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Vienna’s Margelo Joins Poland’s Callstack in a Deal Valuing It Above 20 Million Euros

co-written by newsrooms.ai01. September 2026, 16:06

Marc Rousavy, CEO and Founder of Margelo, and Mike Grabowski, CEO and Founder of Callstack. © CallStack

Polish software company Callstack is acquiring Vienna-based engineering firm Margelo. The two companies announced the deal just now, putting Margelo’s valuation at more than 20 million euros. Both work in the same technical field: React Native, the cross-platform framework that lets teams build iOS and Android apps from a shared codebase.

What Margelo Does

Margelo, headquartered in Vienna, is best known in the React Native community for its open-source libraries. They include Nitro Modules, which lets developers write native modules in C++, Swift or Kotlin (34 million downloads, according to the company), the camera framework VisionCamera (40 million downloads) and the key-value store react-native-mmkv (67 million downloads). On its website, the firm reports more than 420 million npm downloads in total and over 800 contributions to the React Native core.

Alongside the open-source work, the business runs on contract development and performance optimization for app makers. Margelo lists Discord, Coinbase, Expensify, MetaMask, Rainbow, VSCO and WalletConnect among its references, and says it employs around 39 senior engineers. It was founded and is led by Marc Rousavy, also known in the React Native scene as the maintainer of those libraries.

Rousavy describes the start five years ago as a leap into the unknown: “I founded Margelo five years ago and had no idea about business back then, I even dropped out of my economics degree after just three days.” The libraries, he says, were what put the company on the map: “With open-source libraries like VisionCamera (40 million downloads) and react-native-mmkv (67 million downloads) we made a name for ourselves in high-end performance software, and we quickly won clients like Discord, Coinbase, Expensify and VSCO.”

He dates the shift toward architecture to 2024: “Two years ago, with Nitro (34 million downloads), we doubled down on mobile architecture. Nitro was adopted broadly by the community within a very short time, and it allowed us to realize even more of our ideas.”

What Callstack Does

Callstack was founded in 2016 in Wrocław, a few months after Meta released React Native. The company says it employs more than 200 people and serves over 100 enterprise clients. Its focus is helping large organizations adopt, scale and operate React Native. Callstack is also a founding member of the React Foundation, the organization Meta set up to steward React and React Native.

Its finances got a boost last summer, when US fund Viking Global Investors came in with a growth investment that valued Callstack at roughly half a billion zloty (about 120 million euros), according to the company’s own announcement. The current acquisition is presented as a follow-on step to that financing.

How the Deal Is Structured

So far, the valuation of more than 20 million euros is the only figure made public. The companies have disclosed neither the purchase price nor the structure of the consideration, including how much of it is cash and how much is equity. There is also no word on a closing date or regulatory approvals.

The personnel side is clearer. Marc Rousavy stays on, continues to own the development of Margelo’s libraries and will help shape the combined offering. He also points to his future role as a shareholder and to a long backstory behind the deal: “Mike Grabowski from Callstack and I have been friends for a long time. That makes me all the more excited to keep holding a significant stake in the company and to take mobile technology to an entirely new level together.” Addressing his team, he adds: “I want to thank my entire team for making Margelo what it is today.”

Margelo’s existing open-source projects will remain open source, according to the announcement, a detail that regularly draws scrutiny in the community whenever a library maintainer is acquired.

What Comes Next

Callstack wants the acquisition to broaden its technology and open-source portfolio and to support further international growth. Margelo’s technology is to be offered to the same enterprise client base Callstack already serves.

Both companies frame the combination around the way AI is reshaping software development. As AI tools make writing code faster and cheaper, they argue, the value of engineering work shifts further toward architecture, performance, native integration, validation and safe releases. Rousavy puts it this way: the more agentic software development becomes, the more teams need a solid technical foundation on which to build reliable software together with agents.

Callstack CEO Mike Grabowski points to the combination of Margelo’s performance technology and his own company’s track record in complex enterprise environments, arguing that it gives them the team and the reach to solve more of the problems that decide whether software succeeds in production.