Hi there!
We’re RIO (@rioX432) and Apippo (@A5th_Faris) from the AnotherBall Mobile Team.

We work on Android/iOS development for Avvy using KMM and Compose Multiplatform. Since we actively use AI for native code development every day, Kotlin Fest 2025 was packed with topics directly relevant to our daily work.

We attended Kotlin Fest 2025 held on November 1, 2025!

This year featured presentations across 3 rooms, recording the highest number of sponsors and talks ever. AnotherBall participated as a Silver Sponsor!

The entire venue was buzzing with excitement—truly a “festival” atmosphere.

What is Kotlin Fest?

Kotlin Fest is Japan’s largest Kotlin community conference, themed around “Celebrating Kotlin,” aimed at sharing knowledge and fostering connections around Kotlin and its ecosystem.

This year’s venue was the Tokyo Conference Center Shinagawa.

The corporate booths and networking spaces after sessions were packed, with conversations buzzing about implementation details and architecture. What stood out most was the clear increase in companies adopting server-side Kotlin.

Beyond Android and KMP, there was a real sense that Kotlin is expanding as a “general-purpose language.”

Session Highlights

Opening Session

“The Role of Kotlin Developers in an Era Where AI Writes Code”
Taro Nagasawa, Atsushi Mori, Eiji Tamaki / Kotlin Fest Organizers

Even in an age where AI writes code, developers remain the “reviewers and decision-makers.”
The session explained why Kotlin is the most suitable language for this era:

• Simple yet expressive syntax
• AI-assisted ecosystem through JetBrains IDEs
• Consistent training data

Not “AI taking our jobs,” but “evolving design together with AI.”
This direction was shared across the entire Kotlin community in the opening.

[Invited Session] The Technology Behind Kotlin: Language Design and Unsung Heroes

Yan Zhulanow / Kotlin Team

The session covered new features in Kotlin 2.3 and the development background of the Build Tools API.

The Build Tools API is particularly relevant for AnotherBall, which uses KMM.
Improvements in per-module incremental builds and KMP support promise significant improvements in build speed and developer experience.

Kotlin’s evolution values “stability and consistency” over “novelty.”

Custom String Interpolation via Kotlin Compiler Plugin

be-hase

A case study on understanding Kotlin Compiler Plugin internals and customizing string interpolation. The code generation mechanism using IR (Intermediate Representation) extension was thoroughly explained. A session that offered perspective from the “extending” side rather than just “using” the language.

At Least Make It Native - Multiplatform and Exit Strategy

RyuNen344

A pragmatic session focused not just on multiplatform adoption, but on how to localize the pain of exit. It covered gradual replacement strategies while considering Swift Export and Kotlin/Native constraints. The philosophy of “don’t adopt all at once, design for clean rollback, especially define clean interfaces” is crucial for hybrid apps like Avvy.

Implementing CPU/GPU “Cooperative” Performance Management in Kotlin

matuyuhi

An explanation of how to safely handle Android Dynamic Performance Framework (ADPF) in Kotlin. Kotlin-like approaches including async data processing with Flow, type-safe design with sealed classes, and DSL-based control. Immediately worth considering for Avvy. Useful not just for thermal management, but also for optimizing face detection inference on low-spec devices.

Inside of Swift Export - A New Bridge Between Kotlin and Swift

giginet

An explanation of how Swift Export enables calling Kotlin code directly from Swift. Without going through the ObjC bridge, the API experience on iOS improves significantly. A meaningful update for both CMP/KMM adoption and exit strategies. While not production-ready yet, we should start preparing for migration from ObjC now.

Rewind & Replay: Kotlin 2.2 Transforms Coroutine Debugging

daasuu

Introduction to debugging improvements in Kotlin 2.2 + IntelliJ IDEA 2025.1. Enhanced local variable retention, stack restoration, and step execution. Definite usability improvements that will gradually boost daily development efficiency.

Building Full Kotlin! MCP Server, AI Agent, and UI All-in-One

Shuzo Takahashi

An introduction to implementing MCP servers, AI agents, and UI entirely in Kotlin using the Kotlin MCP SDK. The presentation included examples of integrating Figma with MCP servers, which was highly inspiring. Worth exploring for Avvy, especially for UI generation assistance and design automation integration.

Getting Started with AI Agent Development Using Koog

hiro

A detailed session on “Koog,” JetBrains’ AI agent construction library. Langfuse integration, DSL construction, parallel execution—enabling serious AI workflow design. This is a topic to tackle immediately.

Shifting to the mindset of “what to make AI do,” we’re thinking about applications from UI code generation assistance to company-wide OKR progress management and PdM decision support.

A Practical Guide to Transforming Legacy Code into Idiomatic Kotlin with AI Collaboration

nishimy432

A presentation on teaching AI “Kotlin-ness” through prompts when converting Java code to Kotlin. The perspective of reflecting team culture in AI—data classes, scope functions, sealed classes, null safety—was impressive. This approach of conveying code style to AI is something AnotherBall can leverage.

Overall Impressions

Kotlin Fest 2025 was structured around three pillars: AI, Multiplatform, and Developer Experience Evolution.

• Kotlin is becoming the language that drives AI
• Compose Multiplatform and KMM are becoming “technologies for designing exit strategies”
• Kotlin development is heading toward “balancing safety and expressiveness”

At AnotherBall, Avvy prioritizes native UIUX optimization while leveraging KMM for shared business logic, along with Unity for FaceTracking—a highly challenging architecture. We’re also actively using AI for native code development, and many of the technologies presented at this conference directly connect to our development work.

We were reminded of the value of an environment where we can tackle the latest architecture with a full Compose setup.

Final Thoughts

Kotlin Fest 2025 was an event that showcased the depth of the Kotlin community. Both speakers and attendees impressed me with their approach of discussing how to use AI and multiplatform “in the real world.” AnotherBall will continue to push forward in the Kotlin × AI space.

Our team at the event

Have a nice Kotlin!

We’re Hiring

AnotherBall is looking for engineers interested in app development using Kotlin/Unity/AI.
We’re seeking teammates to grow our product together while adopting new technologies like KMM and Compose Multiplatform.

AnotherBall Careers