Senior Backend Engineer

Description

Are you looking for your next professional challenge? Or are you just browsing? Whether curiosity or ambition brought you here, Avantio could be exactly what you're looking for! **Who are we?** We don’t want to bore you with the typical endless corporate biography. Here’s a brief summary of what we do! We develop cloud-based software to help vacation rental agencies manage and distribute their properties through our all-in-one tool. Additionally, we create customized websites for our clients. Want to learn more? Get to know us better here: https://www.youtube.com/watch?v\=n4aeLa2MUnA But technology and innovation are only part of the story. The true engine of Avantio is our people. We are a Valencia-based SaaS company with a global mindset and over 170 professionals from more than 28 different nationalities. Our headquarters are located in sunny Valencia, and we also have an office in Mexico. What unites us? Our experience, professionalism, and passion for what we do. Are you ready to accept the challenge of joining our team? **How we work** At Avantio, AI is not an optional tool nor an experiment—it is embedded in our daily workflow. We use agents to design, implement, test, document, and review. Judgment remains what defines a strong senior profile: breaking down complex problems, making sound architectural decisions, and anticipating weak points in a solution. What changes with AI is where that judgment is applied: much of the code is no longer written directly by you, but generated by an agent under your guidance. Your job is to provide the appropriate context and critically evaluate the output before it reaches production. Responsibility for the code remains yours; AI amplifies your reach, it does not replace it. In practice, our way of working rests on three disciplines: Spec\-driven development. Specifications—both product and technical—are versioned in the repository alongside the code. They constitute the explicit context consumed by both humans and agents: today, a good specification is as decisive as a good implementation. Harness engineering. Using an agent is not enough; you must build the environment that makes it reliable. Repository-level instructions, skills, tools, verification loops, and control mechanisms that allow full task delegation with guarantees. We treat the harness as another engineering asset: versioned, reviewed, and continuously improved. Autonomous agents. We delegate end\-to\-end tasks to code agents that plan, implement, test, and self-correct—with human verification checkpoints at critical stages. The senior role is responsible for designing and implementing these workflows: deciding what to delegate, which control mechanisms to apply, and how to verify the outcome. Additionally, we are also integrating AI into our product. This role will participate in building LLM-based features for our customers. **What you’ll do (and with whom)** Our primary stack is Node.js with TypeScript, complemented by microservices in other languages such as Python or Go. Regarding databases, we use various models (relational, document-oriented, key\-value, time-series) depending on specific needs, with MongoDB as our primary database. For each development, we ask ourselves what the best possible solution is, defining architecture based on the problem’s requirements. All code is tested and deployed via CI/CD pipelines onto our Kubernetes cluster. Maintainability, best practices, and testing are top priorities—more so than ever: as AI accelerates code generation, rigor in review and testing increases accordingly. As a senior profile, you’ll participate in cross-departmental initiatives: * Driving the company’s technological transformation, including AI adoption across the entire development lifecycle. * Building and evolving Avantio’s agent harness: repository instructions, skills, tooling, verification loops, and the shared specification repository used by all teams. * Contributing to the migration to AWS cloud. * Defining global architecture roadmaps. * Maintaining and evolving shared Avantio stack components and microservices. * Supporting other teams in the area: architecture design, validation of technical approaches, technical mentorship (including AI-assisted development), resolution of bottlenecks, proposals for refactoring in response to performance or memory issues, and greenfield architecture design. **Role requirements:** * Senior profile. We do not set a specific number of years of experience; we value high-quality experience: individuals accustomed to environments where delivery excellence is paramount, capable of tackling new challenges autonomously, adaptively, and responsibly. * **Demonstrable, daily use of AI-powered development tools** (e.g., Claude Code, Cursor, Copilot, or similar), beyond simple autocompletion: proven experience delegating full tasks to code agents within agentic workflows. Integrating AI into your daily work is a mandatory requirement for this position. * Evaluation criterion: ability to articulate how functionality correctness is verified (tests, metrics, reviews). * Solid knowledge of JavaScript, specifically Node.js with TypeScript. * Experience with high-volume distributed services (queues, events, etc.). * Solid experience designing and implementing REST APIs. * Experience using and modeling document-oriented (MongoDB) and relational (SQL) databases. * Knowledge and experience in testing, linting, and development best practices. **Nice-to-have:** * Having built autonomous agents in real-world environments using LLM APIs (e.g., Bedrock, Anthropic, OpenAI, or similar): multi\-step planning and tool\-calling loops, RAG, structured outputs, automated evaluation, or human\-in\-the\-loop. * Practical experience in harness engineering: having configured and maintained a code agent’s environment (repository instructions, skills, MCP, verification loops), with the ability to explain the impact achieved. * Experience with spec\-driven development: versioned specifications in the repository serving as the single source of truth for both humans and agents. * Experience managing operational aspects of LLM-based systems in production: prompts and context, model selection, costs, latency, and failure handling. * Experience in other languages such as Go and Python. * Experience with microservice-oriented architectures. * Experience with AWS services. * Kubernetes knowledge. * Experience implementing continuous integration tools. * Contributions to improving developer experience or internal tooling (including AI tooling: skills, internal agents, automations). **What we offer?** Yes, we have fresh fruit in the office, coffee, tea, and several types of milk. We also have a ping\-pong table and fun Christmas dinners. But we know those aren’t what will make you join our team. Let’s get to what truly matters… **Permanent, full-time contract.** ️ **Flexibility tailored to your life:** hybrid work (3 days from home and 2 days in the office each week), flexible hours (arrival between 8:00 and 10:00), intensive schedule in July and August, one afternoon off for your children’s birthdays, and support during their first days of school. We care about your life outside of work. **Keep learning!** Training on tools and skills relevant to your role and team. **Carsharing:** option to drive colleagues to work (with a monthly allowance from the company) or ride to work with them for free. ️ **Vacation days:** 22 paid vacation days (PTO) \+ December 24 and 31 off. **And much more!** Discounts on various services just for being part of Avantio: private health insurance, healthy meals in the office, MyRealFood app, gym, and outdoor sports. Avantio is an equal-opportunity employer. You’re welcome just as you are—regardless of where you come from, what you look like, or who you love. Your story, perspective, and individuality make us stronger and more innovative. Moreover, we provide the tools and opportunities you need to grow professionally. We’ll challenge you to develop your career! If you’ve made it this far… there’s a very good chance you’re interested in joining our team! There’s much more to experience than words alone can convey. Join us and make a difference!