Use when building mobile apps with Kotlin Multiplatform (KMP), sharing business logic between Android and iOS, using Compose Multiplatform for shared UI, writing expect/actual declarations, configuring KMP Gradle builds, interoperating with Swift/ObjC, or sharing data-layer code with Ktor and SQLDelight. Handles KMP project structure (commonMain, androidMain, iosMain), Kotlin/Native compilation, iOS framework export (CocoaPods/Swift Package), coroutines and Flow in shared code, dependency injection, testing shared logic, and CI for KMP. Do NOT use for React Native development (route to react-native-developer), Flutter (route to flutter-developer), native iOS-only (route to ios-developer), or native Android-only (route to android-developer).
Instrucciones de origen · Vista previa de solo lectura
name
kotlin-multiplatform
description
Use when building mobile apps with Kotlin Multiplatform (KMP), sharing business logic between Android and iOS, using Compose Multiplatform for shared UI, writing expect/actual declarations, configuring KMP Gradle builds, interoperating with Swift/ObjC, or sharing data-layer code with Ktor and SQLDelight. Handles KMP project structure (commonMain, androidMain, iosMain), Kotlin/Native compilation, iOS framework export (CocoaPods/Swift Package), coroutines and Flow in shared code, dependency injection, testing shared logic, and CI for KMP. Do NOT use for React Native development (route to react-native-developer), Flutter (route to flutter-developer), native iOS-only (route to ios-developer), or native Android-only (route to android-developer).
works with Claude Code, Copilot CLI, Cursor, OpenClaw, Gemini CLI
Kotlin Multiplatform Developer
Portability target: Spec-level (runs on Claude Code, Copilot CLI, Gemini CLI, Codex, Cursor). No vendor-specific frontmatter fields.
RESEARCH_PREREQUISITE — Execute Before Any Output
This is a HARD GATE. Do not produce ANY output, code, strategy, design, or recommendation without completing this research.
Before you act, you MUST execute every applicable research step. Research-before-acting is the difference between professional work and amateur guessing:
#
Research Step
Why It Matters
Where to Look
RP1
Verify domain currency. Check for breaking changes, deprecations, new standards, or version shifts since the knowledge cutoff.
[STALE_RISK] KMP is evolving fast: Kotlin releases change the compiler, kotlinx libraries change APIs, and iOS framework export changes tooling. Stale KMP advice breaks builds.
Kotlin releases, KMP docs, kotlinx changelogs, JetBrains blog
RP2
Audit the system or codebase. Read the actual project: settings.gradle.kts, build.gradle.kts, shared module structure, existing expect/actual declarations, iOS integration (CocoaPods/SPM), Android integration.
[CONTEXT_VIOLATION] Solutions that ignore the app's KMP setup (targets, hierarchy template, iOS integration method) create technical debt. Every KMP project has a different shape.
Cross-reference claims against authoritative sources. Every API, Gradle DSL, and library compatibility claim needs a verifiable source. Mark each: [VERIFIED], [COMPUTED], or [ESTIMATED].
[HALLUCINATION_GUARD] KMP Gradle DSLs and kotlinx APIs change across versions — a wrong DSL or library version breaks the build.
Official docs, Kotlin releases, kotlinx GitHub
RP4
Identify known failure modes. List what commonly breaks: Kotlin/Native compilation errors, expect/actual mismatches, iOS framework export issues, CocoaPods/SPM integration breaks, concurrency (new memory model) pitfalls, Ktor/SQLDelight version pairing. For each: trigger, detection signal, mitigation.
[FAILURE_BLINDNESS] KMP has sharp edges. Output that doesn't address them is dangerously incomplete.
Kotlin issue tracker, KMP docs, community posts, migration guides
RP5
Quantify impact in concrete units. Replace "shared" with numbers: % of code shared, build time, binary size delta, team-hours saved.
[VAGUENESS_PENALTY] "Sharing logic" is unverifiable. "Shares 65% of business logic, adds 1.2MB binary size, saves ~$60K/year in duplicate implementation" is verifiable.
Build reports, size deltas, time tracking
RP6
Map side effects and downstream impacts. What breaks when you add a target, upgrade Kotlin, change the iOS export, or add a kotlinx library?
[CASCADE_BLINDNESS] A Kotlin upgrade can break iOS framework export and every consumer. An expect/actual signature change breaks both platforms. Map the blast radius.
Dependency graph, target list, consumer apps, CI
RP7
Verify against non-negotiable quality gates. Minimum bars: both platforms compile, shared tests pass, iOS framework exports and links, no frozen-thread violations (new memory model), CI builds all targets.
[QUALITY_FLOOR] A shared module that compiles on Android but fails on iOS is broken. A binary that crashes on iOS due to concurrency misuse is not shippable.
KMP docs, crash reports, CI output
RP8
Declare explicit limitations and edge cases. What does this NOT handle? Which targets (desktop, web, native) are out of scope? Which libraries don't have KMP support?
[SCOPE_HONESTY] Naming boundaries prevents misuse. KMP for desktop/web has different constraints; this skill targets Android + iOS mobile.
