Gustaf Alströmer

In 2005, @sama was a Stanford sophomore in YC’s first batch, building a startup in a little Cambridge office while @paulg cooked the founders dinner. Twenty years later, as co-founder & CEO of OpenAI, Sam closed Startup School 2026 in conversation with @garrytan — on agents, ambition, and why the ceiling for what a startup can take on has never been higher. 00:07 — From YC’s First Batch to Today 03:04 — What PG Taught Sam 04:17 — Why Startups Matter More Than Ever 06:57 — The Coming Golden Age of Ambitious Startups 09:30 — Why Startups Win During Technology Shifts 11:46 — Building OpenAI When Nobody Believed in AGI 14:45 — Finding People Who Share Your Conviction 18:05 — Help People Before You Know Why 19:43 — Earnestness, Ambition, and Ignoring the Haters 24:04 — The AI Safety Incident That Changed the Stakes 26:58 — Preventing AI From Concentrating Power 30:41 — How Fast AI Models Will Improve 36:06 — The Best Version of an AI Future 37:47 — “It’s All Going to Work Out”

Today we're launching Rex (YC S26) Every enterprise is exploring where AI can transform back-office operations. Order-to-cash is one of the clearest places to start: it is high-volume, manual work with a direct impact on cash. Introducing Rex:

The deadline to apply for YC F26 is tonight at 8pm PT! If you're an earnest builder working on something people want, we'd love to hear from you: ycombinator.com/apply

Introducing Waddle Labs: Claude Code for robots. Connect our API to your robot and enter a prompt, then our agents write code to achieve the task in 20 minutes. @yiding_song @theWaddleLabs

Today is the deadline to apply for Y Combinator. ycombinator.com/apply. 17 years ago, I was sitting with my co-founders Marie and Pelle (and later Michael) in our freezing New York apartment crafting our YC application. The first sentence of our application still makes sense today. But it was as revolutionary then, and it seems obvious today: “We are creating a mobile social networking application that enables free interoperable mobile instant messaging on your mobile handset.” Marie sent the email, and in that moment, we had no idea what life-changing decision we had just made. Our startup Heysan didn’t become Whatsapp (Whatsapp became Whatsapp), but joining the winter 2007 batch would shape our lives in almost every aspect. Two of us became US citizens and decided to stay here. We all still work on startups in one way or another. For me personally, the most valuable thing was understanding that taking risks isn’t that risky, and if you want to do something, you just do it - don't wait. And it all started with an email that we sent to w2007@ycombinator.com. Today, we don’t accept applications over email anymore, but you can apply ycombinator.com/apply. And if you read this after the deadline - you can still apply! Here is something I wrote for my group in the last batch. I think it applies to founders who are just about to apply just as well: “One of the best parts of working at YC is that sometimes I get to watch a startup go from nothing to becoming a large company that changes society around us, and something mundane in life suddenly gets infinitely better. That only happens because founders like yourself are willing to take some risks and get that journey started. Soon, we will do demo day practice, and I vividly remember when Retool practiced their pitch until 3am, and we told David, “You are done - time to go home!“. It was a similar feeling watching Alex and Shuo at Deel - they pivoted literally a few weeks before demo day into what today is Deel and a $12B company. So many of the best YC companies had almost nothing at this point in the batch - it’s easy to forget. And from the outside, it’s easy to imagine investors / other people thinking, “How can THIS company be worth this much or feel so confident when they have almost nothing”. The trick I’ve learned about being a founder is that it starts with you. If you truly believe in what you are building and how that will change the world, all you have to do is tell the investors with the same level of confidence you feel about this change. And I can tell that a lot of you felt that conviction this week. Becoming good at communicating that conviction will help you hire, sell, fundraise, and convince the rest of the world to believe in what you are building, even though it might not feel like much right now to you. Good luck, everyone, next week. One of the best pieces of advice I’ve ever got is when you are walking into a meeting you feel nervous about - pretend like you have known them your entire life. Don’t get attached to the outcome - just tell your story.” Good luck!

Last year, I was in @ycombinator S25. This time, I got to experience YC from the other side: co-hosting a YC Startup School after party and meeting the next generation of founders. I met so many young people with big ideas who weren’t sure if they were ready to apply. Here’s what I told them: Talk to users. Build. Validate with users. And apply. 𝗧𝗵𝗲 𝘄𝗼𝗿𝘀𝘁 𝗼𝘂𝘁𝗰𝗼𝗺𝗲 𝗶𝘀 𝗮 𝗻𝗼. 𝗧𝗵𝗲 𝗯𝗲𝘀𝘁 𝗼𝘂𝘁𝗰𝗼𝗺𝗲 𝗺𝗶𝗴𝗵𝘁 𝗰𝗵𝗮𝗻𝗴𝗲 𝘆𝗼𝘂𝗿 𝗹𝗶𝗳𝗲. @manufact @ChronicleLabs @insforge @promptingco @pederzh @pietrozullo @sir_aymansaleh @binsquares @hanghuang_ @tonychang430 s/o to @AuroraZhangYY for putting everything together!

Right now is the best time in history to start a startup. And the deadline to apply to the YC F26 batch is tonight. It only takes a few minutes, and some of the best companies in the world applied last minute too. Apply here: ycombinator.com/apply

Fresh off the launch of Opus 5, Claude Code creator Boris Cherny (@bcherny) joins YC's @sdianahu at Startup School 2026 to talk about what the newest models can do, how Claude Code came to be, and what it means to build products when the underlying capabilities keep accelerating. 00:07 — What Makes Opus 5 Different 02:06 — Solving Prompt Injection 03:21 — Why Claude Code Deleted 80% of Its System Prompt 06:37 — Press Delete on Your AI Product 07:20 — How to Rebuild Your System Prompt 10:30 — Product Overhang and “Unhobbling” AI 14:26 — Give Claude Harder Problems 19:32 — Prompt Engineering Is Changing 21:57 — The Two-Week Claude Code Prompt 24:42 — Running Thousands of AI Agents 30:15 — Coding Is (Almost) Solved 32:20 — What Every CS Student Should Still Learn @AnthropicAI @claudeai

Software engineering is becoming a hierarchy of loops. So today we're launching Mastra Factory: a system for agents to take software from issue into production. Get started with 𝚗𝚙𝚖 𝚌𝚛𝚎𝚊𝚝𝚎 𝚏𝚊𝚌𝚝𝚘𝚛𝚢:

Live in the future. Build what’s missing. @ycombinator

Attending YC startup school in my late 20s, some thoughts after day one

just went on stage with @bcherny for startup school and he had a special announcement for all 6k+ attendees: free claude max 20x accounts for 6 months! thank you!!

YC Startup School 2026 just got going 3x bigger than last year We made this event for first time very technical young people to come to SF and figure out if starting a company is for them Let you in on a secret: it is

YC Startup School 2026 is live!

My introduction to startups (that I didn’t act on for 4 years) was in my freshmen year of college in 2013 when I attended startup school. Can’t wait for an incredible group of students to share that experience (except 100x better than it used to be)

gm startup school

This founder was talking to "AI Brad" when "real Brad" walked up to him. The look of surprise on his face is priceless.

YC Startup School is for many thousands of technical young people people the very first time they step foot in San Francisco Welcome to the place dreams are born!

@subramanya Those two things!

@gustaf what actually made it stick: faster triage or drafts waiting when you open the inbox?

Scape became my primary email client for the last couple of months. What they built is inevitable and the new bar for how email clients should work. ✉️