🔹 What I build I build production backend systems that handle real-time distributed traffic at scale using Golang, Python and Node.js. I focus on systems where latency, reliability, and throughput directly impact production outcomes under high concurrency and event-driven workloads. I design and implement blockchain integration layers that connect backend systems with Ethereum-based transaction and wallet workflows in production environments. 🔹 Production impact Focused on building production-grade systems that improve reliability, throughput, and system resilience under real-world traffic conditions. Built real-time backend systems using Go, Node.js, and Python with Redis Pub/Sub and SSE for high-volume event-driven communication Designed and stabilized high-throughput API systems and microservice architectures under concurrent production traffic Developed blockchain transaction and wallet integration systems using Node.js and Ethereum-based stacks in production environments Implemented smart contract interaction layers and backend services for token and transaction workflows Built payment and external API integration systems with strong consistency, idempotency, and failure recovery requirements Operated cloud backend systems on AWS/GCP with scaling, monitoring, and production observability 🔹 Core engineering focus Distributed backend systems (Go / Node.js / Python) Event-driven architecture (Redis Pub/Sub, SSE) Blockchain transaction systems (Ethereum / Web3 integration) Backend ↔ on-chain data flow design High-concurrency API systems Production reliability, scalability, and latency optimization 🔹 How I work I identify bottlenecks in production systems across latency, throughput, and data consistency. I implement changes that improve system stability and performance across backend and blockchain layers without disrupting production traffic. 🔹 Availability Available for backend architecture, blockchain integration, system scaling, and production refactoring projects.

I build scalable backend systems, data pipelines, and automation platforms used by SaaS, fintech, and e-commerce companies. With 14+ years of backend engineering experience, I specialize in designing high-performance APIs, distributed ETL pipelines, and large-scale automation infrastructure. $500K+ earned on Upwork across 270+ projects building backend systems and data platforms. Key strengths: • Backend architecture (Python, FastAPI, Node.js, .NET, PostgreSQL, Supabase, Celery/RabbitMQ/Redis) • Data pipelines & ETL systems • AI integrations and RAG pipelines on vector databases (pgvector, Pinecone, Qdrant) • Large-scale data extraction and reverse engineered APIs • Distributed workers and automation infrastructure I focus on performance, reliability, and long-term maintainability, delivering backend platforms that grow with the business and remain stable under heavy load. Core technologies Python, FastAPI, Node.js, .NET Core, PostgreSQL, Redis, Supabase, Qdrant, Pinecone, Docker, Celery, RabbitMQ, Playwright, Selenium, Scrapy, REST APIs, GraphQL, ETL pipelines, vector search, distributed workers, cloud infrastructure (AWS/GCP/Azure), LLM, Langchain, Langfuse, BMAD

Welcome to my profile 🤟 I'm a Full-Stack Developer with 10 years of experience. I’ve worked on e-commerce projects, CRM systems, and social networks, using various frameworks to create projects from scratch or update and fix existing ones. I can create multiple types of projects, ranging from landing pages to ERPs, social networks, and more. 💻 Technical Skills: ➡️ Programming Languages & Technologies ▪ PHP ▪ JavaScript ▪ Node.js ▪ SQL ▪ C# ▪ Golang ➡️ Frameworks & Libraries ▪ Laravel ▪ CodeIgniter ▪ Bootstrap ▪ Tailwind CCS ▪ Angular 2+ ▪ React.js ▪ Vue.js ▪ Symfony ▪ Zend 3 ▪ Redis ➡️ RDBMS ▪ MS SQL Server ▪ SQLLite ▪ MySQL ▪ PostgreSQL ▪ Clickhouse NoSQL ▪ MongoDB ▪ Elasticsearch ➡️ Virtualization Tools ▪ Vagrant ▪ Docker ➡️ Methodologies ▪ Agile, Scrum, XP ▪ Pair Programming ▪ Test Driven, Feature Driven, Behavior Driven Development ▪ Waterfall ➡️ Application & Web Servers ▪ Apache ▪ Nginx Cloud Providers ▪ AWS ▪ Azure ➡️ Development Tools ▪ IntelliJ IDEA ▪ Visual Studio + ReSharper ▪ Rider ➡️ Testing Tools ▪ Postman ▪ SoapUI ➡️ Operating Systems ▪ Microsoft Windows ▪ SUSE Linux ▪ macOS I'm available to commit 40 hours per week on an ongoing basis. I can work independently and as part of a team. We can jump on a call to talk more and see if we're a good fit 🤝

