Senior Fullstack Engineer (TypeScript)

Location: Paris, Île-de-France, France

Department: Research & Development

Workplace: hybrid

Employment Type: full

Description

Who we are

CryptoNext Security is a deeptech startup specializing in post-quantum cryptography. Our solutions help businesses and institutions prepare for a future where quantum computers will challenge traditional data security. We work with leading clients in finance, defense, and telecommunications.

On a daily basis, you will

• Own features end to end in TypeScript: Hono and Node.js on PostgreSQL, React SPA on the front.

• Design HTTP APIs consumed through Honos 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.

• Write the unit and integration tests that back your features, and give QA what they need to automate the rest.

• Review code.

• Use AI coding tools as leverage under your own judgement, staying accountable for what lands in the repo.

• Propose upgrades and refactorings, and explain the trade-offs you accepted.

How we work

A team of six engineers, within our 15-person R&D department. A single TypeScript monorepo, with shared types and tooling across services and front. Three days a week in our Paris office, the rest remote. Short feedback loops, high standards. We ship a working version early rather than polishing in isolation, and keep solutions as simple as the problem allows. Our products ship on-premise, sometimes into air-gapped environments, so builds have to be reproducible and dependencies deliberate. We build software our clients rely on for security: secure defaults, reviewed dependencies and tested code are part of shipping, not a later phase. We care more about a decision you can justify than a pattern you can name.

Requirements

The skills youll need

• Strong TypeScript across the stack: strict mode, generics, types shared between server and client instead of duplicated.

• Node.js backend: 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.

• Comfortable 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 on what is worth testing at all.

• Real use of AI-assisted development in a production codebase: agentic workflows, context management, reviewing generated output as critically as a colleagues merge request. You have a view on where these tools pay off and where they create debt.

• Seniority means you can say why you chose an approach, what you rejected, and what it will cost in six months.

Bonus skills

• Go, used in parts of our 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.

Benefits

The team youll be joining

You will join a team of six engineers who own their features across the whole stack, inside our 15-person R&D department, working closely with Product and QA. You will report to the Head of Engineering and collaborate with other departments to build the backend services and interfaces our clients depend on.

Our recruitment process

1. Initial discussion with our HR team to explore your background and motivations.

2. Technical interview with a developer: a live coding session of 1 to 1.5 hours. AI coding tools are required, not merely allowed. We are interested in how you direct and review them. No take-home assignment.

3. Meeting with a manager to ensure alignment with our company culture.

4. If successful, youll receive a final offer!

Whats in it for you at CryptoNext

• A stimulating work environment with cutting-edge technological challenges.

• A competitive salary, aligned with our compensation philosophy.

• A deep dive into the exciting world of post-quantum cryptography and cybersecurity.

Ready to join the adventure? Apply now!