Full Stack Engineer – Backend Focused (Node.js + React.js)

Full Stack Engineer – Backend Focused (Node.js + React.js)

Who we’re looking for

We’re looking for a backend-leaning Full Stack Engineer to join us in building our new App Marketplace — a greenfield product that will integrate Certifier with tools like Zoom, Kajabi, Google Sheets, Typeform, and more. You’ll design backend architecture, build third-party integrations, and help us create seamless automation flows for thousands of global users. While the role is backend-focused, you’ll occasionally contribute to frontend features to ensure a cohesive product experience.

About Certifier

We are a fast-growing 20-person tech startup based in Kraków 🇵🇱, focused on developing B2B SaaS solutions for digital credential management. We are led by experienced founders and backed by leading EU venture funds. In just two years, our team has launched our core product and gained over 1000 organizations as paying customers including Guinness World Records, Salesforce, National Geographic, L'Oréal, and many others. Now we're poised for growth and ready to expand our team.

What you’ll do

As our Full Stack Engineer, you’ll be a key contributor to the evolution of our platform, with a strong focus on backend development and system integrations. You’ll design and implement robust backend services, build seamless integrations with third-party tools, and ensure our infrastructure is scalable and reliable. While your primary responsibilities will center on backend systems, you’ll also contribute to the frontend when needed, helping deliver a cohesive and high-quality user experience.

Your responsibilities

• Design, build, and maintain scalable backend services, APIs, and core platform functionality
• Lead the development of integrations with external systems such as CRM tools, marketing platforms, and identity providers.
• Contribute to our frontend stack – mostly related to supporting backend-driven features.
• Optimize data models, improve application performance, and ensure system reliability.
• Write clean, testable, and well-documented code that supports rapid iteration.
• Collaborate closely with product, QA, and design teams to deliver high-quality features.
• Proactively identify areas for improvement and help define best practices for development and system integration.

Our Tech Stack

Frontend:

• React.js v18+
• React Router
• Ant Design
• Tailwind CSS
• Legend State
• Tanstack Query

Backend:

• Node.js v20+
• AWS (API Gateway, Cognito, Lambda, SQS, SES, SNS, and others)
• Cloudflare
• Serverless
• Prisma ORM
• Zod
• Remeda

Why Certifier?

• A small, focused team building something that matters: You’ll join a small product team (under 10 people), all senior-level and deeply invested in what we’re building. No context-switching, no juggling multiple projects - just one product we care about and want to make genuinely great for our users.
• A codebase you’ll enjoy: Clean, modern, and well-maintained - no legacy nightmares. We refactor regularly, manage tech debt smartly, and have solid E2E test coverage thanks to our QA. 
• Lean, effective collaboration: Just one team meeting a week and more time to focus on your tasks.
• Continuous growth: We value experimentation, curiosity, and learning from failure.
• Transparency & autonomy: We trust you to get the job done without micromanagement. We optimize for output, not hours.
• Respect & flexibility: No time tracking or invasive monitoring - your results speak for themselves.

What you'll need to succeed

Experience and expertise

• 3+ years of experience as a full stack or backend engineer in a SaaS or product environment.
• Strong proficiency in backend development using Node.js.
• Familiarity with frontend development using React.js.
• Proven ability to design and implement third-party integrations (e.g., with CRM, email, payment, or analytics systems). The topics below shouldn’t be completely new to you - you don’t need to master all of them, but if none of this rings a bell, it’s probably not a fit:
• Handling webhooks (retries, verification, error recovery)
• Knowing when to use polling vs webhooks
• Implementing idempotency, retry logic, and error handling
• Working with queues and background jobs
• Managing API rate limits
• Flexible data mapping (e.g., Google Sheets, Typeform)
• Validating and sanitizing incoming data
• Debugging and monitoring automated flows
• Handling long-running workflows and job state tracking
• Experience with AWS is a plus - if you're not an expert, we're happy to help you grow into it.

Skills and competencies

• Ownership & initiative: You take full responsibility for what you build and care about long-term quality, not just quick wins.
• A builder’s mindset: you enjoy owning complex backend projects end-to-end.
• System thinking: ability to balance speed and scalability in architectural decisions.
• Self-sufficiency: proactive, curious, and capable of independently researching and implementing solutions.
• Adaptability: comfortable working in a dynamic startup environment where priorities shift fast.

Product and communication focus

• Product focus: Quickly understand the product’s architecture and user needs to design scalable features and seamless integrations that directly enhance the customer experience.
• Communication: Written and verbal communication in English (B2+ level) to collaborate within an international team.

What we offer 

• Attractive base salary of 18,000 - 24,000 PLN net on a B2B contract depending on your experience 
• Remote, hybrid, or on-site at the office in Krakow - you choose 
• 100% ownership and casual work atmosphere with a flat hierarchy - free from formal relationships and boredom 
• Opportunities for self-development, including training, conferences, and more 
• Work with the best equipment – all the tools you need 
• Off-site team events and trips 
• Medicover health care and sports card 
• 24 paid leave days annually 

Next steps

1. CV Review.
2. HR Screening: Focused on your experience, motivation, and general skills (45-60 minutes).
3. Technical Task: You may be invited to complete a short technical task (~3-4h) where you build a small integration with a third-party service. We value your time and aim to keep this step lightweight and relevant, only including it when helpful for both sides.
4. Technical Interview: a technical call with our Co-Founder and Lead Software Engineer (~2 hours).