👋 𝗛𝗲𝗹𝗹𝗼! ✅ Full-Stack & Back-End Developer with 5+ years of experience ✅ Node.js · NestJS · Django/DRF · React · Next.js · TypeScript · PostgreSQL · Docker I build reliable, scalable web applications — from clean REST APIs and microservices to full-stack products with polished frontends. I work across the entire stack and take ownership of infrastructure too. ⚡️ 𝗪𝗵𝗮𝘁 𝗜 𝗰𝗮𝗻 𝗱𝗼 𝗳𝗼𝗿 𝘆𝗼𝘂: + Build or extend back-end APIs (NestJS, Express, Django/DRF) + Develop full-stack web applications (React, Next.js + Node.js/Django) + Set up and manage server infrastructure (Docker, VPS, Nginx, CI/CD) + Integrate third-party services and payment systems + Improve and refactor existing codebases 𝗖𝗼𝗺𝗽𝗲𝘁𝗲𝗻𝗰𝗶𝗲𝘀: ⚒️ Back-End: NestJS, Express.js, Django/DRF, Node.js, TypeScript, GraphQL, WebSockets, Celery (Beat & Worker), Redis, JWT, AWS Cognito ⚒️ Front-End: React, Next.js (SSR/SSG), Redux, Zustand, D3.js, Chart.js, styled-components, Material UI, Ant Design, SCSS/SASS ⚒️ Databases: PostgreSQL, MySQL, MongoDB, Supabase — TypeORM, Prisma, Sequelize ⚒️ DevOps & Infrastructure: Docker, Docker Compose, Nginx, PM2, GitHub Actions, GitLab CI, VPS setup & management, SSH, DNS, SSL certificates, Cloudflare ⚒️ Cloud & Storage: AWS (S3, EC2), Cloudflare R2, Firebase, Supabase, GoDaddy ⚒️ Monitoring: Sentry ⚒️ Third-Party Integrations: ❇️ Payments: Stripe, Monobank, PayPal, Webhook solutions ❇️ Messaging: SendGrid, Mailjet ❇️ Documents: PandaDoc ❇️ CMS: Strapi, Contentful ❇️ Auth: AWS Cognito, JWT, OAuth2 ❇️ AI: OpenAI API ❇️ Other: AWS Amplify, Firebase On the back end, I build production-grade REST APIs and microservices using NestJS (TypeORM, PostgreSQL, WebSockets, GraphQL, Redis) and Django/DRF (PostgreSQL, Celery for async tasks, background workers). I focus on clean architecture, performance, and maintainability — whether it's a greenfield project or extending an existing codebase. On the front end, I build responsive, performant UIs with React and Next.js — covering SSR/SSG, state management (Redux, Zustand), and third-party API integrations. For infrastructure, I handle the full DevOps cycle: Docker containerization, CI/CD pipelines (GitHub Actions, GitLab CI), Nginx reverse proxy with SSL, VPS provisioning, and cloud storage with AWS S3 or Cloudflare R2. I take pride in clear communication, clean code, and delivering work I'd be happy to put my name on. Looking forward to hearing from you! I'm ready to start 𝗔𝗦𝗔𝗣 🔥

I build production-ready Django REST APIs and React frontends for European and US startups — from greenfield systems to legacy modernization. I've delivered systems trusted by FAO United Nations, KPN Netherlands, and MKB-Jurist (Dutch law firm) — handling everything from architecture to AWS deployment. ─── What I Build ─────────────────────────────────────── 🔧 Backend & APIs Django REST Framework, Fastapi, Node.js, scalable REST API design, legacy refactoring, Django ORM optimization, query tuning and indexing ⚛️ Frontend React.js, TypeScript, TanStack Query, Material UI, Tailwind CSS, Bootstrap, pixel-perfect responsive UI 🗄️ Database & Performance PostgreSQL optimization, Redis caching, Celery async tasks, query tuning, indexing strategies ☁️ DevOps & Cloud AWS EC2/S3/RDS, Docker, Nginx, CI/CD GitHub Actions, Ansible, Terraform, Linux server management ─── Why Clients Choose Me ────────────────────────────── ✅ Delivered for FAO United Nations government portal ✅ EU clients — MKB-Jurist Dutch law firm, KPN Netherlands ✅ Full lifecycle ownership — architecture to deployment ✅ Clear daily communication and on-time delivery ✅ Available for immediate start ─── Tech Stack ───────────────────────────────────────── Python | Django | DRF | Fastapi | Node.js | React | TypeScript PostgreSQL | Redis | Celery | Docker | AWS | Nginx Ansible | Terraform | GitHub Actions | OpenAI API

