Callstack, a React Native software company founded in Wrocław, Poland, acquired Margelo, a Vienna-based engineering firm that builds open-source tools for the same framework. The companies announced the deal on September 1, 2026, and said it valued Margelo at more than €20 million (about $23 million).
Neither side disclosed an exact purchase price or how the consideration was split between cash and shares. Trending Topics reported that no closing date or regulatory approvals were mentioned.
Margelo founder Marc Rousavy is staying on. He will continue to lead development of Margelo's open-source libraries and help shape the combined offering, according to the announcement on Margelo's blog. The existing libraries will remain open source.
What Margelo built
React Native, a framework released by Meta, lets developers write one codebase and ship native apps for both iOS and Android. Margelo, founded about five years ago, made its name with open-source modules that extend the framework and improve app performance.
Its best-known projects are Nitro Modules, which lets developers write native modules in C++, Swift or Kotlin; the camera framework VisionCamera; and the key-value storage library react-native-mmkv. Trending Topics, citing the company, put their downloads at 34 million, 40 million and 67 million respectively, and said Margelo reports more than 420 million npm downloads across its work.
Alongside the open-source projects, Margelo earns money from contract development and performance work for app makers. It lists Discord, Coinbase, Expensify, MetaMask, Rainbow, VSCO and WalletConnect among its clients and says it employs around 39 senior engineers, according to Trending Topics.
“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.”
A founder-owned company
Margelo had no outside shareholders: before the deal it was owned entirely by Rousavy, brutkasten reported. Writing on LinkedIn, he said he would keep a significant stake in the company after the transaction.
Rousavy described starting the business with no commercial background. He said he had dropped out of an economics degree after three days, and that libraries such as VisionCamera and react-native-mmkv brought in early clients. He dated a shift toward mobile architecture to the release of Nitro two years ago.
He also said he and Callstack founder Mike Grabowski had been friends for a long time. Grabowski wrote on LinkedIn that the two had talked about working together for years and had once promised each other to start a joint venture, brutkasten reported.
Why Callstack is buying
Callstack was founded in Wrocław in 2016, a few months after React Native's release. It says it employs more than 200 people and serves more than 100 enterprise clients, helping large organisations adopt and run React Native. It is a founding member of the React Foundation, which Meta set up to steward React and React Native.
Last summer, US fund Viking Global Investors made a growth investment in Callstack that valued the company at about 500 million zloty (roughly €120 million, or about $140 million), according to Trending Topics, which said the Margelo deal was presented as a follow-on step to that financing.
The companies said Margelo's clients will gain access to Callstack's experience running React Native at enterprise scale, while Margelo's technology will be offered to Callstack's enterprise base. Callstack also expects the deal to broaden its open-source portfolio and support international growth.
Betting on AI-written software
Both companies framed the combination around changes AI is bringing to software development. As AI makes writing code faster, they argued, more of the value shifts to architecture, performance, native integration, validation and release safety.
In a press statement quoted by brutkasten, Grabowski said combining the two firms' strengths gave them the team and reach to solve more of the hard problems that decide whether software succeeds in production. Rousavy said that as software development becomes more agentic, the combined company wants to build the foundation teams need to develop reliable software with AI agents.
