Retrospect Featured

Journey to Ethereum Core Dev

Jun Song

28 Jun 2026 • 7 min read

@Olympic National Park

🇰🇷: https://junsong.xyz/ko/core-dev-journey-ko/

Belated career update: I've joined Offchain from this June, as an Ethereum core developer in Prysm. This was one of the milestones for my career, so really happy to share this 😄.

Since I've dived into Blockchain and decided to start my career here (right after completing my military duty at early 2022), my primary goal was to become a blockchain core dev. If I recall myself back then, I couldn't really realize "what is core dev" but from my viewpoint, the term "core dev" was cool (😁). Also I've identified myself as an engineer who is digging into the lower level, so the core dev is pretty fit here.

I chose Ethereum among hundreds of blockchain because of community. Bitcoin and Ethereum are the only two that everyone in the Internet can see what's going on and attend the core development process. And Ethereum has much bigger community and quite friendly to newcomers, thus I thought it's a great place to start something. I also like most of the philosophies that Ethereum pursues.

Aug 2024: First PR which might be a hassle

Back then, the company I was working for at the time used Prysm, and I frequently had to review the Prysm source code. Thus I could find several improvements for the codebase. But the very first PR was super simple: fixing misleading variables in test code. Now we're in AI era, so if I submit this PR today, this should be a hassle for maintainers to review the PR (the impact was low).

After approval of this PR (which I didn't expect to get) and merging it, I felt great: I discovered the charm of open source. What excited me more was that even a single line that I wrote could be part of such a huge codebase, and that my code could be reviewed by experience developers.

Then, I submitted a new PR to Prysm that resolves a small issue in P2P. This PR is quite interesting, because this couldn't get much attention but after one year this issue was discovered so I should rework based on this issue. You can read more about this issue in my blog post above.

During this time, I focused on open source, particularly Ethereum core clients, and opened multiple PRs. (Reth, Lighthouse)

Nov 2024: My first Devcon

Devcon is one of the biggest event related to Ethereum. It took place in Bangkok in 2024. It was inspiring as an open source contributor to meet developers in person whom I only could interact with them on Github. Although a lot of things went online after COVID, I think it is still important to build the offline network. It was great that few devs were recognizing me as a yellow-taco-profiled guy. I also asked how to master P2P to Manu who is one of my colleague in Prysm.

I was really impressed from the talk from Justin Drake (EF) which was about Beam Chain. Beam Chain itself rebranded as lean Ethereum at this moment. I'd say this was one of the turning points of my career as I happened to contribute to Ream which is pushing lean Ethereum initiaives in 2025.

Dec 2024: Found an important issue

After Devcon, I was self-motivated - This was the moment that I started to dream playing borderlessly. (And this is definitely the best advantage for building a career on crypto.) Remote work is pretty common and devs can prove themselves via Github activities.

At December 2024, I found a memory leak issue on Prysm when upgrading to Pectra, and resolved this issue with a couple of PRs(#14697, #14829). Off the record, this was the point that the team members think legit about me.

Jun 2025: Ethereum Protocol Fellowship

In 2025, I finished my two years at Superblock and continued my university work in Daejeon. I had a clear goal to be a core dev, but was bit confused about what and how. Jihoon recommended EPF (Ethereum Protocol Fellowship) at that time. He was also the one who successfully finished the previous cohort and about to start contributing on Ethereum core research.

In April 2025, after a single round of interview with EPF organizers, I was selected as one of 18 initial fellows for the cohort six. I obviously thought I should keep my momentum on Prysm, so decided to commit my several months on the project called "SSZ Query Language". If you're interested, you can read more about my retrospect on my blog post as well.

Dec 2025: Started the hiring process

Before the end of EPF cohort, I wanted to start a new project on Prysm. This was one of the project ideas that Prysm team proposed but no fellows chose. I said Radek who was my mentor in EPF that I want to keep contributing on Prysm with this momentum.

In December 2025, I prepared a PoC and related document, and had a call with Preston (Lead of Prysm team) and Radek. In the slide, I included the rough plan for this project as well as my personal plan for 2026. Going to NYC as an exchange student was already decided, so I also talked this as Offchain has a NYC office.

A week after the call, the company made a decision to start the recruitment process (résumé back then). At first, the company reviewed as a part-time contractor, but since I wasn't eligible to work in US because of my visa status, so it was decided to join the team right after I came back to Korea in mid of May.

After four rounds of interview including HR screening, I finally made it in mid February 2026! Thanks to this, I was able to come back to my country after enjoying my life in NYC. (You might want to read my NYC retrospect here.)

Jun 2026: Join Offchain & Prysm

So I just joined Offchain and Prysm at the first day of June. Time flew, I completed four weeks after that. Thanks to the support from the company and team, I think I spent pretty productive time these days.

Before I join the team, I asked the members that "who do you want to work with" and they said one word: proactiveness. Here in Prysm, unlike other team, we normally find things that intrigue most by ourselves, and just do it. It's the biggest win that I can fully devote myself to the project that I'm interested in. In fact, I have dozens of topics so it is pretty hectic in my working hours. (but it's fun though)

You can see most of my work in my Github.

I'm excited to my days in Prysm and as a core dev. See you at the end of this year, I'll definitely bring my retrospect!