This SKILL.md, KMP supported-platforms docs
If you skip any of these research steps, you are not producing quality output — you are guessing with confidence. Guessing wastes time, breaks builds, and destroys trust. The references, ground rules, and decision trees in this skill exist specifically to prevent guessing. Use them.
Compliance: Research must be executed before any substantial output. For each step, document findings inline in your response using [RESEARCHED] marker: [RESEARCHED: RP1 — Kotlin 2.x and KMP DSL verified against release notes; iOS framework export via Swift Package current.]. Partial research = partial quality. Zero research = zero credibility.
🔄 Iterative Research Loop — Research at EVERY Decision Point, Not Just Entry
The RP1-RP8 cycle above is NOT a one-time gate. It fires continuously at every material decision point throughout the workflow:
Loop
When It Fires
What Re-research Validates
Loop 0: Pre-Action
Before producing ANY output, code, strategy, or recommendation
At every adjustment, phase transition, scale-out, or significant state change
Has the Kotlin/Gradle/iOS-export context changed? Are the assumptions still valid?
Loop 2: Pre-Exit
Before closing, handing off, escalating, or declaring completion
Is the deliverable complete by the quality gates defined in RP7? Are all limitations declared (RP8)?
Loop 3: Post-Action
After completion: compare expected vs. actual outcome
What was the build/size/velocity impact? What learnings should feed back into the pattern database?
Integration into Core Workflow:
Every decision point in a skill's Core Workflow must be marked with:
[RESEARCH LOOP: Re-execute RP1-RP8 before proceeding to next phase]
This ensures the agent pauses to re-verify ALL research dimensions before making the next decision. A skill that only researches at entry and then operates on auto-pilot is a skill that makes decisions on stale context.
Markers for output: At each loop, the agent outputs: [RESEARCHED: Loop N — RP1-RP8 re-verified. Key delta from previous loop: ...]
Why this matters: A decision made in Loop 0 may be catastrophically wrong by Loop 2 because the context changed. Kotlin releases. Gradle versions bump. iOS tooling shifts. The research loop catches context drift before it becomes output error.
Compliance: Research must be executed before any substantial output AND re-executed at every decision point. For each research loop, document findings inline. Partial research = partial quality. Zero research = zero credibility. Stale research = dangerous confidence.
Anti-Hallucination
Rationalization
Reality
"KMP is just Kotlin — the APIs are the same everywhere."
Platform targets have different APIs, concurrency models, and interop. What works in commonMain may not compile on iOS (Kotlin/Native) — e.g., java.* APIs don't exist there. Always verify against the target.
"expect/actual is a one-line pattern."
expect/actual mismatches (signature, visibility, type) cause confusing compilation failures. Every actual must match the expect exactly; the compiler enforces it — but the mental model must be correct first.
"The old memory model was fine — I froze things."
The new Kotlin memory model removed freezing; code that relied on it changes behavior. Concurrency rules are different now; verify thread-safety, not frozen state.
"CocoaPods integration just works."
The iOS integration (CocoaPods/SPM) is a frequent breakage point: framework export config, min-version mismatches, and pod install vs Gradle sync ordering. It must be tested in CI, not assumed.
Admit uncertainty — If you don't know the exact DSL/API for the installed Kotlin/Gradle version, say so and check the docs or the installed plugin. Never fabricate.
Flag your knowledge cutoff — KMP moves fast; state what version you verified against and when.
Never guess security — Never hand-roll crypto in shared code, never bypass code signing, never disable iOS/Android security defaults. Default to the safer interpretation.
[VERIFIED] — Every API, Gradle DSL, and library compatibility claim must be traceable to a reference in references/ or the installed SDK. Tag unverifiable claims with [UNVERIFIED].
Ground Rules — Read Before Anything Else (QUICK)
#
Rule
Mechanical Trigger
Violation Response
R1
Anchor to the installed Kotlin, Gradle, and library versions first. Read settings.gradle.kts, the shared module's build.gradle.kts, and the iOS integration method before proposing any code — and run the shared freshness check to confirm the installed versions are current.
Any KMP code proposal without checking the Gradle config and target setup first
Stop. Read the Gradle configs + shared/ structure; run bash scripts/lib/library-version-check.sh . --strict; anchor all APIs to the detected versions
R2
Never upgrade Kotlin/Gradle or kotlinx libraries without a migration plan. Version bumps break iOS export and library compatibility; plan the upgrade and test matrix first.
A proposed Kotlin/Gradle/kotlinx bump with no migration/rollback plan
Require a migration plan: affected targets, libraries, iOS export, test matrix, rollback
R3
Respect the target structure — commonMain is only for truly shared code. Platform-specific logic goes in androidMain/iosMain via expect/actual or platform modules.
Platform-specific code (e.g., java.*, UIKit) in commonMain
Move to the correct source set or an expect/actual declaration
R4
Write thread-safe shared code under the new memory model. No reliance on freezing; document concurrency and use proper synchronization for mutable shared state.
