Senior Software Engineer/Architect – Full Stack & Agentic AI

About the opportunity

We’re looking for a senior, hands-on Software Engineer/Archiect to join as our second engineering hire and help grow our founding engineering team. You’ll bring a strong full-stack engineering background and practical experience building agentic AI applications.

Our preferred stack is Node.js, JavaScript/TypeScript and React, but we’re also open to engineers with deep experience in other modern stacks who are comfortable transitioning.

We’re building an agentic system for accounting firms that processes documents arriving through email, WhatsApp, SMS and a mobile app. It classifies invoices and receipts, extracts structured data, communicates with clients and routes uncertain cases to human reviewers.

Working alongside our first engineer and the founders, you’ll build across the product—from backend services and integrations to user-facing interfaces and AI workflows. You’ll take ownership of substantial parts of the system and help shape technical decisions, with implementation and delivery shared across the team.

What you'll do:

• Build and evolve the full-stack product, including backend services, APIs, integrations and user-facing interfaces

• Develop agentic workflows involving tool use, state management, model routing, retries, fallbacks and human review

• Connect the platform to messaging channels and third-party business systems

• Build evaluation and monitoring capabilities to understand AI quality, reliability, latency and cost

• Implement validation and guardrails around AI-driven actions and outputs

• Develop retrieval workflows using customer-specific knowledge where needed

• Extend the human-in-the-loop review experience and incorporate feedback safely

• Collaborate on architecture, prototype solutions and take features through to production

We're looking for someone who has:

• Production experience automating business workflows with AI, supported by measurable improvements.

• Strong senior-level experience building and shipping full-stack software, with ownership of architecture, implementation and production reliability

• A solid foundation in Node.js and JavaScript/TypeScript, ideally with React—or substantial experience with another modern stack and a willingness to transition

• Strong backend engineering skills, including API design, databases, integrations and asynchronous workflows

• Hands-on experience developing agentic AI applications beyond prompt engineering, such as systems that use tools, manage state, execute multi-step workflows or involve human intervention

• Practical experience with an agent framework or SDK, such as LangGraph, Google ADK or Claude Agent SDK, or comparable custom implementations

• An understanding of how to test, evaluate and monitor AI behaviour, and handle unreliable outputs or failures

• The ability to work through ambiguous product requirements, make sound technical decisions and turn prototypes into maintainable software

• A collaborative approach and the confidence to take responsibility for significant parts of an early-stage product

It would also be useful to have experience with:

• RAG, vector databases and customer-specific retrieval

• Building MCP servers or clients and connecting agents to external tools

• Managed model platforms or deploying and evaluating open-weight models

• AI evaluation and observability tools, such as Langfuse or Arize Phoenix

• Multimodal document processing, OCR or intelligent classification

• Accounting, ERP or workflows involving sensitive customer data

What you'll get:

• The opportunity to build a product from the ground up, by joining as our second engineering hire and helping shape the founding team.

• Hands-on collaboration with our first engineer and experienced founders

• Significant ownership of key technical areas and influence over engineering decisions

• The chance to build real-world AI applications that solve meaningful business problems

• A fast-moving environment with minimal bureaucracy

• Office based in Belgrade, working closely with the founding product team.

Why this role is different:

You’ll join at an early stage, with room to shape both the product and how we build it. Working alongside our first engineer and the founders, you’ll explore problems, develop prototypes and turn them into production systems for accounting firms.

This role combines substantial technical ownership with close collaboration in a small, growing engineering team. If you enjoy building products from scratch, experimenting with AI and taking responsibility for the systems you develop, we’d love to hear from you!