ceo and founder @callstackio, @reactjs foundation, codex ambassador @openai. i love cars ποΈ
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When Viking became a shareholder in Callstack, I knew we were setting out to deliver an ambitious plan. I wanted to build Callstack into a globally significant technology company. Viking believed in what we had already built and in how far we could go.
Today is the next chapter of that plan! πͺ
@mrousavy and I have been friends for years, and we have talked about working together for a long time. At one point, we even promised each other that we would start a venture together one day. Today, with Margelo joining Callstack, we finally get to build together.
The timing feels right. AI is changing how software gets built. Teams can produce more, faster, but architecture, performance, validation and safe releases remain difficult to get right.
Those are the problems both teams have spent years solving.
Margelo is a high-end engineering company that has built some of the strongest performance technology in React Native. Together, Nitro Modules, VisionCamera and react-native-mmkv have recorded more than 142 million npm downloads combined.
Callstack has spent a decade helping large organizations adopt and scale React Native while contributing to the framework and open-source ecosystem.
Together, we are bringing Margelo's performance-first technologies and Callstack's enterprise delivery experience to the same clients. We will continue developing Margelo's open-source libraries while expanding what we can offer enterprises building, scaling and operating high-performance cross-platform products.
Marc and the @margelo_com team: welcome!
Time to build the next chapter! πͺ
π @margelo_com Joins Callstack to Advance High-Performance React Native Engineering
Vienna, Austria. 1 September 2026.Β Callstack, a leader in React Native and cross-platform development, today announced that Margelo, the Vienna-based high-end engineering company behind Nitro Modules, VisionCamera and react-native-mmkv, is joining Callstack in a transaction valuing Margelo at more than EUR 20 million.
Margelo builds high-performance technology for React Native. Nitro Modules (34M downloads) lets developers write fast, type-safe native modules in C++, Swift or Kotlin, and quickly gained widespread traction across the ecosystem. VisionCamera (40M downloads), Margelo's enterprise-grade open-source Camera framework and react-native-mmkv (67M downloads), a key-value storage library, power countless React Native apps today.
Callstack has spent a decade helping large organizations adopt, scale and operate React Native, contributing to the framework and its open-source ecosystem along the way.
Together, the two teams give clients the strongest React Native performance expertise in the market, backed by delivery proven in complex enterprise environments. Margelo's technology and Callstack's delivery experience will now be available to the same clients.
The timing reflects a fundamental change in how software is built. AI is making implementation faster and cheaper, but architecture, performance, native integration, validation and release safety are becoming more important, not less. These are the fundamentals on which both companies have built their reputations and on which they will continue to build together.
"Margelo has built some of the strongest performance technology in React Native. Callstack has spent a decade helping large organizations adopt and scale the framework. Bringing those strengths together gives us the team and reach to solve more of the hard problems that determine whether software succeeds in production," said Mike Grabowski, CEO and Founder of Callstack.
"At Margelo, weβve built high-performance technology that developers already rely on. Weβre excited to partner with Callstack to bring it to more clients and scale its impact. As software development becomes increasingly agentic, we want that technology to provide the foundation teams need to build reliable software with agents," said Marc Rousavy, CEO and Founder of Margelo.
Margelo's existing open-source libraries will remain open source. Marc will continue to lead their development and help shape the combined company's offering.
The transaction follows Callstack's 2025 growth investment and supports its strategy to expand technology and open-source capabilities while growing internationally.
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Mike retweeted
I became a T3 Code user recently - itβs great especially if you have many subs and many machines to manage. I recommend giving it a shot if you havenβt already π. I also like in-app simulator preview experience support powered by @expo device hub (for streaming) and @agent_device (for agentic control) π
T3 Code now has a device hub to let you and your agent see and manage iOS and Android simulators while working!
One of the coolest things (I think) is that the simulator doesn't even have to run on your device! You can setup remote hosts to let agents running on Linux boxes trivially test iOS changes on a Mac Mini!
Still early but wanted it out there, lmk any feedback! Available now on nightly!
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Mike retweeted
Meta just announced new ways to build for Meta VR devices, and React Native is part of that story.
The new Meta VR Layout SDK brings spatial, multi-panel experiences to React Native, while the new Expo Launch integration makes the path from an Expo project to Meta's Release Channels and Store much simpler.
We've been working with @Meta on React Native support for VR, and today we're updating our developer documentation alongside the announcements.
More coming soon in our React Native reference app as we add examples for the new spatial APIs.
