General Partner @YCombinator, Co-Founder @FutureAdvisor (YC S10), MIT EECS
San Francisco, CA
Joined June 2007
- Tweets400
- Following380
- Followers3.9K
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Jon Xu retweeted
Startup School is coming to Taipei on November 12! 🇹🇼
Hear from founders like @andyfang of DoorDash, @jeffchanginvest of Vest, @moseslo of Xendit, @mchiang0610 of Ollama, and more.
And join builders and hackers from across Asia for a day of talks and sessions with YC partners.
So pumped to work with Vivian and Raphael! Excited for the founders that get to partner with them.
We’re excited to welcome @vivianmshen and @raphaelschaad as YC’s newest General Partners!
Vivian and Raphael first came to YC as founders.
Vivian co-founded Juni Learning (W18), which teaches kids coding and math, and later founded Acely, an AI tutor for college prep. Raphael founded Cron (W20), the calendar app that became Notion Calendar.
They built products used by millions of people, then returned to YC as Visiting Partners to help other founders. Now they're doing it full-time.
ycombinator.com/blog/welcome…
Fresh off their Series A announcement, @Jerr_Wu and @wgm752 from @HalluminateAI joined me to discuss the future of post-training, simulating entire companies, and why alignment and safety start with the environments models learn in. Really enjoyed their perspective from building at the frontier of model training.
Halluminate is building reinforcement learning environments and benchmarks to help AI models do knowledge work beyond coding, starting with finance.
With a team of fewer than 10 people, the company works with four of the top five closed-source US AI labs and recently raised a $30M Series A.
In this fireside, co-founders @Jerr_Wu and @wgm752 sit down with YC's @xuster to share how testing browser agents led them to building the environments models now use to practice tasks like financial modeling. They explain why better verification, not just more training tasks, is critical to making models smarter, and how that work could eventually lead to simulated companies where teams of AI agents learn to work together.
00:00 — Halluminate’s Pivot From Evals to Training Environments
03:48 — Teaching AI to Do Financial Work
07:13 — Why Verification Matters More Than Volume
10:13 — From Finance to Simulated Companies
13:30 — AI Safety and Building the Team
Congrats to @TungaBayrak and the @FreyaVoice team on launching the most human-like AI voice model.
Today we’re releasing Adam and Eve, the most human-like AI voices ever built.
Adam ranks #1 among AI models on @DesignArena’s AudioRealismBench.
We believe we’ve crossed the uncanny valley. API access on our Website!
Listen: freyavoice.ai
Leaderboard: designarena.ai/leaderboard/a…
Special thanks to @alpsencerozturk, @ahmeterdempmk, and the entire Freya team for making this possible.
We’re just getting started. Join us!
You know a team is humble when “5x’d ARR” off of a strong base is just another bullet point. Loved working with @alexytung and @leonyaolh.
Officially a YC Alum! Experience of a lifetime.
> Met the best mentor ever @xuster
> 5x’d ARR
> Built something people want
> Took 25 flights all over the US
> Met many friends and future unicorn founders
Sometimes the best defense requires a strong offense.
We got into @ycombinator 3 months ago with zero revenue and today @FabraixHQ is at $1.04M annualised run rate.
Traditional cybersecurity was built for software that behaves within defined boundaries. AI agents can reason, use tools, take actions, and find ways around those boundaries.
The cost asymmetry we're seeing in irl warfare is also now a software problem.
So we built autonomous red-teaming agents that find real vulnerabilities in AI agents that work 24/7, powered by our own state-of-the-art offensive models that outperform frontier models.
AI agents are automating all knowledge work.
@FabraixHQ is the adversarial testing infrastructure that will secure every production agent.
The best way to predict the future is to create it.
So excited for the companies presenting at YC Demo Day today. Robots entering the real world, frontier agents tackling industrial logistics, new possibilities in personalized medicine, and much more.
Can’t wait to live in the future these founders are building.
