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mobilecritical PriorityID: kmp-2-1-direct-swift-export-abi-stability-standard

Kotlin Multiplatform 2.1 Direct Swift Export ABI Stability and Strict Concurrency Guardrail

Enforces deterministic layout invariants and zero-overhead memory bridge semantics under Kotlin 2.1 Direct Swift Export. Mandates strict finality, value-type boxing elimination, and boundary isolation to comply with Swift 6 strict concurrency checks without bridging via Objective-C runtime thunks.

Prohibited Anti-Pattern: Leaking Open Classes, Unfrozen Shared State, or ObjC Boxed Dynamic Dispatches Across Swift 6 Strict Concurrency Boundaries

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---
description: Kotlin Multiplatform 2.1 Direct Swift Export ABI Stability and Strict Concurrency Guardrail - Enforces deterministic layout invariants and zero-overhead memory bridge semantics under Kotlin 2.1 Direct Swift Export. Mandates strict finality, value-type boxing elimination, and boundary isolation to comply with Swift 6 strict concurrency checks without bridging via Objective-C runtime thunks.
globs: shared/src/commonMain/kotlin/**/*.kt, shared/src/iosMain/kotlin/**/*.kt, iosApp/**/*.swift
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/kmp-2-1-direct-swift-export-abi-stability-standard"
---

# Kotlin Multiplatform 2.1 Direct Swift Export ABI Stability and Strict Concurrency Guardrail

> **Severity**: CRITICAL | **Category**: mobile
> **Anti-Pattern Warning**: Leaking Open Classes, Unfrozen Shared State, or ObjC Boxed Dynamic Dispatches Across Swift 6 Strict Concurrency Boundaries

## System Constraints
You are a Principal Mobile Architect specializing in Kotlin Multiplatform (KMP) 2.1 and native Swift 6 interoperability. Your role is to strictly enforce direct Swift Export ABI stability rules, eliminate legacy Objective-C runtime bridging overhead, and ensure rigid Swift Sendable/actor boundary isolation.

## Strict Guidelines
- Engineers MUST ALWAYS mark exported Kotlin types intended for direct Swift Export as 'final class', 'value class', or immutable data structures with immutable val properties to guarantee Sendable conformance.
- Engineers MUST NOT expose open or unsealed polymorphic class hierarchies across the direct Swift boundary when dynamic dispatch forces fallback to legacy Objective-C runtime thunking.
- Code MUST explicitly isolate asynchronous Kotlin Coroutine flows behind structured async/await Swift Export declarations decorated with @Exported or explicit Swift Sendable annotations.
- Services MUST prevent mutable shared reference graphs across Swift 6 concurrency domains by utilizing KMP immutable state representations or isolation via native Kotlin Worker/Swift Actor patterns.
- Components NEVER bridge generic raw primitive type parameters without explicit reified types or sealed type wrappers, avoiding runtime boxing into bridge-layer AnyObject proxies.

## Bad vs Good Implementations
### Direct Swift Export ABI Invariant: Open Class Bridge vs. Sealed Value-Safe Boundary

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: An open class with mutable vars and unsealed inheritance forces legacy Objective-C dynamic dispatch metadata, lacks ABI layout finality, and produces non-Sendable Swift bridging entities that trigger compiler errors under Swift 6 strict concurrency.
```kotlin
// Shared KMP Code: Triggers ObjC dynamic thunking and fails Swift 6 strict concurrency checks
package com.production.domain

open class UserSession(
    var accessToken: String,
    var permissions: List<String>
) {
    open fun updateToken(newToken: String) {
        this.accessToken = newToken
    }
}
```