Read more π clstk.com/3VvaKMc
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Meet TraceSift β‘οΈ
An open-source tool that turns CPU and React profiles into actionable performance insights without flooding your agentβs context.
Early benchmark (GPT-5.6 Terra):
β’ Codex: 45K tokens
β’ TraceSift: 1.6K tokens
Thatβs ~28Γ fewer tokens π
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Building with agents brings back the feeling I had in the early days of React Native. So much to figure out, so many people trying things! π₯
Bringing those people together at Agent Conf felt like a very natural next step. Proud we made it happen π
Software development is moving from hand-written implementation toward systems where engineers direct agents, shape context, and judge the result. In the Agent Conf 2026 keynote, Mike Grabowski and MichaΕ PierzchaΕa share how @callstackio is adapting to that shift.
Follow @apex__ai and @agent_device for real-time updates.
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Mike retweeted
cameo at Agent Conf
Software development is moving from hand-written implementation toward systems where engineers direct agents, shape context, and judge the result. In the Agent Conf 2026 keynote, Mike Grabowski and MichaΕ PierzchaΕa share how @callstackio is adapting to that shift.
Follow @apex__ai and @agent_device for real-time updates.
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Mike retweeted
We deliberately added an invalid commit reference to an agentic workflow. The Reviewer rejected it, the Developer corrected it, and the Reviewer verified the fix before human approval. A controlled recovery loop sent verified work forward. Read more about itπ
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Mike retweeted
Grateful for @o_kwasniewski sending over 20 PRs to agent-device while testing their e2e package π
tester.army/e2e
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Open source has been part of our DNA since 2016. To give back to those who shared their RN experience for RN at Scale 2026, we donated $1.3K to hot-updater, lerna-lite, changesets, release-it, react-native-maps, and react-native-keychain. Thank you to everyone maintaining them π
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The man is cooking
Swift can now convert Swift closures to πππ::ππππππππs.
It's very simple in theory; the Swift closure is passed to C++ as a ππππ* that will be called with a little bit of ptr magic (ππ πππππππ).
The closure's capture context (aka any values from "outside" the closure that are used "inside" the closure) is also a ππππ* that will just be passed along.
This is almost exactly how you would bridge a closure/stateful function over to C, with the difference being that in C you would also need a ππππ(..) function - here we use ππ πππ_πππππππ(..)
github.com/mrousavy/swift/blβ¦
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I want to bring C++ exceptions to Swift.
My proposal is to add a new πππΈπ΅π_ππ·ππΎππ annotation to C++ methods, which imports them as πππππ π in Swift.
The πππ::ππ‘πππππππ::π πππ() will be copied into Swift to represent a Swift π΄ππππ - fully on-stack.
With the newly proposed compiler option -ππ‘π‘-ππ‘πππππππ-ππππ=ππππππ, every C++ method that is not marked ππππ‘ππππ will be imported as πππππ π in Swift - no πππΈπ΅π_ππ·ππΎππ needed.
This highlights why Swift's explicit πππππ π requirement is a better language design decision than C++'s implicit "everything can throw".
It's interesting that C++ exceptions are zero overhead if they are not throwing, while Swift is - because it essentially wraps everything that can throw in a ππππππ<π>.
forums.swift.org/t/handling-β¦
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This talk has the best tip I heard at the conference
How to build self-improvement loops? You ask agents for feedback!
My talk about AI at @expo inspired some folks so sharing here too.
piped.video/watch?v=mkJlmG1rβ¦
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Mike retweeted
Cool meetup in NYC tomorrow! Wonβt be there but you should come if youβre into mobile dev luma.com/fegsycpl
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Mike retweeted
Introducing Appduct: Call your app functionality from the outside, instantly.
Appduct doesn't use accessibility information or vision. It integrates directly into your app. More setup, instant interactions.
Works with Swift, Kotlin & React Native.
incubator.callstack.com/appdβ¦
Your E2E test isn't slow because of the thing you're testing.
It's slow because of the login, the onboarding, the cookie banner, and the cart you rebuild tap by tap every single run.
Call `seedCart(3)` instead. Start the test at the part that matters.
incubator.callstack.com/appdβ¦
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Mike retweeted
great stuff here if you missed it!
All talks from @AgentConf are live on YouTube.
Here's the full playlist: piped.video/playlist?list=PLβ¦
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Mike retweeted
Agent Conf is now on YouTube.
Every speaker. Every talk. Ready to watch.
A huge thank you to everyone who took our stage and gave the community new ideas to test and conversations to continue.
Hit play π
clstk.com/4Axb1P0
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