Jon Xu retweeted
CNBC + @garrytan filming live from the hardware floor of @ycombinator Demo Day!
They are sitting within 20 feet of teams building jet-powered interceptors, military exoskeletons, humanoids, cargo rockets, drones that can move clouds...
Jon Xu retweeted
We're giving enterprise buyers an early look at YC companies building AI tools for them. Apply now if you're interested in joining and attending our first event with the Fall '26 batch at YC SF, Oct 22.
Introducing the YC Early Access Network.
4x a year, senior technology leaders get early looks at YC's enterprise AI companies and help shape their roadmaps.
Invite-only, by application. Fall '26 kicks off Oct 22 in SF.
Apply now: events.ycombinator.com/yc-ea…
Jon Xu retweeted
Legora's revenue has increased by over 9x from this time a year ago. That's an astounding growth rate for a company of this size. It's not unusual for a startup to grow that fast in the first 6 months, but Legora is 3 years in.
The momentum continues at @WeAreLegora ...we beat our original Q3 '26 plan a full month early. July was the strongest quarter-opening month in Legora's history, and August was even larger.
Total net new ARR grew 36% MoM.
But the story of the month is in-house teams. ARR from in-house is up 15x+ YoY, average ARR per in-house customer is up 2.5x+ YoY, and in-house teams are now almost half of our total new customers.
Most importantly, you earn ARR by building the best product on the market. Our usage climbs every month: 16+ hours per monthly active user.
In September, we expect to add more ARR than in all of Q1 '26 combined, which would make Q3 the best quarter in our history and capture yet another ARR milestone.
LFG!
Jon Xu retweeted
Introducing @robocurve, a Public Benefit Corporation to measure and report frontier robotics capabilities.
We build real-world evaluations for robots and publish results as a neutral third party.
Jon Xu retweeted
We’ve started moving onto longer-horizon tasks.
Table bussing - π0.5 completed 48/50 rollouts on a consistent setup. Very much in distribution, but a useful baseline.
Jon Xu retweeted
YC invented the SAFE a decade ago, simplifying the process of raising seed money for founders across the world.
Today, we’re releasing a SAFE tool to make it even easier to raise your seed round on SAFEs. And it’s free!
In 2013, @cjoneslevy created the SAFE at YC. It's since been used by YC founders to raise $15B+ and has become the standard way startups raise money.
So we're launching Send a SAFE: a free tool to generate, sign, & send one in ~2 minutes, built to be used by founders (and their AI agents).
Great founders see the world as it should be and pull us forward into the future. Aerogen is building toward chip fabs that no longer need expensive cleanrooms 🚀
We’re launching Aerogen Systems on YC today.
We put modular reactor purifiers inside chipmaking tools to keep wafers clean without a cleanroom.
Talos cleans the tools. The Cleanpipe connects them.
Find us here: aerogensystems.com
Jon Xu retweeted
Today we're launching @akonlabs backed by @ycombinator
We build GitNexus, an open-source knowledge graph for your repository. Coding agents query it to get exactly the context a task needs symbols, call paths, dependencies instead of grepping around and stuffing half your codebase into a context window.
Context is the bottleneck for coding agents. Bigger windows haven't fixed it. Buyers of agents are paying more per token to get worse code, and the agents themselves have no map of the repo they're editing.
We're fixing this. On DeepSWE, the same agent with GitNexus vs. without:
1. 37% → 68.4% of tasks passed
2. Half the cost per fix
3. On the hardest tasks, the gap more than triples
One stack: knowledge graph, coding agent, automated PR review. Managed, self-hosted, or air-gapped.
45k GitHub stars. 65k weekly npm downloads. 10+ enterprise deployments.
Sign up at: app.akonlabs.com
Jon Xu retweeted
If you're an MIT alum or MIT intern in the bay, and you're interested in startups:
Come by the YC office next week for happy hour!
events.ycombinator.com/mit-y…
