Software Engineer Fullstack AI-Modernisation

About the role

Theodo operates as a group of software engineering companies, structured around a shared ambition to position Artificial Intelligence as the core driver of software development innovation. Founded in 2009, the group has grown to over 850 employees, generating 100 million euros in revenue in 2025, and has established a presence through renowned projects such as TF1+, LCL Pro, and the France Identité Numérique application. The specific objective of the AI Modernisation practice is to act as the group's spearhead in this ambition, focusing on the modernization of legacy systems for clients ranging from rapid-growth scale-ups like Qonto and Doctolib to established enterprises such as Carrefour, Pernod Ricard, Les Mousquetaires, Urgo, Cdiscount, and Edenred. This role exists to deliver on that promise by applying a proprietary methodology that combines mastered AI with a "Lean Tech" approach, aiming to accelerate delivery speeds by a factor of three for critical migration projects. You will join a human-scale team within a community of over 120 specialized Theodoers, working directly with clients to redefine the future of their digital infrastructure.

What you'll do

As a Software Engineer specializing in AI Modernisation, your primary responsibility is to conceive, develop, and deploy digital products by transforming legacy software into performant, maintainable, and sustainable solutions. You will conduct in-depth analysis of existing codebases to identify technical debt and define precise modernization pathways, such as migrating from frameworks like Loopback 2 to NestJS. A central part of your daily work will involve replacing frontend architectures, for instance shifting from Vue Express to React, to ensure robust and high-performance user interfaces.

You will leverage Large Language Models (LLMs) as integral tools within the development lifecycle, utilizing them for code generation, automated refactoring, and enhancing testing procedures to achieve a significant acceleration in delivery timelines. You will work in close collaboration with Product Managers and stakeholders to translate complex business requirements into precise technical specifications, leading the execution of intricate migrations while guaranteeing business continuity and system integrity.

Using AI-powered debugging instruments, you will diagnose and resolve complex software defects, establishing automated testing suites to ensure long-term stability and performance. Additional essential duties include conducting in-depth technical investigations, performing code reviews, and creating technical documentation to facilitate knowledge transfer and ensure the longevity of the applications you build. The role further involves exploring emerging technologies and mentoring junior developers to foster a culture of continuous learning and the adoption of best practices in AI-assisted software engineering.

Requirements

To be considered for this position, you must hold a degree in computer science, engineering, or a related technical discipline. A mandatory requirement is proven expertise in JavaScript and its ecosystem, strictly adhering to ES6+ standards, alongside hands-on experience in building server-side applications using Node.js. You must demonstrate advanced proficiency in React to effectively manage complex user interface states. Furthermore, familiarity with backend frameworks, specifically NestJS, is a prerequisite to address the demands of enterprise-level legacy modernization.

Candidates must possess a comprehensive understanding of the full software development lifecycle and Agile methodologies, combined with the capacity to rapidly assimilate new technologies and adapt to evolving project constraints. Strong communication skills are essential to align technical and non-technical stakeholders around shared project goals. Practical experience with AI development tools such as GitHub Copilot or Cursor, containerization technologies like Docker, and deployment pipelines on cloud platforms is explicitly valued as a core competency for this role.

Practical notes

The position is based in Paris and is offered under a permanent contract (CDI). It provides clear pathways for career advancement and internal mobility within Theodo group. Given the international scope of the clients served, fluency in both French and English is mandatory to ensure seamless collaboration. The role requires a commitment to the group's methodology of "Lean Tech," which emphasizes learning as a priority, and the capacity to thrive in a fast-paced, dynamic environment where technical challenges and technologies vary significantly from one project to another.