Senior Fullstack Engineer (TypeScript)

As a Senior Fullstack Engineer at CryptoNext Security, you own features end to end across TypeScript backend and React frontend. You join a team of six engineers within a 15-person R&D department, working on post-quantum cryptography solutions that help businesses and institutions prepare for a quantum-enabled future. Your code ships on-premise into security-critical environments where secure defaults, reviewed dependencies, and tested code are non-negotiable.

What you'll do

• Own features end to end in TypeScript: Hono and Node.js on PostgreSQL, React SPA on the front
• Design HTTP APIs consumed through Hono's typed RPC client, with generated OpenAPI specs
• Model and query PostgreSQL: schema, migrations, and query performance
• Build interfaces with Vite, TanStack, Tailwind and shadcn/ui
• Work directly with Product and QA to ship working versions early
• Write unit and integration tests that back your features, and give QA what they need to automate the rest
• Review code and propose upgrades and refactorings with clear trade-off analysis
• Use AI coding tools as leverage under your own judgement, staying accountable for what lands in the repo

What you bring

• Strong TypeScript across the stack: strict mode, generics, types shared between server and client instead of duplicated
• Node.js backend expertise: API design, auth, error handling, observability
• Asynchronous processing: queue consumers and background workers, idempotency, retries and failure handling, backpressure
• PostgreSQL: schema design, migrations and rollbacks, indexing, reading a query plan
• Strong React: hooks, composition, clear state boundaries, client-side routing and server state caching
• Comfort with a modern frontend toolchain: fast bundling, utility-first styling, component libraries you extend rather than fight
• API contract design: REST, OpenAPI, typed clients, versioning
• Unit and integration testing, with a defensible position on what belongs at which level and what is worth testing
• Real use of AI-assisted development in a production codebase: agentic workflows, context management, reviewing generated output critically
• Seniority demonstrated by explaining why you chose an approach, what you rejected, and what it will cost in six months

Nice to have

• Go, used in parts of the codebase
• Hono typed RPC and end-to-end type inference
• Secure coding practices and application security
• Basic understanding of cryptography and cybersecurity
• Experience in a startup or deeptech environment

What we offer

• A competitive salary, aligned with the compensation philosophy
• Three days a week in the Paris office, the rest remote
• A stimulating work environment with cutting-edge technological challenges in post-quantum cryptography

About CryptoNext Security

CryptoNext Security is a deeptech startup specializing in post-quantum cryptography. The company works with leading clients in finance, defense, and telecommunications to prepare for a future where quantum computers will challenge traditional data security.