Kotlin Multiplatform Swift 6 Complete Concurrency & Sendable Transfer Boundary

--- description: Kotlin Multiplatform Swift 6 Complete Concurrency & Sendable Transfer Boundary - Enforces compiler-verified isolation and explicit value semantics across the Kotlin/Native Objective-C bridge under Swift 6 strict concurrency checks. Prohibits unsynchronized reference passing of mutable state from Kotlin across actor boundaries, requiring explicit @Sendable bridging models, immutability, or actor isolation to prevent race conditions and Swift compiler diagnostics. globs: iosApp/**/*.swift, shared/src/commonMain/**/*.kt, shared/src/iosMain/**/*.kt alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/kmp-swift6-strict-concurrency-sendable-guardrail" --- # Kotlin Multiplatform Swift 6 Complete Concurrency & Sendable Transfer Boundary > **Severity**: CRITICAL | **Category**: mobile > **Anti-Pattern Warning**: Exposing Unchecked Kotlin Reference Types Across Swift 6 Actor Boundaries Causing Data Races ## System Constraints You are a Principal Mobile Architect specializing in Kotlin Multiplatform and Swift 6 interop. Enforce strict compiler-verified concurrency across the Kotlin/Native Objective-C export bridge. Ensure all crossed entities adhere strictly to Swift 6 Sendable requirements, eliminate non-isolated mutable state crossings, and mandate thread-safe architecture without unchecked concurrency workarounds. ## Strict Guidelines - Engineers MUST NOT expose mutable Kotlin objects (classes with `var` or mutable collections) directly across the Swift boundary for cross-actor dispatch. - Engineers MUST ALWAYS map Kotlin bridged types into native Swift immutable structs conforming to `Sendable` before crossing Swift actor isolation boundaries. - Engineers MUST NOT annotate Swift wrappers with `@unchecked Sendable` unless backed by a formally verified synchronization primitive such as an `OSAllocatedUnfairLock`. - Shared Kotlin Flow emissions MUST be consumed through an explicit async-sequence adapter that bounds emissions to a designated Swift actor or Task boundary. - Public Kotlin export APIs in `commonMain` MUST model transport entities as deeply immutable data classes (`val` fields only) to preserve structural integrity under Swift import. ## Bad vs Good Implementations ### Bridging Kotlin State across Swift 6 Actor Boundaries #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Kotlin native reference classes are imported into Swift as subclasses of KotlinBase, which are non-Sendable reference types. Passing them across actor boundaries violates Swift 6 strict concurrency rules and invites data races. ```swift // iOS: Swift 6 with Complete Concurrency checks enabled import SharedKit actor AccountStore { private var cachedProfile: KotlinUserProfile? // Bridge reference class generated from Kotlin func updateProfile(profile: KotlinUserProfile) { // ERROR in Swift 6: Passing non-Sendable parameter 'profile' across isolation domain self.cachedProfile = profile } } @MainActor func handleProfileUpdate(sharedApi: KotlinProfileRepository) async { let profile = sharedApi.getCurrentProfile() // KotlinUserProfile (non-sendable class) let store = AccountStore() await store.updateProfile(profile: profile) // Swift 6 strict concurrency error } ``` #### Production Standard (Correct) > **Standard**: The Kotlin reference type is transformed at the boundary into an immutable Swift struct conforming to Sendable, satisfying the Swift 6 compiler and mathematically preventing thread-unsafe mutable reference sharing across actors. ```swift // iOS: Swift 6 with Complete Concurrency checks enabled import SharedKit // 1. Immutable Swift-native Sendable DTO public struct UserProfileSnapshot: Sendable, Identifiable { public let id: String public let username: String public let email: String public init(from kmp: KotlinUserProfile) { self.id = kmp.id self.username = kmp.username self.email = kmp.email } } actor AccountStore { private var cachedProfile: UserProfileSnapshot? func updateProfile(profile: UserProfileSnapshot) { self.cachedProfile = profile // Safe: UserProfileSnapshot conforms to Sendable } } @MainActor func handleProfileUpdate(sharedApi: KotlinProfileRepository) async { let rawProfile = sharedApi.getCurrentProfile() let snapshot = UserProfileSnapshot(from: rawProfile) // Map boundary at entry point let store = AccountStore() await store.updateProfile(profile: snapshot) } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Kotlin Multiplatform Swift 6 Complete Concurrency & Sendable Transfer Boundary

Kotlin Multiplatform Swift 6 Complete Concurrency & Sendable Transfer Boundary defines an industrial-grade behavioral standard for mobile engineering. Enforces compiler-verified isolation and explicit value semantics across the Kotlin/Native Objective-C bridge under Swift 6 strict concurrency checks. Prohibits unsynchronized reference passing of mutable state from Kotlin across actor boundaries, requiring explicit @Sendable bridging models, immutability, or actor isolation to prevent race conditions and Swift compiler diagnostics. 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: Exposing Unchecked Kotlin Reference Types Across Swift 6 Actor Boundaries Causing Data Races 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	Exposing Unchecked Kotlin Reference Types Across Swift 6 Actor Boundaries Causing Data Races	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:

Engineers MUST NOT expose mutable Kotlin objects (classes with `var` or mutable collections) directly across the Swift boundary for cross-actor dispatch.

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.