Pete Koomen
General Partner, Y Combinator; Angel Investor; Co-Founder, Optimizely at Y Combinator
Reviewed Updated Jun 22, 2026This profile is AI-generated. If you spot an error, please help us fix it by sharing a URL to the correct information.
Background
Pete Koomen is a General Partner at Y Combinator and the co-founder and former CTO of Optimizely, the A/B testing and experimentation platform 12. He co-founded Optimizely with Dan Siroker in January 2010 after both had worked together at Google; the company went through YC’s Winter 2010 batch 3. Before Optimizely, Koomen was a product manager at Google from 2006 to 2009, where he helped launch and grow Google App Engine 4.
Koomen helped Optimizely grow from inception to over $100M in annual recurring revenue 1. The company raised a $28M Series A led by Benchmark in 2013, a $57M Series B led by Andreessen Horowitz in 2014, and a $58M Series C led by Index Ventures in 2015 3. Optimizely was acquired by Episerver in 2020, with the combined company rebranded as Optimizely in January 2021 3. Koomen co-authored “A/B Testing: The Most Powerful Way to Turn Clicks Into Customers” 1.
In October 2023, Koomen announced he was joining Y Combinator as a Group Partner; he was later promoted to General Partner 15. He holds a master’s degree in Computer Science from the University of Illinois at Urbana-Champaign 16.
Stated Thesis
Koomen has not published a formal investment thesis, but his writing on AI-native software design points to where he focuses his attention. In his April 2025 essay “AI Horseless Carriages,” he argues that most current AI applications are poorly designed because they mimic pre-AI software patterns 7. He writes:
“Most AI apps should be agent builders, not agents.” 7
He uses Gmail’s Gemini integration as a representative failure case, arguing AI-native software should “maximize a user’s leverage in a specific domain” rather than generating generic outputs the user must then edit 7.
At YC, Koomen has advised notable alumni including PostHog, Ditto, Treble.ai, Waterplan, and TigerEye; companies he has worked with at YC are collectively valued at approximately $1.3B 1.
Inferred Thesis
Sample size caveat: Public records of Koomen’s angel checks are limited. Signal by NFX lists him with roughly 100+ recorded investments, but only a handful of named deals are publicly verifiable. The picture below is qualitative rather than statistical.
Based on the publicly named investments — Cerbos, Sublime Security, Milk Video, and others — Koomen’s angel checks cluster around:
- Stage: Pre-seed and seed only. Typical check size is reported as $5K–$50K with a sweet spot near $25K 8.
- Sector: Developer infrastructure, security, and enterprise SaaS dominate the named deals. Cerbos (authorization-as-code), Sublime Security (email security platform), and Milk Video (video creation tooling) all fit the “tools for technical or product teams” pattern that aligns with his Optimizely background 8.
- AI angle: Koomen’s recent writing and YC work emphasize AI-native software design, suggesting newer checks skew toward AI-first developer tooling.
- Geography: US-centric, with most named investments in the San Francisco Bay Area.
Note that since joining YC as a Group/General Partner in late 2023, the primary mechanism by which Koomen backs companies is through YC’s standard investment, not personal angel checks. Founders seeking him should expect the YC application path rather than a direct angel pitch.
Portfolio
This table is intentionally conservative — it includes only investments verifiable in independent public sources. Aggregator counts (Signal, Tracxn, PitchBook) suggest Koomen has made significantly more checks than are listed here, but most are not individually disclosed in primary sources.
| Company | Year | Stage | Source |
|---|---|---|---|
| Cerbos | 2021 | Seed ($3.5M, led by Crane) | FinSMEs, Nov 2021 13 |
| Sublime Security | 2023 | Angel | Signal by NFX 8 |
| Milk Video | ~2021 | Angel/Seed | Signal by NFX 8 |
| Natural | 2026-07-20 | Series A ($30M, participated as individual investor) | PR Newswire 14 |
Tracxn and PitchBook list additional positions but without per-deal citations these are not included here 910.
