Staff Software Engineer, Backend - Fintech Platform (m/f/n)

In. Post is building its own fintech platform from the ground up. It’s a large-scale, international initiative where engineers will have the opportunity to shape the architecture, build core fintech components, and create solutions designed to operate at In. Post’s scale. We’re building a lean, highly effective engineering organization in Poland, with AI-native engineering at its core .

We’re looking for strong engineers who are comfortable with agentic engineering, from experienced professionals building complex systems and engineering tooling to a new generation of engineers who have embraced AI-driven development from the start of their careers. In. Post Pay is looking for a Staff Engineer to define the technical direction for one of our platform areas

• Payments Core, Merchant, or Consumer. You'll multiply team output through architecture clarity, engineering standards, and cross-team technical leadership, while helping evolve our agentic engineering harness in a data-driven way. You'll own the evolution of fundamental technology decisions across your area - ensuring they're sound, defensible, and viable for the long term - and act as the technical throughline between engineering teams, product strategy, and the organisation's wider architecture standards. → Hybrid working model: preferably 1 day per week in the Warsaw or Cracow office .

Responsibilities

Define and maintain the backend technical architecture for your area: domain boundaries, event contracts, state machine patterns, data consistency models, and cross-service API design across multiple teams Author Architecture Decision Records (ADRs) and RFCs, facilitating technical decision-making across teams with documented reasoning and clear trade-off analysis Evaluate and decide on fundamental technology trade-offs: database selection, cloud services, message broker topology, and consistency models appropriate for financial data Own the evolution of the agentic development harness - instrument it, measure adoption and friction, and drive improvements with data Drive org-wide technical change: identify patterns to adopt or deprecate broadly, build the case, and see adoption through across teams without direct authority Provide technical input into product strategy; mentor Staff-track engineers through design reviews, pairing, and technical writing feedback 8+ years in software engineering, including 2+ years in cross-team technical leadership or architecture authority 3+ years designing distributed payments or fintech systems Hands-on experience with multiple tech stacks and expertise in backend Solid grounding in distributed systems architecture - event sourcing, CQRS, domain events, saga pattern - and state machine / financial entity lifecycle design Experience designing APIs at service-boundary level

• REST, async, and event-based contracts Comfort with database trade-off reasoning (relational, time-series, document, ledger-style) and cloud architecture evaluation (AWS/GCP/Azure) Experience designing or evolving an agentic engineering harness — instrumentation, adoption measurement, iteration Working knowledge of LLM tool-use patterns and context management in engineering workflows Excellent technical writing
• ADRs, RFCs, and design docs for multiple audiences English proficiency at B2 or above Nice to have: 2+ years in platform or internal developer tooling engineering 1+ years of agentic/LLM system design experience 2+ years in BNPL or lending domain architecture PCI DSS / PSD2 architecture compliance experience from the design phase Kafka or equivalent streaming architecture experience at scale Open-source contributions or technical publishing Polish language skills Opportunity to work in a diverse , international , and cross-functional environment, along with leading experts . You work and learn from a senior leader and their leadership team, expanding your experience and exposure in the e-commerce and logistics industries. In. Post is a growing company that offers its people an increasing number of opportunities in several locations in Europe. Fulfilling careers with a range of benefits for people and investing in providing training opportunities for their development. You will feel a part of the In. Post community that makes an impact on sustainability, convenient deliveries, and the circular economy every day. Excellent working environment and flexible hours We offer B2B type of contract