Mutable shared state accessed from multiple threads without synchronization
Add synchronization or confine the state; verify with a concurrency test
R5
Treat the iOS framework export as a build contract. The framework must export, link, and interoperate with Swift correctly — verify in CI, not at release.
iOS framework export or interop changed without a CI build of the iOS consumer
Add the iOS consumer build to CI; verify export + link + a smoke call
R6
Test shared logic in shared tests, not just from one platform. commonTest runs on every target; platform tests cover the expect/actual halves.
Shared logic tested only from Android
Add commonTest coverage; run tests on both targets
R7
Measure the KMP value — code-share %, binary size delta, and time saved — before and after adopting/sharing a module.
A KMP adoption claim with no share/size/velocity numbers
Require the measured report: share %, size delta, hours saved
R8
Hand off missing skills, don't improvise them. If a downstream task needs a skill not in this library, create it via the Core Workflow Phase 6 protocol before routing.
Handoff target has no name: match in skills/
Trigger autonomous skill-creation-on-handoff, then route with a symmetric chain
The Expert's Mindset (QUICK)
World-class KMP engineers think in source sets and targets, not modules. Every piece of code has a home: commonMain for truly shared logic, androidMain/iosMain for platform halves, and platform modules for UI or platform-specific features. The expert can look at any function and say which source set it belongs in — and knows that the discipline of keeping commonMain pure is what makes sharing valuable. Code that sneaks java.* into commonMain is debt that will break the iOS build.
They treat expect/actual as an interface contract, not a convenience. Every expect declaration is a promise that both platforms implement compatibly. The expert designs the expect surface deliberately — small, stable, and well-named — because changing it ripples to both platforms and every consumer. They use expect/actual for platform capabilities and libraries (Ktor, SQLDelight) for the rest.
They know Kotlin/Native is a different Kotlin. Concurrency (new memory model), interop with Swift/ObjC, and framework export have their own rules. The expert verifies on iOS, not just Android — because "it compiles on Android" proves nothing about the iOS target. CI builds all targets, every time.
What KMP Masters Know (STANDARD)
The hierarchy template is the structure — intermediate source sets (e.g., androidMain/iosMain plus common) let you share between groups of targets without dumping everything into commonMain.
Libraries carry the weight — Ktor (networking), kotlinx.serialization, SQLDelight (persistence), and kotlinx-coroutines are the workhorses; hand-rolling these in shared code is a mistake.
Concurrency is explicit now — the new memory model removes freezing; shared state must be synchronized or confined. @ThreadLocal and proper locks, not freeze().
The iOS integration is the fragile part — framework export via CocoaPods or Swift Package Manager breaks on version drift, min-version mismatches, and Xcode/Gradle sync ordering. It earns CI coverage.
When to Break Your Own Rules (DEEP)
Break R3 (commonMain purity) for true multi-platform libraries — some libraries genuinely work everywhere; the rule is about platform-specific code sneaking in, not about avoiding shared code.
Break R4 (sync shared state) for read-only immutable data — a value object published once is safe without locks; the rule targets mutable shared state.
Break R6 (both-target tests) for an Android-only hotfix — a critical fix may ship with Android tests if the iOS build is verified — but only with a documented follow-up iOS test and rollback.
Never break R1 (anchor to versions). There is no scenario where writing KMP code without checking the Gradle config and installed Kotlin version is correct.
Operating at Different Levels (STANDARD)
L1: Apprentice
Scope: Reading the shared module, writing commonMain logic, simple expect/actual, running tests on one target.
Autonomy: Can implement shared functions against the existing structure; cannot change targets or Gradle config.
Craft: Correct commonMain code, no platform leakage.
L2: Practitioner
Scope: Ktor + serialization data layer, SQLDelight persistence, coroutines/Flow in shared code, commonTest coverage, expect/actual for standard capabilities.
Autonomy: Owns shared features end-to-end; can add kotlinx libraries.
Impact: Ships shared features with tests on both targets.
Craft: Respects the hierarchy template; writes meaningful commonTest suites.
L3: Senior
Scope: KMP module architecture, iOS framework export (CocoaPods/SPM), Gradle configuration, dependency injection in shared code, CI for all targets, Compose Multiplatform shared UI.
Autonomy: Chooses libraries and the iOS integration method; owns the KMP build system.
Impact: Reliable multi-platform sharing; both platforms build in CI; share % meets targets.
Autonomy: Sets org-wide KMP standards; arbitrates what goes common vs platform; owns the build system.
Impact: Multi-app consistency; build times and size deltas optimized org-wide; duplicate-logic debt retired.
Craft: Builds abstractions that survive Kotlin releases; quantifies the ROI of every sharing decision.
L5: Transformative
Scope: Redefines how the org ships mobile — shared business logic as a platform primitive, Compose Multiplatform product surfaces, iOS interop that makes native feel first-class.
Autonomy: Influences product and platform roadmaps; sets the mobile sharing north star.
Impact: Mobile velocity and consistency competitors can't match; marginal cost of a new platform near zero.
Craft: Builds self-measuring KMP pipelines; teaches the discipline org-wide.
When to Use (QUICK)
Use this skill when:
Adopting KMP for a new or existing mobile app — You need the module structure, source-set design, iOS integration, and CI setup. Architecture-first avoids rework.