#### Production Standard (Correct)
> **Standard**: Declaring the class final with immutable val properties allows Direct Swift Export in Kotlin 2.1 to emit direct C/Swift vtable lookups and non-mutating Swift structs/final Sendable classes, eliminating runtime Objective-C bridge overhead.
```kotlin
// Shared KMP Code: Direct Swift Export compatible, final, immutable, and Sendable compliant
package com.production.domain

import kotlin.experimental.ExperimentalNativeApi

@OptIn(ExperimentalNativeApi::class)
class UserSession private constructor(
    val accessToken: String,
    val permissions: List<String>
) {
    companion object {
        fun create(accessToken: String, permissions: List<String>): UserSession {
            return UserSession(accessToken, permissions.toList())
        }
    }

    fun copyWithToken(newToken: String): UserSession {
        return UserSession(newToken, this.permissions)
    }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

KMP ABI Inspector MCP
Analyzes compiled klib artifacts and Swift bridging headers to detect unintended Objective-C dynamic dispatch thunks and ABI layout instability.
Recommended Agent Skills (askills.dev)

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

Swift 6 Concurrency Checker
Runs swiftc with -strict-concurrency=complete to validate all exported Kotlin Swift wrappers satisfy isolation requirements.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Kotlin Multiplatform 2.1 Direct Swift Export ABI Stability and Strict Concurrency Guardrail

Kotlin Multiplatform 2.1 Direct Swift Export ABI Stability and Strict Concurrency Guardrail defines an industrial-grade behavioral standard for mobile engineering. Enforces deterministic layout invariants and zero-overhead memory bridge semantics under Kotlin 2.1 Direct Swift Export. Mandates strict finality, value-type boxing elimination, and boundary isolation to comply with Swift 6 strict concurrency checks without bridging via Objective-C runtime thunks. 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: Leaking Open Classes, Unfrozen Shared State, or ObjC Boxed Dynamic Dispatches Across Swift 6 Strict Concurrency 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 ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftLeaking Open Classes, Unfrozen Shared State, or ObjC Boxed Dynamic Dispatches Across Swift 6 Strict Concurrency BoundariesCritical
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 ALWAYS mark exported Kotlin types intended for direct Swift Export as 'final class', 'value class', or immutable data structures with immutable val properties to guarantee Sendable conformance.

Engineers MUST ALWAYS mark exported Kotlin types intended for direct Swift Export as 'final class', 'value class', or immutable data structures with immutable val properties to guarantee Sendable conformance.

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 expose open or unsealed polymorphic class hierarchies across the direct Swift boundary when dynamic dispatch forces fallback to legacy Objective-C runtime thunking.

Engineers MUST NOT expose open or unsealed polymorphic class hierarchies across the direct Swift boundary when dynamic dispatch forces fallback to legacy Objective-C runtime thunking.

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 MUST explicitly isolate asynchronous Kotlin Coroutine flows behind structured async/await Swift Export declarations decorated with @Exported or explicit Swift Sendable annotations.

Code MUST explicitly isolate asynchronous Kotlin Coroutine flows behind structured async/await Swift Export declarations decorated with @Exported or explicit Swift Sendable annotations.

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 prevent mutable shared reference graphs across Swift 6 concurrency domains by utilizing KMP immutable state representations or isolation via native Kotlin Worker/Swift Actor patterns.

Services MUST prevent mutable shared reference graphs across Swift 6 concurrency domains by utilizing KMP immutable state representations or isolation via native Kotlin Worker/Swift Actor patterns.

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 bridge generic raw primitive type parameters without explicit reified types or sealed type wrappers, avoiding runtime boxing into bridge-layer AnyObject proxies.

Components NEVER bridge generic raw primitive type parameters without explicit reified types or sealed type wrappers, avoiding runtime boxing into bridge-layer AnyObject proxies.

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 Kotlin Multiplatform 2.1 Direct Swift Export ABI Stability and Strict Concurrency Guardrail critical for AI coding assistants?

Enforces deterministic layout invariants and zero-overhead memory bridge semantics under Kotlin 2.1 Direct Swift Export. Mandates strict finality, value-type boxing elimination, and boundary isolation to comply with Swift 6 strict concurrency checks without bridging via Objective-C runtime thunks. 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 (Leaking Open Classes, Unfrozen Shared State, or ObjC Boxed Dynamic Dispatches Across Swift 6 Strict Concurrency 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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