Senior Backend Engineer; LLM​/AI

Full-time | Tech Team
📍 Barcelona (hybrid) or remote within Europe
💰 €50,000 – €65,000

threesixfive is working with a Barcelona-based AI product company that builds production‑grade conversational AI systems used by enterprise customers.

The company operates at the intersection of LLMs, real‑time backend systems, and cloud‑native architecture, delivering intelligent AI experiences at scale rather than prototypes or experiments. Their platform is already live with paying customers and continues to grow rapidly.

As part of this partnership, we’re hiring a Senior Backend Engineer to play a key role in designing, building, and operating the backend systems that power these AI‑driven experience.

• Build and scale backend systems powering production conversational AI
• Hands‑on senior role balancing architecture and real‑world delivery
• Work deeply with LLMs, RAG pipelines, and real‑time systems

You’ll help shape the backend foundations of a cutting‑edge AI platform, where your architectural and performance decisions directly impact real‑time AI experiences used by enterprise customers.

This is not an R&D or prototype‑heavy role — it’s about shipping, scaling, and operating intelligent systems in production.

We’re looking for a Senior Backend Engineer with recent, real‑world LLM product experience who combines strong architectural thinking with hands‑on development.

You’ll work closely with senior engineering leadership, contributing to technical direction and system design, while remaining deeply involved in day‑to‑day coding, feature delivery, and production operations.

This is a role for someone who designs systems and builds them.

• Contribute to architectural decisions and backend system design
• Collaborate with infrastructure engineers on:
  • Observability (monitoring, logging, alerting)
  • Deployment and scaling strategies
  • Latency, throughput, database queries, caching, resource usage
• Own production operations:
  • Provide technical guidance through:
    Code reviews
  • Collaborate on real‑time and streaming‑oriented architectures
• Build and maintain backend services in production‑grade Python (and Go)
• Develop conversation‑related features:
  • State management
  • History
• Implement multi‑document knowledge bases and retrieval systems
• Integrate LLM APIs and build RAG pipelines
• Design and maintain APIs and service integrations (gRPC, REST)
• Work on core backend services:
  • Orchestration
  • Caching
  • Platform APIs
• Own testing, CI/CD pipelines, and deployment automation
• Python (FastAPI) and Go (gRPC services)
• AWS (S3, EC2, Lambda, managed services, LLM platforms)
• LLM APIs (e.g. OpenAI, managed cloud LLM services)

• 5+ years of backend engineering experience
• Strong background in:
  • Distributed systems
  • Microservices
• Proven experience building and deploying high‑performance Python services
• 2+ years working with LLM‑powered systems in production (2022–2025)
• Hands‑on use of LLM APIs
• Experience building RAG or retrieval‑based systems
• Comfortable balancing architecture design with hands‑on implementation
• Strong AWS experience with a focus on:
  • Performance optimisation
  • Observability
  • Production reliability
• Demonstrated ability to improve production systems:
  • Throughput
• Excellent collaboration skills across backend, infrastructure, and AI‑focused teams

• Experience with real‑time media, streaming, or conversational platforms
• Data engineering or ML model‑serving infrastructure exposure

• Ownership of critical backend services established
• Knowledge transfer completed
• Core conversation and knowledge features live in production
• Clear performance and observability improvements delivered

• Backend services and RAG pipelines running reliably at scale
• Other engineers supported and unblocked through your technical guidance
• Recognised internally as a go‑to expert for backend + LLM implementation

🏠 Hybrid work in Barcelona or remote within Europe

• Ownership of backend systems used daily by enterprise customers
• Hands‑on technical role with real architectural responsibility
• Deep technical challenges across:
  • LLMs
  • Real‑time systems
  • Scalability and reliability
  • High‑impact role in a small, senior engineering team
  • Close collaboration with engineering, AI, and infrastructure specialists
• Opportunity to deepen expertise in applied AI systems
• Central Barcelona office
• Lunch support when in the office
• Private health insurance
• Travel allowance (distance‑based)
• Flexible benefits under Spanish tax legislation
• Wellness / fitness discounts

To apply or learn more: email kris with your CV or Linked In profile.