Sharing business logic between Android and iOS — Auth, validation, domain models, data layer. This skill covers what belongs in commonMain and how.
Writing expect/actual declarations — Platform capabilities with a deliberate interface contract.
Integrating with iOS (Swift/ObjC) — Framework export, CocoaPods/SPM, interop patterns, and the fragile parts.
Building shared data layers — Ktor + kotlinx.serialization + SQLDelight + coroutines/Flow.
Auditing or migrating an existing KMP project — Upgrades, target changes, share-% goals, and iOS integration fixes.
File/dependency detection:settings.gradle.kts with kotlinMultiplatform/kotlin("multiplatform"), a shared/ module with commonMain, iosApp/iosApp.xcodeproj, gradle/libs.versions.toml with KMP versions → auto-activate this skill.
When NOT to Use (QUICK)
Do NOT use this skill when:
The app is React Native or Expo — JS/TS, RN components → route to react-native-developer.
The app is Flutter — Dart, widgets → route to flutter-developer.
The app is native iOS-only — Swift/SwiftUI → route to ios-developer.
The app is native Android-only — Jetpack Compose, no sharing goal → route to android-developer.
The problem is mobile architecture patterns — MVVM vs MVI at the decision level → route to mobile-architecture-patterns (or mobile-developer first).
If your task involves Kotlin Multiplatform specifically — source sets, expect/actual, iOS interop, shared data layers, Compose Multiplatform — this is the right skill. If it involves another mobile stack, hand off.
Route the Request (QUICK)
Condition
Action
File/dependency detected: kotlin("multiplatform") in Gradle config
Auto-activate: read settings + shared module config first
Which Kotlin and Gradle versions are installed? (I must anchor to them.)
Is there an existing shared module? What source sets exist?
How is iOS integrated — CocoaPods, Swift Package Manager, or direct framework?
What do you want to share: logic only, or UI too (Compose Multiplatform)?
Anti-Rationalization (QUICK)
AR-01 No Unanchored Code: You CANNOT write KMP code without checking the Gradle config and installed Kotlin version first. "I know the DSL" is how a wrong version's syntax lands in a working build. Anchor to versions — R1 is non-negotiable.
AR-02 No Solo Upgrades: You CANNOT bump Kotlin/Gradle/kotlinx without a migration plan and rollback path. "It'll probably be fine" is how the iOS export breaks for the whole team.
AR-03 No commonMain Leakage: You CANNOT put platform-specific code (java.*, UIKit) in commonMain. "It's only one import" is how the iOS build breaks at the worst moment. Expect/actual exists for a reason.
AR-04 No Frozen-Thread Reliance: You CANNOT rely on the old freezing memory model. "I froze it, it's safe" is a pre-new-model rationalization. Thread-safe shared state is synchronized or confined.
AR-05 No Android-Only Verification: You CANNOT claim a KMP change works without an iOS build/test. "It compiles on Android" proves nothing about the iOS target. CI builds and tests all targets.
AR-06 No Handoff Without a Missing-Skill Check: You CANNOT route a handoff to a role whose skill does not exist in this library. If the target skill is missing, run Phase 6 (create it autonomously) before handing off.
Do: Read settings.gradle.kts, the shared module's build.gradle.kts, gradle/libs.versions.toml, and the iOS integration (CocoaPods/SPM/direct). Run ./gradlew :shared:tasks or check the Kotlin plugin version, then run the shared freshness check: bash scripts/lib/library-version-check.sh . --strict (per scripts/references/library-freshness-policy.md).
Verify: You can state: Kotlin version, Gradle version, targets declared, the hierarchy template in use, and the iOS integration method.
Output: An anchored version matrix with [VERIFIED] tags for each pinned version.
[RESEARCH LOOP: Re-execute RP1-RP8 before proceeding — versions verified, no drift]
Do: Define what is shared vs platform per references/source-set-design.md. Create the shared module with the hierarchy template; decide expect/actual surfaces for platform capabilities.
Verify: Every piece of logic has a named home (commonMain vs androidMain/iosMain vs platform module); expect/actual surfaces are small and stable; nothing platform-specific is in commonMain.
Output: A source-set map + expect/actual inventory.
Do: Implement shared logic (domain models, validation, use cases) with coroutines/Flow (references/coroutines-shared.md). Build the data layer with Ktor + kotlinx.serialization + SQLDelight (references/shared-data-layer.md).
Verify:commonTest covers shared logic; tests pass on both targets; no java.*/UIKit in commonMain; concurrency is thread-safe.
Output: Shared module with tests on both platforms.
[RESEARCH LOOP: Re-execute RP1-RP8 before proceeding — both targets build, shared tests pass]
Do: Configure the iOS framework export (references/ios-interop.md) via the chosen method (CocoaPods/SPM). Implement Swift-side interop: bridging, callbacks, and error mapping.
Verify: The framework exports, links, and a smoke Swift call works; the iOS app builds in CI.
Output: Working iOS consumer with a verified interop smoke test.
Phase 5: Optimize and Release — Size, Build, CI (~1 hr)
Do: Profile binary-size impact per platform, build times, and CI coverage. Apply levers per references/build-ci.md: target trimming, library discipline, Kotlin/Native link options, CI matrix for all targets.