I build production AI systems with LLMs, RAG pipelines, and voice agents - backend-first engineering on Python (FastAPI) and Node.js (NestJS), with full-stack delivery when you need it. Most AI projects break the moment real users hit them: hallucinations, slow retrieval, broken function calls, runaway token costs, voice agents that lose context. I fix that. I've shipped end-to-end AI systems across RAG pipelines, document processing, conversational chatbots, voice AI, multi-step agents, and fine-tuned models - all built to hold up in production, not just demo well. What I build: - RAG pipelines - embeddings, vector search (Pinecone, FAISS, pgvector), re-ranking, retrieval evaluation - LLM API integrations - OpenAI, Claude, structured JSON outputs, function calling, tool use, guardrails - Document processing - PDF parsing, OCR, structured extraction at scale - AI chatbots & conversational agents with memory, context management, and grounded responses - Voice AI systems - speech-to-text, TTS, real-time voice agents (Twilio, Vapi, LiveKit-style architectures) - AI agents - multi-step workflows, tool calling, planning, recovery from failure - Fine-tuning - custom model training on domain data (OpenAI, open-source) - Backend systems - FastAPI (Python) and NestJS (Node.js) with clean architecture, auth, queues, observability - Full-stack delivery - Next.js / React frontends when the project needs a working product, not just an API - AWS deployment - EC2, Lambda, S3, RDS, with cost and latency monitoring Recent results on Upwork: ✓ AI/ML Full-Stack project delivered in a week - 5-star review, client called the work "professional" and would hire again without hesitation ✓ Full-Stack contract over 25 hours - 5-star review for clean code, on-time delivery, and clear communication Tech stack: Python, FastAPI, Node.js, NestJS, Next.js, React, OpenAI, API, Anthropic Claude API, LangChain, LlamaIndex, Pinecone, FAISS, pgvector, PostgreSQL, Redis, AWS (EC2, Lambda, S3, RDS), Docker, Twilio, Vapi. If you're past the prototype stage and need an AI system that's reliable, accurate, and ready for paying users - message me with what you're building or where it's breaking. I'll reply with how I'd approach it and what the first milestone looks like.

I am a Senior Full-Stack Engineer with 7+ years of production experience building SaaS platforms across fintech, AI communications, legal tech, e-commerce, and CRM. Backend stack: Python (FastAPI, Django/DRF, Flask), PostgreSQL, MongoDB, Redis, Celery, TaskIQ, RabbitMQ. Frontend: React.js, Next.js, Vue.js. Infrastructure: AWS (EC2, EBS, S3, Cognito). Key projects shipped: - Omnichannel cloud communication platform handling bulk SMS, WhatsApp, and cloud calling at scale (Django, FastAPI, Celery, Redis, NATS, RabbitMQ) - Two AI agent calling systems (FastAPI, Twilio, custom knowledge base and workflow engine) - Compliance data platform (FastAPI, PostgreSQL, S3, document processing pipeline) - Multi-role legal SaaS with AI-assisted chat (Django, React.js) - Rental station app with payment integrations (FastAPI, MongoDB, AWS, PayPlus) I work well with clients who have clear requirements and need someone who can own the backend end to end.

