Good question. I'll try to answer as objectively as possible, despite my bias towards Skip's approach.
Kotlin Multiplatform (KMP) enables you to target different platforms with your Kotlin. In the context of mobile apps, it allows you to compile your Kotlin to a native framework for iOS, so you can reuse your business logic. On iOS, the Kotlin is running in its own little garbage-collected runtime, but it sets up a bridge to Objective-C and Swift, so the iOS developers can communicate with it from their apps (the interface of which will typically be written separately for each platform). It is neat technology, and Skip integrates with it[1]. We were on their Talking Kotlin podcast in 2024 talking about it[2].
When targeting just the shared business logic and not the UI, Skip is, in some ways, the inverse of KMP: whereas they let you share Kotlin logic between the iOS and Android app, Skip lets you share the Swift logic. Skip operates in two different modes[3]: Skip Lite and Skip Fuse. Skip Lite is the original version of Skip, and transpiles your Swift into Kotlin. Skip Fuse is a later iteration and resulted from the formation of the Swift Android workgroup[4], of which we are founding members. In both modes, you can share your Skip business logic layer between multiple apps, and this is a popular application of Skip (e.g., see this talk at NSSpain[5]).
So that's the story for shared logic. Now onto the user interface part:
While I mentioned that Skip _can_ be used just for sharing business logic, it really shines when you build your whole app with it. You write your app in conventional SwiftUI, and Skip will translate it into the equivalent Jetpack Compose (which is now Android's official recommended way to build apps). Launching your app from Xcode will bring up both your iOS app in the simulator, and the equivalent Android app in the emulator. It is designed to be a single vertically-integrated app creation solution, and enables a single team (or a single developer) to iterate on both platforms at the same time, without any of the coordination overhead of building two separate apps for the two platforms.
KMP itself doesn't have an equivalent, but it does have a sibling project "Compose Multiplatform" (CMP), which is built on top of KMP and sort of does the opposite: it lets you write your app in Kotlin and Jetpack Compose and run it on iOS. But the way that it achieves this is different from Skip's approach: it doesn't use native controls on iOS, but instead paints pixels on the screen that mimic the native iOS UI (à la Flutter). The results are predictable: an uncanny valley UI that doesn't feel _quite_ right, and that struggles to keep up with the platform conventions. Notably, like Flutter, they won't be able to support Liquid Glass in any convincing form, and so apps built with it are going to be stuck on outdated iOS UI conventions. In short: CMP is native on Android but alien on iOS, whereas Skip is native on both platforms.
That's our take on the difference between the two. In fairness to KMP, they do have some distinct advantages in terms of reach: whereas Skip is squarely focused on just mobile platforms, KMP can target desktops and the web as well. If that is a priority for you, or you already have a lot of Kotlin experience or are invested in the ecosystem, then KMP might be a good fit for your needs. But if you like Swift and SwiftUI, and are happy working with the Apple developer tools, then you should give Skip a try. It really is magic.
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Good question. I'll try to answer as objectively as possible, despite my bias towards Skip's approach.
Kotlin Multiplatform (KMP) enables you to target different platforms with your Kotlin. In the context of mobile apps, it allows you to compile your Kotlin to a native framework for iOS, so you can reuse your business logic. On iOS, the Kotlin is running in its own little garbage-collected runtime, but it sets up a bridge to Objective-C and Swift, so the iOS developers can communicate with it from their apps (the interface of which will typically be written separately for each platform). It is neat technology, and Skip integrates with it[1]. We were on their Talking Kotlin podcast in 2024 talking about it[2].
When targeting just the shared business logic and not the UI, Skip is, in some ways, the inverse of KMP: whereas they let you share Kotlin logic between the iOS and Android app, Skip lets you share the Swift logic. Skip operates in two different modes[3]: Skip Lite and Skip Fuse. Skip Lite is the original version of Skip, and transpiles your Swift into Kotlin. Skip Fuse is a later iteration and resulted from the formation of the Swift Android workgroup[4], of which we are founding members. In both modes, you can share your Skip business logic layer between multiple apps, and this is a popular application of Skip (e.g., see this talk at NSSpain[5]).
So that's the story for shared logic. Now onto the user interface part:
While I mentioned that Skip _can_ be used just for sharing business logic, it really shines when you build your whole app with it. You write your app in conventional SwiftUI, and Skip will translate it into the equivalent Jetpack Compose (which is now Android's official recommended way to build apps). Launching your app from Xcode will bring up both your iOS app in the simulator, and the equivalent Android app in the emulator. It is designed to be a single vertically-integrated app creation solution, and enables a single team (or a single developer) to iterate on both platforms at the same time, without any of the coordination overhead of building two separate apps for the two platforms.
KMP itself doesn't have an equivalent, but it does have a sibling project "Compose Multiplatform" (CMP), which is built on top of KMP and sort of does the opposite: it lets you write your app in Kotlin and Jetpack Compose and run it on iOS. But the way that it achieves this is different from Skip's approach: it doesn't use native controls on iOS, but instead paints pixels on the screen that mimic the native iOS UI (à la Flutter). The results are predictable: an uncanny valley UI that doesn't feel _quite_ right, and that struggles to keep up with the platform conventions. Notably, like Flutter, they won't be able to support Liquid Glass in any convincing form, and so apps built with it are going to be stuck on outdated iOS UI conventions. In short: CMP is native on Android but alien on iOS, whereas Skip is native on both platforms.
That's our take on the difference between the two. In fairness to KMP, they do have some distinct advantages in terms of reach: whereas Skip is squarely focused on just mobile platforms, KMP can target desktops and the web as well. If that is a priority for you, or you already have a lot of Kotlin experience or are invested in the ecosystem, then KMP might be a good fit for your needs. But if you like Swift and SwiftUI, and are happy working with the Apple developer tools, then you should give Skip a try. It really is magic.
[1]: https://skip.dev/blog/skip-and-kotlin-multiplatform/
[2]: https://talkingkotlin.com/going-from-swift-to-kotlin-with-sk...
[3]: https://skip.dev/docs/modes/
[4]: https://www.swift.org/android-workgroup/
[5]: https://www.youtube.com/watch?v=EIGl6GOo210