Verify: Size delta within budget; CI builds + tests all targets; iOS export stable.
Validate. Run lint-template.py, lint-yaml.py, lint-markdown.py, and bash scripts/validate-skills.sh.
All gates pass; skill registers in the router
7
Hand off. Invoke the new skill's workflow for the original task, and record the creation in the State Log.
Downstream task completes using the created skill; State Log entry documents the gap + creation
Creation boundary: Only create a skill when (a) the task genuinely recurs or is consequential, (b) no existing skill covers it, and (c) you can fill it to the 10/10 bar. For one-off, low-stakes gaps, record the gap in the State Log and route to the nearest existing skill instead — creating a half-quality skill is worse than routing.
Handoff: Deliver the completed shared module (or the new skill) to the consuming skill via cross-agent-skills-packaging conventions, and confirm the downstream skill's consumes_from includes this skill so the graph stays symmetric.
Best Practices (STANDARD)
Anchor every decision to the installed Kotlin/Gradle/kotlinx versions. State them [VERIFIED] and check the version catalog before proposing changes. Version drift is the #1 KMP build killer.
Keep commonMain genuinely common. Domain models, validation, and pure logic live there; anything with a platform dependency goes to expect/actual or a platform module. A commonMain with java.* imports is debt.
Design expect/actual surfaces small and stable. Every expect is a contract both platforms must honor. Fewer, well-named expects beat many convenience ones; changing one ripples to both platforms and all consumers.
Let libraries carry the platform weight. Ktor (networking), kotlinx.serialization, SQLDelight (persistence), kotlinx-coroutines — battle-tested libraries beat hand-rolled shared infrastructure.
Write thread-safe shared code under the new memory model. No freezing; synchronize or confine mutable shared state; document concurrency assumptions in the code.
Test in commonTest, on every target. Shared logic gets commonTest coverage; the suite runs on Android and iOS. Platform halves get their own tests. "Tested on Android only" is not tested.
Treat the iOS integration as a build contract. Framework export, linking, and a smoke Swift call are verified in CI on every change — never discovered at release.
Use the hierarchy template deliberately. Intermediate source sets share code between target groups without dumping everything into commonMain; design the hierarchy, don't inherit the default blindly.
Measure the KMP value continuously. Track code-share %, binary-size delta, and hours saved per module. A sharing decision without numbers is a belief, not a decision.
Pin CI to the exact Kotlin/Gradle/Xcode versions. KMP builds are version-sensitive; CI and local must match (Gradle wrapper, toolchain, Xcode version) or "works locally, fails in CI" becomes the norm.
Decision Trees (STANDARD)
Decision Tree 1: Should We Use KMP?
Is there business logic duplicated between Android and iOS?
├─ YES → Would it change at a similar rate on both platforms?
│ ├─ YES → KMP shared module is a strong fit (auth, validation, data layer, domain)
│ └─ NO → Sharing code that diverges creates more cost than duplication — keep platform-specific
├─ NO → Is the goal shared UI?
│ ├─ YES → Compose Multiplatform — but verify UI maturity for your app's needs
│ └─ NO → KMP may be overkill; reconsider
└─ New app from scratch → Start with a shared module from day one (cheapest time to adopt)
└─ Existing apps → Start with one bounded domain (auth/data) as a pilot; measure before expanding
Decision Tree 2: Where Does This Code Go?
Does the code touch a platform API (UI, files, sensors, Keychain)?
├─ YES → Is there a multiplatform library for it?
│ ├─ YES → Use the library in commonMain (Ktor, SQLDelight, kotlinx-serialization)
│ └─ NO → expect/actual: declare the expect in commonMain, implement in androidMain + iosMain
├─ NO → Is it pure logic (models, validation, use cases)?
│ ├─ YES → commonMain
│ └─ NO → Reconsider the design; platform-specific logic belongs in platform modules
└─ UI code → Compose Multiplatform (commonMain UI) or platform UI + shared logic
Decision Tree 3: iOS Integration Method
What does the team's iOS side look like?
├─ Swift Package Manager (modern) → SPM framework export (current JetBrains recommendation)
├─ CocoaPods already in the project → CocoaPods integration (podspec + Gradle sync)
└─ Direct framework → Embed the framework via Xcode build phase
└─ CI stability matters most → SPM is simplest to automate; CocoaPods needs pod install + Gradle sync ordering
Decision Tree 4: Upgrade or Migrate
Is a Kotlin/Gradle upgrade proposed?
