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mobilehigh PriorityID: kmp-compose-multiplatform

Kotlin Multiplatform (KMP) Architecture Standard

Guardrails for Kotlin Multiplatform: isolate platform specifics behind expect/actual, share 100% of domain and data layers, and manage Ktor HTTP client engines per platform.

Prohibited Anti-Pattern: AI leaks Android/iOS platform APIs into commonMain, causing multiplatform compilation failure.

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---
description: Kotlin Multiplatform (KMP) Architecture Standard - Guardrails for Kotlin Multiplatform: isolate platform specifics behind expect/actual, share 100% of domain and data layers, and manage Ktor HTTP client engines per platform.
globs: commonMain/**/*.kt, androidMain/**/*.kt, iosMain/**/*.kt
alwaysApply: false
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/kmp-compose-multiplatform"
---

# Kotlin Multiplatform (KMP) Architecture Standard

> **Severity**: HIGH | **Category**: mobile
> **Anti-Pattern Warning**: AI leaks Android/iOS platform APIs into commonMain, causing multiplatform compilation failure.

## System Constraints
You are a Kotlin Multiplatform (KMP) architect. Code in commonMain must be 100% pure Kotlin; never import Android Context or UIKit in commonMain. Use expect/actual declarations only when pure multiplatform abstractions (like Kermit or Ktor) cannot suffice.

## Strict Guidelines
- commonMain must remain strictly platform-agnostic; no java.* or platform-specific libraries.
- Use Ktor Client with platform-specific engines (OkHttp for Android, Darwin for iOS).
- Share database operations using SQLDelight or Room KMP across Android and iOS.

## Bad vs Good Implementations
### KMP Multiplatform Expect/Actual Pattern

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Directly referencing Android SDK in commonMain causes iOS compilation to fail immediately.
```kotlin
// Anti-pattern in commonMain:
fun getDeviceId(): String {
    // COMPILATION ERROR: android.os.Build does not exist on iOS target!
    return android.os.Build.MODEL
}
```

#### Production Standard (Correct)
> **Standard**: Provides clean compile-time contract via expect/actual with dedicated implementations per platform.
```kotlin
// commonMain/Platform.kt
expect class PlatformContext {
    val platformName: String
}

// androidMain/Platform.kt
actual class PlatformContext {
    actual val platformName: String = "Android ${android.os.Build.VERSION.SDK_INT}"
}

// iosMain/Platform.kt
actual class PlatformContext {
    actual val platformName: String = "iOS ${platform.UIKit.UIDevice.currentDevice.systemVersion}"
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

Filesystem MCP
Inspect multiplatform Gradle source sets.
Recommended Agent Skills (askills.dev)

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

KMP Module Scaffold
Scaffold clean architecture KMP shared repositories and network clients.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Kotlin Multiplatform (KMP) Architecture Standard

Kotlin Multiplatform (KMP) Architecture Standard defines an industrial-grade behavioral standard for mobile engineering. Guardrails for Kotlin Multiplatform: isolate platform specifics behind expect/actual, share 100% of domain and data layers, and manage Ktor HTTP client engines per platform. 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: AI leaks Android/iOS platform APIs into commonMain, causing multiplatform compilation failure. 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 DriftAI leaks Android/iOS platform APIs into commonMain, causing multiplatform compilation failure.High
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

commonMain must remain strictly platform-agnostic

commonMain must remain strictly platform-agnostic; no java.* or platform-specific libraries.

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

Use Ktor Client with platform-specific engines (OkHttp for Android, Darwin for iOS).

Use Ktor Client with platform-specific engines (OkHttp for Android, Darwin for iOS).

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

Share database operations using SQLDelight or Room KMP across Android and iOS.

Share database operations using SQLDelight or Room KMP across Android and iOS.

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 (KMP) Architecture Standard critical for AI coding assistants?

Guardrails for Kotlin Multiplatform: isolate platform specifics behind expect/actual, share 100% of domain and data layers, and manage Ktor HTTP client engines per platform. 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 (AI leaks Android/iOS platform APIs into commonMain, causing multiplatform compilation failure.) 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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Kotlin