Jon Xu

General Partner @YCombinator, Co-Founder @FutureAdvisor (YC S10), MIT EECS

[Ansh @ansh_chokshi]

introducing land search and enrich api + mcp

billions of real world data points, structured for agents to make decisions.

now your agents can:

> find matching land across 160M+ parcels.
> filter and rerank using 550 available data fields.
> compare assets using structured data with sources.

fetch, our location data api, got 4M+ calls. and we just made it completely free.

built for agent speed and scale. dirt cheap.

sign up and get $10 for free: https://t.co/acQmRkYcdx

[Jon Xu @xuster]

@mynameisyahia @generalcatalyst @orangecollectv Insane product and team💥📈🚀

[Jon Xu @xuster]

@zakariaornot Super cool!

[Jon Xu @xuster]

Insane team!

[Y Combinator @ycombinator]

Applied Electrodynamics is building cameras that can see through walls and other opaque materials.

Using radio waves, they can locate studs, pipes, cables, and junction boxes behind surfaces like drywall or marble and turn it into a dimensionally accurate 3D model.

[Y Combinator @ycombinator]

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.

[Jon Xu @xuster]

So pumped to work with Vivian and Raphael! Excited for the founders that get to partner with them.

[Y Combinator @ycombinator]

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.

https://t.co/8dvzPwxZlU

[Jon Xu @xuster]

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.

[Y Combinator @ycombinator]

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

[Jon Xu @xuster]

@Jerr_Wu and @wgm752 have built one of the fastest growing companies out of YC Summer 25 batch.@HalluminateAI is defining the core training infrastructure for how frontier models get smarter. Very excited for what's coming next from them. Congrats on the Series A! 🚀

[Jerry Wu @Jerr_Wu]

Today we're excited to announce @HalluminateAI's $30M Series A led by @oakhcft with participation from @YCombinator, @orangecollectv, @FTPartners, @heavybit, and more.

[Jon Xu @xuster]

Congrats to @TungaBayrak and the @FreyaVoice team on launching the most human-like AI voice model.

[Tunga Bayrak @TungaBayrak]

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: https://t.co/gHXurz0gSc

Leaderboard: https://t.co/cQmbwyBqrB

Special thanks to @alpsencerozturk, @ahmeterdempmk, and the entire Freya team for making this possible.

We’re just getting started. Join us!

[Jon Xu @xuster]

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.

[Alex @alexytung]

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

[Tyler Bosmeny @bosmeny]

people are saying

[Jon Xu @xuster]

Sometimes the best defense requires a strong offense.

[Zach AL @zachdotai]

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.

[Y Combinator @ycombinator]

Ashton joined us at Demo Day!

[Jon Xu @xuster]

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.

[Tyler Bosmeny @bosmeny]

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...

[Harj Taggar @harjtaggar]

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.

[Paul Graham @paulg]

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.

[Jay Chooi @chooi_jeq]

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.

[James Stevens @jamesw_stevens]

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.

[Ankit Gupta @agupta]

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!