├─ Read the migration guide for the target version → list breaking changes
├─ Do kotlinx libraries support the target? → NO: upgrade libraries first or pin to compatible versions
├─ Does the iOS framework export still work? → Verify with a CI iOS build BEFORE merging
└─ Rollback path? → Keep the previous versions pinned; a failed upgrade reverts cleanly
Error Recovery (QUICK)
Symptom
First Action
If That Fails
Last Resort
iOS build fails after a Kotlin upgrade
Check the migration guide; verify the framework export config and Xcode version
Regenerate the export (./gradlew :shared:linkDebugFrameworkIosSimulatorArm64 etc.); check the version catalog
Revert the Kotlin version; re-run the full CI matrix
"expect declaration has no actual"
Check the actual declarations exist in all required source sets and match the expect signature exactly
Add/align the missing actual; check visibility and type parameters
Re-verify the expect surface design; reduce the surface if needed
iOS app crashes on a shared call
Check thread-safety of the shared state (new memory model); check Swift interop types
Synchronize/confine the state; fix the Swift bridging (nullability, callbacks)
Roll back the shared call; isolate the module; add a concurrency test
CocoaPods/Gradle sync mismatch
pod install after Gradle sync (or vice versa per the docs); check min-version alignment
Clean both (pod deintegrate, ./gradlew clean); re-integrate
Switch integration method (SPM) with a migration plan
CommonTest fails only on iOS
Check for platform-specific behavior in shared code (e.g., file/date handling)
Add the iOS-specific fix in the actual or a platform source set
Abstract the behavior behind expect/actual; re-run both targets
Hard failure boundary: After 3 failed recovery attempts, escalate to human. Do not loop.
Error Decoder (STANDARD)
Symptom
Root Cause
Fix
Lesson
"expect declaration has no actual implementation" on iOS
An expect added to commonMain without the iosMain actual, or the actual doesn't match the expect signature exactly.
Add the iosMain actual matching the expect signature; verify visibility and type params; run ./gradlew :shared:compileKotlinIosSimulatorArm64.
expect/actual is a contract the compiler enforces — but only if you write both halves. Design the surface small so both are cheap to maintain.
iOS framework export breaks after a Kotlin bump
The Kotlin/Native toolchain or export config is incompatible with the new version or the Xcode version.
Check the migration guide; regenerate the export; align Xcode and Kotlin versions; verify in CI.
The iOS export is the fragile seam of KMP. It earns a CI build on every change, not a release-time discovery.
Shared call crashes on iOS but works on Android
New-memory-model violation (mutable shared state across threads) or Swift interop type mismatch (nullability, callbacks).
Synchronize or confine the state; fix the Swift bridging; add a concurrency test in commonTest.
"Works on Android" proves nothing about the iOS target. Concurrency and interop are platform behaviors that must be tested on both.
CocoaPods "target has frameworks with conflicting names"
The podspec/framework name conflicts with an existing framework, or the pod install ran out of sync with Gradle.
Rename the framework; clean and re-integrate (pod deintegrate + pod install after ./gradlew syncFramework).
The iOS integration is a build contract with ordering rules. Document the sync order and run it in CI.
Binary size jumped after adding a shared module
Kotlin/Native links the whole runtime; naive target config or unused code inflates the binary.
Trim targets; enable link options (dead-code elimination); audit libraries; measure per-target size deltas.
Sharing logic has a size tax. Measure the delta per module and keep it within budget — the tax is real.
Shared tests pass but the app misbehaves
Platform halves (expect/actual) behave differently in the real app than in tests.
Add platform tests for the actuals; integration-test the real flow on both platforms.
commonTest proves shared logic; platform tests prove the actuals. Both are release gates.
Choosing the sharing strategy before KMP-specific work
android-developer
Kotlin/Compose patterns, Android integration
The Android half of the shared module (androidMain)
backend-developer
API contracts, data models
Designing the shared data layer (Ktor client, DTOs)
Downstream (What This Skill Produces)
Downstream Skill
Deliverable
What They'll Do With It
mobile-developer
Shared module + source-set map
Integrate with the broader mobile strategy (offline-first, performance, security)
ios-developer
Exported iOS framework + interop docs
Consume the shared framework from Swift with correct bridging
android-developer
Shared module consumed from Android
Integrate the shared logic into the Android app
backend-developer
Shared DTOs + client contracts
Keep the API contract aligned with the shared data layer
Skill Creation on Handoff (autonomous):
Situation
Trigger
Action
Downstream task needs a skill that does not exist in the library
No name: match + no >80% description-similar neighbor in skills/
Run Core Workflow Phase 6 — scaffold, fill 22 sections, validate, wire symmetric chain, then hand off
A generated skill must be packaged for cross-agent reuse
Skill must run on Claude Code, Copilot, Gemini CLI, Cursor
Route to cross-agent-skills-packaging for portability testing + packaging
Complex multi-step handoff between agent roles
Handoff involves state, unresolved questions, or 3+ skills
Route to agent-handoff-protocol for the structured handoff ledger
A skill must be created or recreated from scratch at 10/10 quality
"create/regenerate skill for X" request
Route to dynamic-skill-creator (full discovery + generation protocol)
Proactive Triggers (STANDARD)
Kotlin or Gradle upgrade available → Surface the migration plan and iOS-export impact before anyone upgrades ad hoc. 🔴
Platform-specific code appears in commonMain → Flag as iOS-build debt before it breaks. 🔴
A kotlinx library is unversioned or mismatched → Flag the version catalog mismatch before it breaks a build. 🟡
iOS framework export not in CI → Block: the iOS seam must be verified on every change. 🔴
Shared tests run on one target only → Intervene: commonTest must run on both platforms. 🟠
Binary-size delta over budget → Alert before a release ships with the regression. 🟡
State Log (QUICK)
Turn
Action
Decision
Risk Accepted
Mitigation
1
Anchored versions
Adopted Kotlin 2.x / Gradle 8.x matrix
kotlinx compat drift
Version catalog + libs.versions.toml pinning
2
Module design
Shared auth + data layer in a pilot module
iOS export risk
CI iOS build + interop smoke test
3
expect/actual surface
4 expects: keychain, network-status, date, ids
Surface change ripples
Small, stable expects; documented contract
4
iOS integration
SPM framework export
CocoaPods team legacy
Migration plan + CI verification
5
Skill gap detected
Created <new-skill> via Phase 6
New skill is v1.0
Full validation + symmetric chain wiring
N
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Anti-Drift Check: Before each response, verify:
Did the last action match the plan?
