Block Joins the Kotlin Foundation

We’re excited to announce that Block has officially joined the Kotlin Foundation, a non-profit organization created to protect, promote and advance the development of the Kotlin programming language. This milestone highlights a special, shared history between our engineering teams and the Kotlin community. In fact, JetBrains’ (the company that developed and supports Kotlin) new multiplatform I/O library, Kotlinx-io , is built on Square’s own Okio library. Okio’s design for efficient data streams has been a foundation of our software for years, and seeing it live on in Kotlinx-io underscores the continuity between our past contributions and Kotlin’s future. By joining the Kotlin Foundation, we’re deepening this relationship – not only to support the language we love and rely on, but to ensure that our products and customers benefit from Kotlin’s continued evolution.

Kotlin at Block: A Longstanding Commitment

Block’s journey with Kotlin began early and runs deep. Square was among the early adopters of Kotlin for Android, embracing “Project Kotlin” in production back in 2015. This forward-thinking choice improved our Android development speed and code quality. By 2017, we had officially selected Kotlin as the language of choice for new backend services, bringing Kotlin’s clarity and reliability to our server-side systems that power millions of transactions. In the following years, our enthusiasm for Kotlin only grew – we experimented with Kotlin/Native to share code on iOS in 2019, and by 2021 even our web clients were tapping into Kotlin/JS for shared logic. In short, Cash App and Square have used Kotlin on virtually every platform we operate (mobile, web, and server), making Kotlin a cornerstone of Block’s technology stack.

Importantly, we haven’t just used Kotlin – we’ve actively contributed to its ecosystem. Our engineers at Square and Cash App have created many popular open-source libraries in Kotlin or for Kotlin developers. For example, OkHttp , our modern HTTP client for Java/Kotlin, became so widely adopted that it’s now the default HTTP client on Android devices. Libraries like OkHttp (and its companion Okio) set benchmarks in reliability and performance, and have been Kotlin-friendly from the start. We’ve also created open source multiplatform tools such as SQLDelight (for database management) and adopted Kotlin Multiplatform early on, which helped prove out Kotlin’s capability to share business logic across Android and iOS. In fact, one of our engineers noted that using Kotlin Multiplatform let our Android, iOS, and even backend teams collaborate on a single codebase – a powerful testament to Kotlin’s versatility. All of these efforts – from mobile libraries to backend services – reflect a long-standing commitment to Kotlin’s success. This history of adoption and contribution made joining the Kotlin Foundation a natural next step.

Why We Joined the Kotlin Foundation

The Kotlin Foundation is the non-profit organization established by JetBrains and Google to steward the Kotlin language’s development. By becoming a Foundation member, Block is affirming our support for the broader Kotlin community beyond our own walls. We believe that when Kotlin thrives, everyone – from individual developers to companies like ours – benefits. Joining the Foundation allows us to take a more active role in that success.

What does Foundation membership entail? It means Block will contribute resources and expertise to help Kotlin’s ecosystem grow and prosper. In particular, this membership allows us to:

• Help guide Kotlin’s future direction: We’ll have a voice in Kotlin’s evolution by participating in Foundation committees and working groups, collaborating with JetBrains, Google, and other members on the language roadmap. This helps ensure Kotlin continues to meet the needs of developers and industries (including fintech) in the years ahead.
• Support community initiatives through funding: As a member, Block contributes financially to the Kotlin Foundation. These funds go toward community programs like grants for open-source Kotlin projects and Kotlin education contests for students. In other words, we’re directly investing in the Kotlin community’s growth – supporting the maintainers, educators, and rising developers who make the ecosystem strong.
• Share our domain expertise: Block’s engineering team brings deep experience in building large-scale, secure fintech applications. We look forward to sharing our insights with the Kotlin community – for example, offering feedback on new features or libraries based on our real-world experience in payments and banking. As an official member, our voice can help ensure Kotlin continues to excel in performance, security, and developer productivity for all use cases. The Foundation explicitly values such expertise from industry leaders, and we’re eager to contribute to Kotlin’s ongoing innovation.

Ultimately, joining the Kotlin Foundation is about giving back to the community that has given us so much. Kotlin has been a strategic technology for Block’s businesses (from Square’s payment systems to Cash App’s consumer finance tools), and this membership is our way of helping to guide and nurture that technology for the benefit of all.

Open Source at Block: Building for Our Customers

Supporting Kotlin through the Foundation is a natural extension of Block’s broader commitment to open source and to our customers. Open source technology has always been central to how we build products at Block. It provides transparency, security, and freedom for our customers, and accelerates innovation for our engineers. Kotlin is a perfect example – its expressive, safe design allows our developers to create new features quickly while maintaining the high reliability standards that financial services demand. By investing in Kotlin’s community, we’re investing in our ability to continue delivering exceptional products and services.

This commitment also aligns with the mission of Block’s Open Source Program Office, which seeks to foster deeper relationships with open-source foundations and standards bodies . Our Kotlin Foundation membership is a prime example of that mission in action. It’s not just about influencing a programming language; it’s about contributing to an ecosystem that underpins countless applications, including our own. Every improvement in Kotlin, (whether in developer experience, performance, or multi-platform capability) can translate into better experiences for the people and businesses who use Block’s products.

Many modern fintech apps and services (inside and outside of Block) rely on Kotlin’s technology, so strengthening Kotlin means empowering those developers to build more ambitious solutions. From enabling safer digital payments to accelerating development of new financial tools, a robust Kotlin ecosystem benefits everyone. We believe that supporting open source communities like Kotlin leads to “new solutions that drive economic empowerment” – which is Block’s ultimate purpose.

Kotlin has played a strategic role in many of Block’s innovations, and now we’re helping ensure it continues to do so for years to come. We’re excited to guide and grow Kotlin together with the community, because when developers and companies unite around open source, everybody wins – from the engineers writing the code to the customers benefiting from faster, richer, and more secure financial applications. The future of Kotlin at Block, and in the wider community, looks bright, and we’re proud to be part of it.

To learn more about Block’s open source initiatives, visit the Block Open Source website.