In Their Own Words
On evaluating founders, from a 2026 interview with the University of Illinois Grainger College of Engineering:
“You’re trying to bet that these are the super smart, relentlessly resourceful founders who are going to realize their idea is probably bad and keep at it and find another one and find another one until they find something that works.” 11
“It’s a bet on the people more than anything else.” 11
On the current opportunity for founders:
“I can’t think of a time when there’s been more good ideas lying around for founders.” 11
On what makes a great founder, from a 2018 Y Combinator Q&A:
“The two most important skills in a founder are 1. resilience and 2. the ability to attract people that are better than you are. Develop those and you can solve any problem.” 12
On the design of AI applications, from “AI Horseless Carriages” (April 2025):
“Most AI apps should be agent builders, not agents.” 7
“Many of them are infuriatingly useless in the same way that Gmail’s Gemini integration is.” 7
“It’s a world where I don’t have to spend time doing mundane work because agents do it for me.” 7
What Founders Say
No independently sourced founder testimonials specific to Pete Koomen’s angel investing or YC partnership work were found in this research pass. YC alumni he has advised (PostHog, Ditto, Treble.ai, Waterplan, TigerEye) are listed on his YC partner page but without quoted testimonials 1.
Connections
- General Partner, Y Combinator (joined Oct 2023 as Group Partner; later GP) 15
- Co-founder & former CTO, Optimizely — co-founded with Dan Siroker, 2010–2017 operating role 13
- YC W10 alumnus — Optimizely went through Winter 2010 batch alongside Siroker 3
- Former Google PM (2006–2009) — worked on Google App Engine 4
- University of Illinois Urbana-Champaign — MS Computer Science; recognized as a distinguished alumnus by the Siebel School of Computing and Data Science 6
Sources
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“Pete Koomen: YC Partner,” Y Combinator. Accessed June 2026. https://www.ycombinator.com/people/pete-koomen↩↩↩↩↩↩↩↩↩
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Pete Koomen, personal site. Accessed June 2026. https://koomen.dev/↩
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“Optimizely,” Wikipedia. Accessed June 2026. https://en.wikipedia.org/wiki/Optimizely↩↩↩↩↩
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“Pete Koomen,” Golden. Accessed June 2026. https://golden.com/wiki/Pete_Koomen-633JVRV↩↩
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Pete Koomen, “Thrilled to announce that today I’m joining Y Combinator,” LinkedIn post, October 2023. Accessed June 2026. https://www.linkedin.com/posts/petekoomen_thrilled-to-announce-that-today-im-joining-activity-7116108166156713984-FsHZ↩↩
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“Pete Koomen,” Siebel School of Computing and Data Science, University of Illinois. Accessed June 2026. https://siebelschool.illinois.edu/about/awards/alumni-awards/alumni-awards-past-recipients/pete-koomen↩↩
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Pete Koomen, “AI Horseless Carriages,” koomen.dev, April 2025. Accessed June 2026. https://koomen.dev/essays/horseless-carriages/↩↩↩↩↩↩
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“Pete Koomen’s Investing Profile,” Signal by NFX. Accessed June 2026. https://signal.nfx.com/investors/pete-koomen↩↩↩↩
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“Pete Koomen — Portfolio & Founded Companies,” Tracxn. Accessed June 2026. https://tracxn.com/d/people/pete-koomen/__xaT1DJdJiOFLYVd2JTmo0yTfyf1x0kiOQjmJK4VAhvQ↩
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“Peter Koomen Investment Portfolio,” PitchBook. Accessed June 2026. https://pitchbook.com/profiles/investor/225733-24↩
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“Betting on people: A startup founder and investor’s perspective on entrepreneurship,” The Grainger College of Engineering, University of Illinois, 2026. Accessed June 2026. https://grainger.illinois.edu/news/stories/2026-betting-on-people↩↩↩
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“Q&A with Pete Koomen, Cofounder of Optimizely,” Y Combinator blog, October 17, 2018. Accessed June 2026. https://www.ycombinator.com/blog/qa-with-pete-koomen-cofounder-of-optimizely/↩
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“Cerbos Raises $3.5M in Seed Funding,” FinSMEs, November 30, 2021. Accessed June 2026. https://www.finsmes.com/2021/11/cerbos-raises-3-5m-in-seed-funding.html↩
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“Natural Raises $30M Series A to Build Payments Infrastructure for AI Agents,” PR Newswire, July 20, 2026. Accessed July 2026. https://www.prnewswire.com/news-releases/natural-raises-30m-series-a-to-build-payments-infrastructure-for-ai-agents-302829855.html — Series A led by Forerunner Ventures; Pete Koomen listed as individual investor. ↩