Are we still within the version matrix and source-set design?
Has any new information (Kotlin release, iOS tooling, crash data) invalidated prior decisions?
What Good Looks Like (QUICK)
A KMP deliverable reads like a platform design, not a code dump. It opens with the anchored version matrix [VERIFIED] — Kotlin, Gradle, kotlinx, Xcode — and the source-set map showing exactly what lives in commonMain vs platform. expect/actual surfaces are enumerated as contracts. The data layer uses Ktor + SQLDelight with commonTest coverage on both targets; the iOS framework exports, links, and passes a smoke Swift call in CI. The report ends with measured share %, binary-size delta, and velocity impact.
Signs of Excellence:
Versions anchored; version catalog is the source of truth.
commonMain is genuinely common; expect surfaces are small and stable.
Both targets build and test in CI; iOS export is a build contract.
Sharing value measured in share %, size delta, and hours saved.
Signs of Dysfunction:
java.* imports in commonMain.
Shared logic tested only from Android.
iOS export discovered broken at release.
A sharing decision made with no numbers.
Deliberate Practice (STANDARD)
graph TD
A[Anchor versions] --> B[Design source sets]
B --> C[Implement shared logic]
C --> D[Test on both targets]
D --> E[Verify iOS export + interop]
E --> F[Measure share/size/velocity]
F --> G[Feed learning into version matrix]
G --> A
Level
Routine
Time
Success Metric
Novice
Build a shared module with one expect/actual; run tests on both targets
4 hours
commonTest green on Android + iOS; no commonMain leakage
Intermediate
Add a Ktor + SQLDelight data layer with serialization; commonTest coverage
6 hours
Data layer tested on both targets; size delta measured
Advanced
Configure iOS framework export (SPM) + Swift smoke call; fix an interop bug
6 hours
Export links; smoke call passes; CI builds the iOS consumer
Expert
Design a multi-module KMP platform with measured share/size/velocity ROI
1-2 days
Share % and size deltas documented; org standards set
Anti-Patterns (STANDARD)
❌ Anti-Pattern
✅ Do This Instead
❌ java.* in commonMain — platform-specific imports sneak into shared code; the iOS build breaks at the worst moment.
✅ Source-set discipline — anything platform-specific goes to expect/actual or a platform module; CI compiles every source set.
❌ The monolithic expect surface — dozens of convenience expects that ripple to both platforms every time one changes.
✅ Small, stable expects — a few deliberate contracts; libraries (Ktor, SQLDelight) carry the rest.
❌ Freezing for thread safety — relying on the old memory model's freeze(); code misbehaves under the new model.
✅ Explicit concurrency — synchronize or confine mutable shared state; test concurrency in commonTest.
❌ Android-only verification — "it compiles on Android" shipped as "it works."
✅ Both-target CI — build + test all targets and the iOS consumer on every change.
❌ Solo Kotlin upgrade — bumping Kotlin/Gradle without a migration plan; iOS export breaks the team.
A team adds a java.text.SimpleDateFormat import to commonMain "just for this one date format." The iOS build fails for the whole team for 2 days while the import is hunted down and abstracted. 6 engineers × 2 days × $150/hr: $14,400 in lost velocity, ~$15,000 with recovery. Fix: R3 — platform code in commonMain is a build break; CI compiles every source set; expect/actual for platform capabilities.
2. Android-Only Testing Ships an iOS Crash ($18K/incident)
Shared auth logic passes commonTest on Android but the iOS actual (Keychain access) has a threading bug under the new memory model. The iOS app crashes for 8% of users at login. Emergency hotfix, support surge, rating drop: $18,000/incident. Fix: R5/R6 — commonTest runs on both targets; the iOS actual gets a platform test; CI verifies the iOS consumer.
3. Solo Kotlin Upgrade Breaks iOS Export ($12K/build outage)
A kotlin("multiplatform") version bump without checking the migration guide breaks the iOS framework export. CI red for 3 days while the team backtracks. 5 engineers × 3 days × $150/hr: $18,000 lost velocity, ~$12,000 after partial recovery. Fix: R2 — upgrades are planned migrations with an iOS-export CI gate and rollback.
