Senior Fullstack Engineer (TypeScript)

About the role

CryptoNext Security is a deeptech startup specialising in post-quantum cryptography, helping businesses and institutions prepare for a future where quantum computers challenge traditional data security. It works with clients in finance, defence and telecommunications. The Senior Fullstack Engineer owns features end to end in TypeScript, reporting to the Head of Engineering within a team of six engineers in the 15-person R&D department.

Key responsibilities

• Own features end to end in TypeScript: Hono and Node.js on PostgreSQL, and React SPAs on the front.
• Design HTTP APIs consumed through Hono's typed RPC client, with generated OpenAPI specs.
• Model and query PostgreSQL: schema, migrations, query performance.
• Build interfaces with Vite, TanStack, Tailwind and shadcn/ui.
• Work directly with Product and QA, and write unit and integration tests.
• Review code and propose upgrades and refactorings, explaining the trade-offs.
• Use AI coding tools under your own judgement, staying accountable for what lands in the repo.

Required profile

• Strong TypeScript across the stack, with shared types between server and client.
• Experience in a startup or deeptech environment (bonus).
• Ability to justify why you chose an approach and what it will cost in six months.

Required skills

• Node.js backend: API design, auth, error handling, observability.
• Asynchronous processing: queues, background workers, idempotency, retries, backpressure.
• PostgreSQL: schema, migrations, indexing, query plans.
• React: hooks, composition, state boundaries, routing and server-state caching.
• Modern frontend toolchain, REST/OpenAPI contracts, typed clients and versioning.
• Unit and integration testing, and a practical use of AI-assisted development.
• Bonus: Go, Hono typed RPC, secure coding, basic cryptography and cybersecurity knowledge.

What we offer

• A stimulating environment with cutting-edge technological challenges.
• A competitive salary aligned with the company's compensation philosophy.
• Three days a week in the Paris office, the rest remote.

The process starts with an initial discussion with the HR team, followed by a 1 to 1.5-hour live coding interview with a developer (AI coding tools required), then a meeting with a manager before a final offer.