Senior Fullstack Engineer (TypeScript)

CryptoNext Security; Paris; Hybride; EUR 85 000 - 120 000; Plein temps

Résumé du poste

CryptoNext Security, a Paris-based deeptech startup focused on post-quantum cryptography, seeks a senior full-stack engineer to own features end-to-end. You will work across TypeScript, Node.js, React, PostgreSQL, and OpenAPI, collaborating with Product and QA in a hybrid Paris team.

This role emphasizes production-ready testing and architectural decision-making, with AI coding tools as an aid. Join a 6-engineer team within a 15-person R&D group; three days in the Paris office, and remote for

Qualifications

• Strong TypeScript across the stack with strict mode and shared types.
• Node.js backend: API design, auth, errors, observability.
• Asynchronous processing, queues, retries and backpressure.
• PostgreSQL schema design, migrations, indexes, query plans.
• Strong React with hooks and client/server state management.
• Frontend toolchain: fast bundling, utility-first styling, component libs.
• API contract design: REST and OpenAPI, typed clients.
• Unit and integration testing with clear guidance on what to test.
• Experience with AI-assisted development and responsible tooling.
• Senior candidate can justify choices and costs over time.

Responsabilités

• Own features end to end in TS: API, DB, UI.
• Design HTTP APIs consumed via typed RPC client with OpenAPI specs.
• Model and query PostgreSQL; manage migrations and performance.
• Build interfaces with Vite, TanStack, Tailwind, and shadcn/ui.
• Collaborate with Product and QA; write tests and automate.
• Review code and use AI coding tools judiciously.
• Propose upgrades and explain trade-offs and costs.

Connaissances

TypeScript

Node.js

React

PostgreSQL

OpenAPI

REST APIs

Testing

Code review

AI in coding

Senior reasoning

Outils

Hono

OpenAPI

TanStack

Tailwind

Vite

shadcn/ui

PostgreSQL tooling

Description du poste

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.

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 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.
• 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 you'll 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 colleague's 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 you'll 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, you'll receive a final offer!

What's 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?