Kevin Kho

Community Engineer @PrefectIO
@fugue_project Maintainer
Orlando Machine Learning and Data Science Organizer

[Kevin Kho @kvnkho]

@juliafedorin @calcsam Respect! Building one successful open source project is hard enough

[Julia Fedorin @juliafedorin]

Fewer than five people on Earth have built two open-source JavaScript framework companies. @calcsam is one of them.

The book that made him known is now one of the go-to texts for building agents. You’ve seen it: Principles of Building AI Agents.

But the part that’s important to his story is the low point before @Mastra. When @GatsbyJS faded, the acquisition went sideways. They went off to build sales AI and failed for months, then came back to open-source dev tools and instantly felt like they were back on track.

His word for it: the "unknown knowns." The things you know so well you forget you know them. They'd forgotten they were world-class at the exact thing they walked away from.

I sat down with him at @ycombinator, where Mastra went through the Winter 2025 batch.

We got into:
•Why "that's interesting" is the most dangerous thing a user can tell you
•Why planning in quarters is dead, it's actions-per-minute now
•Why it's okay to quit your idea
•Writing a print book in an era that moves too fast to print
•The loneliness nobody talks about: teams stopped pair programming because everyone's driving their own agent

His rule, from a YC partner: generosity breeds luck.

𝐓𝐈𝐌𝐄𝐒𝐓𝐀𝐌𝐏𝐒
(00:00) Intro — Y Combinator
(01:30) What Mastra Is
(03:00) Content Wants to Be Frequent
(05:00) Shipping a Tier-One Feature Every Day
(07:00) The $13M Seed and $22M Series A
(09:00) Turn-Based vs. Real-Time Strategy
(12:00) Why Experience Beats Fresh Eyes
(15:00) "That's Interesting" Means They're Not Interested
(18:00) It's Okay to Quit Your Idea
(21:00) The Book That Started as a Joke
(25:00) From Journalism to Open Source
(28:00) The Netlify Acquisition, and What It Cost
(32:00) The Unknown Knowns
(36:00) The YC Winter '25 Takeoff Story
(40:00) Remote Work Is Underrated in the Age of AI
(44:00) The New Loneliness of Building With Agents
(47:00) What He Still Can't Figure Out About Agents

[Kevin Kho @kvnkho]

@thatcort @DataPolars Hi @thatcort, no numbers on Ray. Just change the engine from Spark to "ray" and it will use the initialized Ray. I don't think it uses Polars under the hood.

[Kevin Kho @kvnkho]

@shakudo_io https://t.co/AzZHpykhb9

[Kevin Kho @kvnkho]

@Marvindesmond1 @PrefectIO @Al_Grigor Thanks for working on it! @Marvindesmond1 Keep it up!

[Kevin Kho @kvnkho]

@CSReuter @PrefectIO 👋this guy also