(Freelance) Senior Backend Engineer (m/f/d) - Agent SDK

voize; Kraków; Hybrid; PLN 240,000 - 360,000; Full time

Benefits offered by this job

Stock options

30 vacation days + birthday off

Germany Transport Ticket

Urban Sports Club

Regular company off-sites

Blinkist/Audible learning platforms

Job summary

voize, a fast-growing startup, is building AI companions to streamline frontline workers' workflows. As Senior Backend Engineer on the Agent SDK, you will own the Kotlin foundation that powers our AI agents, shaping the runtime and client-facing contracts.

Join a team that values observability, testing, and scalable API design, with hybrid work options across Germany or remote. You’ll collaborate closely with product and ML teams to ensure a robust SDK that scales with evolving AI capabilities.

Qualifications

• Production backend experience on the JVM (Kotlin/Java/Scala) with Kotlin focus.
• Experience designing APIs and internal SDK or platform contracts.
• Experience with testing and observability; interest in AI agent systems.

Responsibilities

• Build and maintain the Kotlin SDK as an internal product with its runtime and contracts.
• Design and ship the identity/auth plumbing if needed.
• Wire telemetry, error reporting, and data privacy/anonymization for agents.
• Create architecture tests and the evaluation harness integration.
• Manage Koog version bumps and format compatibility; set shared tool output limits.

Skills

JVM backend

Kotlin

API design

Testing & observability

AI agent systems

Job description

Why voize? Because we're more than just a job!

At voize, we believe the greatest gift to frontline workers is time - time to care, connect, and be present. Today, that time is lost to busywork and complex systems that pull them away from what matters most: people.

Our vision is to change that by building AI companions that seamlessly take over digital workflows. We don't replace humans with technology - we amplify their impact.

Our mission is backed with a $50M Series A funding led by Balderton Capital, with support from HV Capital, Y Combinator and other leading VCs. Today, 2,000+ facilities trust voize, and over 200,000 users rely on our AI companion to ease their daily workload.

As a dynamic team, we combine first-in-class technology with meaningful social impact. And now, we're looking for you to join us on this mission!

Your Mission: Build the layer every agent moves onto

As Senior Backend Engineer on the team behind our Agent SDK, you build and maintain the Kotlin foundation that our AI agents move onto. Agents have grown into a product surface across verticals and markets, and the layer underneath them needs a permanent owner. Your work makes the cost of the second and third agents visibly less than the first, so feature teams can ship at their own pace.

Your Daily Business - No two days are alike

• Build and maintain the SDK as an internal product. You shape the runtime and the client-facing contracts that consuming teams build on. The SDK has a roadmap of its own, and you help set it.
• You build the identity layer that agents run on, so customer-facing agents can write with a real user identity. Your responsibilities may include designing and shipping the auth plumbing, depending on what we have already built by your start date.
• Make every agent observable from the day it launches. You wire telemetry and error reporting into the SDK by default, and you integrate inference and anonymization so the SDK enforces them for every consumer.
• Build the test kit that the SDK still needs. You create it along with the architecture tests that protect the runtime internals, and you build the path that runs every SDK agent against our ML team's evaluation harness before a model change ships.
• Absorb ecosystem change on behalf of consuming teams. You own Koog version bumps and format compatibility, and you set the shared limits on tool output and token usage, so an upgrade lands once in the SDK and reaches everyone from there.

Your Skillset - What you bring to the table

• Several years of production backend experience on the JVM - Kotlin, Java, Scala or similar. Our stack is Kotlin, and you are ready to work in it.
• You have designed APIs that other engineers build against. An SDK or an internal platform where the contract is the product.
• You have built and maintained integrations, and kept them stable when the system on the other side changed.
• Testing and observability are part of how you build.
• You are drawn to AI agent systems, and to the orchestration and evaluation work underneath them.

Growing together - what you can expect at voize

• We are a fast-growing startup, that means you will take on real problems, grow quickly, and make a real impact, giving frontline workers more time for people
• We foster an open, collaborative culture with regular team events, whether you work from our Berlin office or remotely across Germany
• Become a co-creator of our success with stock options
• Generous perks: 30 vacation days plus your birthday off, Germany Transport Ticket, Urban Sports Club, regular company off-sites, and access to learning platforms such as Blinkist and Audible, plus free language courses
• You decide when you work best, that means flexible working hours and a good hybrid set-up.

Ready to talk?

We look forward to your application and can't wait to meet you - no matter who you are or what background you have!