Senior Backend Engineer (Kotlin / Java)

About Fourthwall

Creatives, brands and startups run shops, memberships and donations on Fourthwall, on their own domain and with their own data. MKBHD, Y Combinator, ThePrimeagen, Coffeezilla, Acquired, Vox and the Smithsonian are among them. More than 500,000 creatives and companies use it. Investors: Lightspeed, Initialized Capital and Seven Seven Six. Offices in Los Angeles and Poznań.

• Company: Fourthwall, a product company. You contract with us directly.
• Level: Senior individual contributor. Kotlin or Java in production qualifies; the work is in Kotlin.
• Location: Remote in Poland. Meeting in Poznań from time to time is a plus, not a requirement.
• Hours: Your own, with agreed overlap with the team.
• On-call: Part of the role. You join the backend rotation.
• Rate: 22,500 to 38,000 PLN net per month on B2B (invoice amount excluding VAT), long-term.
• Stack: Kotlin, Spring Boot, jOOQ on PostgreSQL, Arrow, Google Cloud, Codex / Claude Code.
• Process: Intro call, paid exercise of about three hours, 75 minutes with the team, decision within five working days.

About the product and culture

Our promise is that we will make creating and selling products as easy as having a great idea. To deliver on that promise, our product must be comprehensive, including handling product design, manufacturing, shipping, ecommerce platform, customer support, and taxes.

The only way to do that is with a superstar team who want to work hard, own large areas of responsibility, and who care deeply about making customers happy. And that’s exactly the type of engineers we’re looking for. Smart, motivated, and looking for a chance to make a big impact on a product.

The service you would join has a first commit from 2019. It has run the business since and still goes to production several times a day.

Fourthwall is the merchant of record. We take the payment, handle sales tax and VAT, get the product made in one of 17 locations, ship it and pay the seller. All of that runs through this service: one Kotlin codebase, one deployable. It talks to payment, tax, manufacturing and shipping partners and exposes a public API with webhooks.

A lot of what it does is payments, refunds, payouts and tax. A bug there costs real money, so idempotency and reconciliation matter more here than in a typical CRUD service. The codebase is seven years old and has the usual leftovers. We clean up as we go, but that is not the main job. We are on JDK 26, Kotlin 2.4 and Spring Boot 4.

Why now

AI is changing the world unbelievably quickly, and we believe Fourthwall is in an amazing position to be a key part of the new agentic economy. We have an opportunity to become the MCP that agents use to design and order physical products, making it easy for them to connect with world-class manufacturers.

We’ve built a strong foundation to scale from, including having a product our customers love and being profitable, without losing our startup roots and desire to move fast.

How we ship

Fourthwall is placing bets: features creatives have asked for, agentic interfaces, groundwork for new products, new partner integrations, changes to payments and payouts, security work. Leadership picks them together with the engineers who build them. Teams of one to five form around a project and re-form when it is done. You would be in those discussions and have a say in what gets built.

Before anything gets built we want to know what it would change for users or revenue, and whether there is a cheaper way to find out. Agents with access to all our data surfaces help with the numbers. You make the case to leadership and to the people who talk to customers every day. Sometimes the answer is a smaller version, or no.

Then you build it, schema to rollout flag, and delete the old path once it is live. A week later you check what it changed.

When production breaks, switch the feature off, tell support what to say, size the impact, then fix and reconcile. Support, growth and fraud will ask you questions, and you will have some for them.

Coding agents

The team works through coding agents and builds its own harness: AGENTS.md files, skills, subagents, hooks. You direct the agent, read what it wrote and run the checks. What you merge is yours. When an agent repeats a mistake, fix the skill or the hook rather than the prompt. We pay for the tooling.

We will ask where an agent let you down on production code and what you changed afterwards. If you paste a prompt and merge whatever comes back, or keep agents out of your work, this job will not suit you.

What we look for

We test all of this in the process: the code in the exercise and the pairing session, the rest in the intro call and the discussion.

• Kotlin or Java on a Spring backend in production.
• SQL. With or without an ORM, you know what SQL runs and what the abstraction hides.
• When a failure belongs in the return type (Either, Kotlin Result, a sealed type) and when an exception is right.
• One third-party API you integrated end to end: contract, retries, idempotency, how it fails.
• For a query, an HTTP call or a background job: what it costs, when it gets slow, how it fails.
• One feature you shipped and what it changed for users or revenue.
• One request you did not build as asked: what was wanted, what you did instead, and why.
• One piece of work outside your usual area, say a data pull, a Terraform module or a partner’s sandbox: how you got up to speed and what you tried first that did not work.
• Clear written English.

Learnable: jOOQ, JobRunr, Pub/Sub, Terraform, MCP tools with Spring AI.

The offer

• 22,500 to 38,000 PLN net per month on B2B, long-term. What moves you up the range: changes you carried end to end, tickets and merge requests others can pick up cold, and bets you proposed that shipped.
• 26 days off a year at the full monthly fee.
• MacBook, private healthcare with dental, sports card, English lessons, an educational budget and accounting support.
• Equity in the US-based company.

Hiring process

About five hours in total.

• CV or profile, plus one change you carried end to end and what it changed for users or revenue.
• Intro call: the role, your situation, questions both ways.
• Paid exercise, about three hours in your own time, no strict limit. We pay 500 PLN for it.
• 75 minutes with the team: 30 building a small feature together with coding agents, then 45 of discussion.
• Decision within five working days.