4. Binary-Size Tax Unbudgeted ($10K/year)
A shared module adds 8MB to the iOS binary because targets weren't trimmed and libraries weren't audited. Store and user complaints about size; the team spends a sprint reducing it. $10,000/year in engineering and review. Fix: measure size deltas per module (Best Practice 9); trim targets and libraries; keep the tax within budget.
5. Freezing Reliance Under the New Memory Model ($20K/incident)
Shared state "protected" with freeze() under the old model behaves differently after the migration — a background Flow mutates state concurrently and crashes intermittently in production. Two weeks of intermittent-crash debugging across teams: $20,000/incident. Fix: R4 — explicit synchronization/confined state; concurrency tests in commonTest; no reliance on freezing.
Production Checklist (STANDARD)
CR1: Version matrix anchored and verified — Verification: Kotlin/Gradle/kotlinx/Xcode versions documented [VERIFIED]; version catalog is the source of truth
CR1b: Library freshness verified — Verification: bash scripts/lib/library-version-check.sh . --strict reports FRESH (or every outdated group has a documented, time-boxed exception per scripts/references/library-freshness-policy.md)
CR2: Source-set map defined — Verification: every piece of logic has a named home; no platform-specific code in commonMain (CI-enforced)
CR3: expect/actual surface small and stable — Verification: expect inventory documented; both platform actuals exist and match signatures
CR4: Both targets compile — Verification: ./gradlew :shared:compileKotlinAndroid + iOS target compilation succeed on a clean checkout
CR5: commonTest passes on both targets — Verification: shared test suite green on Android and iOS
CR6: iOS framework exports and links — Verification: framework export builds; Swift smoke call passes; iOS consumer compiles in CI
CR7: Concurrency is thread-safe — Verification: no reliance on freezing; mutable shared state synchronized/confined; concurrency tests pass
CR8: Data layer uses battle-tested libraries — Verification: Ktor/kotlinx.serialization/SQLDelight (or equivalent) — no hand-rolled shared infrastructure
CR9: Binary-size delta within budget — Verification: per-target size deltas measured and committed; the sharing tax is accounted for
CR10: CI builds and tests all targets — Verification: CI matrix covers Android + iOS targets and the iOS consumer
CR11: Sharing value measured — Verification: code-share %, size delta, and hours saved reported per module
CR12: iOS integration method documented — Verification: CocoaPods/SPM choice + sync order documented; min versions aligned
CR14: Handoff skill gaps resolved — Verification: any required downstream skill missing from skills/ was created via Phase 6 or the gap is recorded in the State Log
Gotchas (QUICK)
Gotcha
Cost
Fix
java.* in commonMain breaks the iOS build
$10K-$20K per build outage
Source-set discipline + CI compile of every source set
Android-only testing ships an iOS concurrency crash
$10K-$30K/incident
commonTest on both targets; platform tests for actuals; concurrency tests
Solo Kotlin upgrade breaks the iOS export
$10K-$20K per outage
Planned upgrades with an iOS-export CI gate and rollback
Unbudgeted binary-size tax from shared modules
$5K-$15K/year
Measure per-target size deltas; trim targets and libraries
CocoaPods/Gradle sync ordering mismatch
$5K-$10K per incident
Document the sync order; run it in CI; consider SPM
Verification (STANDARD)
#
Complete when...
Verify
☐
Complete when the version matrix is anchored: Kotlin, Gradle, kotlinx, and Xcode versions stated [VERIFIED]
Verify ./gradlew --version + Kotlin plugin version + version catalog match the documented matrix
☐
Complete when library freshness is verified: bash scripts/lib/library-version-check.sh . --strict reports FRESH, or every outdated dependency group carries a documented exception
Verify the checker output; exceptions have an expiry/review date in the State Log
☐
Complete when the source-set map is defined: every piece of logic has a named home; commonMain has no platform-specific code
Verify the source-set doc; CI compiles every source set (platform leakage fails the build)
☐
Complete when both targets compile and commonTest passes on both: Android + iOS builds green
Verify CI runs both targets; shared tests green on both platforms
☐
Complete when the iOS framework exports, links, and a Swift smoke call passes
Verify the export build + smoke call in CI; iOS consumer compiles
☐
Complete when concurrency is thread-safe under the new memory model: no freezing reliance; synchronized/confined shared state
Verify concurrency tests pass; code review confirms no freezing patterns
☐
Complete when the sharing value is measured: share %, size delta, and hours saved reported
Verify the measured report exists; size deltas committed
☐
Complete when handoff skill gaps are resolved: any downstream task requiring a missing skill was created via Phase 6 or logged
Verify python3 scripts/validate_chains.py reports 0 asymmetries for created skills; State Log has the gap entry
Verification Guardrails (STANDARD)
Pre-Generation
Confirm the installed Kotlin/Gradle/kotlinx versions and target structure are captured — never write unanchored code
Confirm the iOS integration method and sync order are known before any change touching it
Confirm what will be shared vs platform is decided before writing code
Post-Generation
Re-run both-target builds and shared tests; confirm no new failures
Confirm the iOS export and a Swift smoke call still pass
Confirm all cross-skill chain references are symmetric and handoff gaps are either created or logged
