers MUST NEVER expose raw mutable Kotlin classes across Swift Task boundaries without explicit actor isolation or value-type mapping.
- Engineers MUST transform bridged Kotlin Flows and Suspend functions into native Swift AsyncSequence or async/await primitives using SKIE or explicit Swift wrappers conforming to Sendable.
- Engineers MUST pin any Kotlin object bridging UI state explicitly to @MainActor on the Swift boundary to prevent cross-thread ARC deallocation races.
- Engineers MUST project Kotlin DTOs into native Swift structs conforming to Sendable before crossing Swift structured concurrency boundaries (TaskGroup, Task.detached).
- Engineers MUST NOT bridge Kotlin Job or CoroutineScope lifecycles into Swift through unmanaged ARC strong references; lifecycle bindings MUST use weak references or explicit onTermination cancellation handlers.
- Services MUST compile with -Xfrontend -strict-concurrency=complete in Swift 6 mode without using @unchecked Sendable as a workaround for Kotlin reference types.
- Components NEVER capture bridged Kotlin completion callbacks inside escaping Swift closures without marking closures as @Sendable and ensuring captured Kotlin state is thread-confined.

## Bad vs Good Implementations
### Bridging Kotlin StateFlow to Swift 6 Concurrency Boundary

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Kotlin reference types generated by Kotlin/Native are not Sendable. Capturing raw Kotlin objects in unstructured Swift Tasks causes runtime crashes under Swift 6 strict concurrency, data races on background worker threads, and memory leaks when ARC and Kotlin's GC race to deallocate the boundary bridge.
```swift
import SharedKit

// ANTI-PATTERN: Raw Kotlin reference crossing actor boundaries without Sendable or @MainActor
final class ProfileViewModel: ObservableObject {
    @Published var state: ProfileState? // Kotlin reference type
    private var cancellable: Cancellable? // Kotlin Job reference
    
    func observe(repository: KotlinUserRepository) {
        // Unsafe: Escaping non-Sendable closure crossing to background Task
        Task {
            self.cancellable = repository.userFlow.watch { [weak self] rawState in
                // DATA RACE & ARC HAZARD: Mutating UI state on non-isolated background thread
                // Kotlin reference deallocated across runtime boundary during Task cancellation
                self?.state = rawState
            }
        }
    }
}
```

#### Production Standard (Correct)
> **Standard**: The fix enforces @MainActor isolation for UI state mutation, maps the non-Sendable Kotlin state into a deeply immutable Swift Sendable struct at the ingestion point, and ensures two-way cancellation cleanup via AsyncStream.onTermination to release Kotlin Coroutine Jobs without ARC deadlocks.
```swift
import SharedKit

// PRODUCTION FIX: Explicit @MainActor boundary, Sendable Swift DTO, and bridged cancellation
struct UserProfile: Sendable, Identifiable {
    let id: String
    let name: String
    
    init(from kmpState: SharedKit.ProfileState) {
        self.id = kmpState.userId
        self.name = kmpState.displayName
    }
}

@MainActor
final class ProfileViewModel: ObservableObject {
    @Published private(set) var profile: UserProfile?
    private var observationTask: Task<Void, Never>?
    
    func observe(repository: KotlinUserRepository) {
        observationTask?.cancel()
        
        observationTask = Task { [weak self] in
            // SKIE or native AsyncStream projection ensuring cancellation propagation
            let stream = AsyncStream<UserProfile> { continuation in
                let job = repository.userFlow.watch { kmpState in
                    continuation.yield(UserProfile(from: kmpState))
                }
                continuation.onTermination = { @Sendable _ in
                    job.cancel()
                }
            }
            
            for await updatedProfile in stream {
                guard !Task.isCancelled else { break }
                self?.profile = updatedProfile
            }
        }
    }
    
    deinit {
        observationTask?.cancel()
    }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Xcode Build Server MCP
Inspects Swift compiler diagnostics and verifies zero warnings under -strict-concurrency=complete.
Recommended Agent Skills (askills.dev)

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

Swift Concurrency Boundary Auditor
AST validator checking Swift files for Sendable conformance violations and non-isolated KMP bridges.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: KMP to Swift 6 Strict Concurrency & ARC Actor Boundary Standard

KMP to Swift 6 Strict Concurrency & ARC Actor Boundary Standard defines an industrial-grade behavioral standard for mobile engineering. Enforces thread-safe, deadlock-free interop between Kotlin Multiplatform (KMP) coroutines and Swift 6 Strict Concurrency. Eliminates ARC reference leaks, cross-runtime actor deadlocks, and Sendable violations across Kotlin/Native runtime boundaries and Swift cooperative thread pools. 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: Crossing Swift 6 Actor Boundaries with Raw Kotlin Reference Types and Unisolated Completion Closures 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 ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftCrossing Swift 6 Actor Boundaries with Raw Kotlin Reference Types and Unisolated Completion ClosuresCritical
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 InconsistencyAI 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 NEVER expose raw mutable Kotlin classes across Swift Task boundaries without explicit actor isolation or value-type mapping.

Engineers MUST NEVER expose raw mutable Kotlin classes across Swift Task boundaries without explicit actor isolation or value-type mapping.

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 transform bridged Kotlin Flows and Suspend functions into native Swift AsyncSequence or async/await primitives using SKIE or explicit Swift wrappers conforming to Sendable.

Engineers MUST transform bridged Kotlin Flows and Suspend functions into native Swift AsyncSequence or async/await primitives using SKIE or explicit Swift wrappers conforming to 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.
3

Engineers MUST pin any Kotlin object bridging UI state explicitly to @MainActor on the Swift boundary to prevent cross-thread ARC deallocation races.

Engineers MUST pin any Kotlin object bridging UI state explicitly to @MainActor on the Swift boundary to prevent cross-thread ARC deallocation races.

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

Engineers MUST project Kotlin DTOs into native Swift structs conforming to Sendable before crossing Swift structured concurrency boundaries (TaskGroup, Task.detached).

Engineers MUST project Kotlin DTOs into native Swift structs conforming to Sendable before crossing Swift structured concurrency boundaries (TaskGroup, Task.detached).

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

Engineers MUST NOT bridge Kotlin Job or CoroutineScope lifecycles into Swift through unmanaged ARC strong references

Engineers MUST NOT bridge Kotlin Job or CoroutineScope lifecycles into Swift through unmanaged ARC strong references; lifecycle bindings MUST use weak references or explicit onTermination cancellation handlers.

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

Services MUST compile with -Xfrontend -strict-concurrency=complete in Swift 6 mode without using @unchecked Sendable as a workaround for Kotlin reference types.

Services MUST compile with -Xfrontend -strict-concurrency=complete in Swift 6 mode without using @unchecked Sendable as a workaround for Kotlin reference types.

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

Components NEVER capture bridged Kotlin completion callbacks inside escaping Swift closures without marking closures as @Sendable and ensuring captured Kotlin state is thread-confined.

Components NEVER capture bridged Kotlin completion callbacks inside escaping Swift closures without marking closures as @Sendable and ensuring captured Kotlin state is thread-confined.

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 KMP to Swift 6 Strict Concurrency & ARC Actor Boundary Standard critical for AI coding assistants?

Enforces thread-safe, deadlock-free interop between Kotlin Multiplatform (KMP) coroutines and Swift 6 Strict Concurrency. Eliminates ARC reference leaks, cross-runtime actor deadlocks, and Sendable violations across Kotlin/Native runtime boundaries and Swift cooperative thread pools. 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 (Crossing Swift 6 Actor Boundaries with Raw Kotlin Reference Types and Unisolated Completion Closures) 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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