Senior Fullstack Engineer

Senior Fullstack Engineers at fonio take end-to-end responsibility for building product features that deliver measurable value to customers, working in a remote-first, high-growth team focused on a German-language AI phone assistant used by thousands of European companies.

What you'll do

• Own features from idea and prototype through architecture, implementation, and release.
• Engage continuously with customers, partners, and Sales to surface real pain points.
• Run product discovery by forming hypotheses and validating them with quick experiments.
• Instrument and gather the metrics relevant to your area to enable data-driven decisions.
• Build pragmatic, scalable software with a balance between immediate value and long-term maintainability.
• Prioritize shipping over excessive meetings and communicate asynchronously where possible.
• Act with radical ownership and push initiatives forward without waiting for permission.
• Work remotely with CET overlap or from the Vienna office as needed in a remote-first setup.

What fonio is looking for

• Proven experience as a full-stack developer with end-to-end software delivery responsibility.
• Strong experience with Node.js, TypeScript, NestJS, and React; experience integrating LLMs is a major plus.
• Product mindset with an ability to measure success by customer value rather than lines of code.
• Customer-focused: skilled at extracting actionable insights from direct user conversations.
• Experiment-driven and pragmatic: prefers lightweight validation before building large systems.
• Comfortable owning tracking, analytics, and data needed to support product decisions.
• Native-level German proficiency to support a German-language AI phone assistant.
• Strong CS fundamentals and the judgment to trade off effort, quality, and speed appropriately.