👋 Hi there! I’m a results-driven Senior Full Stack Developer with 8+ years of experience building scalable cloud-based applications, APIs, dashboards, and production-grade business systems. I help startups, growing businesses, and enterprise teams turn raw data and business ideas into reliable, scalable, and high-performance solutions. Whether you need a *data pipeline, cloud data warehouse, analytics dashboard, backend API, web application, desktop app, or full business automation system*, I can take ownership from planning to deployment. 🌟 What I Can Help You With 🔹 Cloud Data Platforms I work with modern cloud platforms including AWS, GCP, and Azure to build secure, cost-efficient, and production-ready data systems using services such as S3, Redshift, Lambda, Glue, Athena, BigQuery, Dataflow, Azure Data Factory, Synapse, and Blob Storage. 🔹 Full Stack Web & Desktop Applications I build modern, scalable applications using JavaScript, TypeScript, Angular, Node.js, NestJS, ElectronJS, .NET Core, MongoDB, PostgreSQL, and REST APIs*. From MVPs to full-scale enterprise systems, I focus on clean architecture, performance, and long-term maintainability. 🔹 Data Engineering & ETL/ELT Pipelines I design and build scalable data pipelines for ingestion, transformation, validation, and reporting using tools like Python, SQL, Apache Airflow, Spark, Kafka, dbt, Pandas, Snowflake, BigQuery, Redshift, and AWS Glue. 🔹 Dashboards, Reporting & Analytics I create business-focused dashboards and reporting systems using Power BI, Tableau, Superset, SQL, and cloud data warehouses, helping teams make faster and better decisions. 🔹 Automation & Workflow Optimization I automate manual business processes, reporting workflows, data syncs, API integrations, and backend operations to save time, reduce errors, and improve efficiency. 🔧 Tech Stack Languages: Python, SQL, JavaScript, TypeScript, Bash Frontend: Angular, HTML, CSS, responsive UI development Backend:Node.js, NestJS, .NET Core, REST APIs Desktop Apps: ElectronJS Databases: PostgreSQL, MySQL, MongoDB, SQL Server, Snowflake, BigQuery, Redshift Data Tools: Airflow, Spark, Kafka, dbt, Pandas, NumPy Cloud:AWS, GCP, Azure AWS Services: S3, Lambda, Glue, Redshift, Athena, ECS DevOps: Docker, Kubernetes, GitHub Actions, Terraform, CI/CD Visualization: Power BI, Tableau, Superset 💼 Why Work With Me? ✅ End-to-End Ownership — I can handle everything from data architecture and backend development to deployment and reporting. ✅ Production-Ready Delivery — I build clean, reliable, documented, and scalable solutions. ✅ Business-Focused Approach — I don’t just write code; I build systems that solve real business problems. ✅ Strong Communication— I keep projects clear, organized, and easy to understand. ✅ Fast & Reliable Execution — I focus on quality delivery with practical timelines and clean results. 🚀 Whether you need a data engineering solution, cloud pipeline, full stack application, business dashboard, API integration, or automation system, I’m ready to help you build it properly. 📩 Let’s connect and discuss how I can help bring your project to life.

My longest client has been with my team for 7 years and over $1M of continuous development. Another project started as a $7K MVP and grew into a $70K product — because it was architected to survive success from day one. I'm Sergei — founder and CTO of a senior engineering team that designs and builds AI-powered SaaS platforms: multi-tenant architecture, AI agents, and the unglamorous production infrastructure that keeps them alive under real users. Here's the uncomfortable truth about how founders usually find me: I'm the second team they hire. Recently a client brought us a "finished" AI SaaS platform before launch. Our architecture audit found 4 critical blockers and 11 high-risk issues the previous team had left behind — each one cheaper to fix on paper than in production. Cheap code isn't cheap. It's a mortgage you didn't know you signed. WHAT I ACTUALLY DO 1) AI SaaS Product Engineering Multi-tenant platforms with AI agents built in — not bolted on. My rule: LLMs go where they create value users pay for; deterministic, testable code goes everywhere else. That discipline is the difference between an AI demo and an AI product. 2) Architecture Audits & Production Rescues A fixed-price, fixed-scope review of your codebase before launch or investment: architecture assessment, risk register, production-readiness score, and a prioritized roadmap. You own the report whether or not we continue together. This is the single highest-ROI engagement I offer — every hour of audit saves 10–20 hours of rework. 3) Long-Term Product Development (T&M) For founders who want a technical team that thinks in years, not sprints. Time-and-materials with full transparency: you see who works on what, at which rate, every week. Fixed-price rewards vendors for cutting corners you won't see for months; T&M rewards us for exactly one thing — keeping you happy enough to continue. Our 7-year client relationship exists because of this model, not despite it. HOW THE TEAM WORKS You'll be working with me directly on architecture and key decisions — and with my senior engineers on delivery, at rates below mine, with full visibility into who does what. You get founder-level attention with team-level throughput. I personally review the architecture of everything we ship. I build for non-technical founders deliberately. You're an expert in your domain — you shouldn't need to learn what a message queue is to make good decisions. Every technical choice comes to you as a business decision: what it costs, what it risks, what it unlocks. STACK (the boring part) Python, Node.js, React/Next.js, PostgreSQL, Redis, AWS, Docker, CI/CD. AI: Claude & OpenAI APIs, RAG pipelines, multi-agent orchestration, evaluation and guardrails for production LLM systems. NEXT STEP If you're building an AI SaaS product — or rescuing one — message me with a short description of where you are. If there's a fit, we'll start with a 15-minute call, and usually from there with an Architecture Audit: fixed price, fixed timeline, a report you keep either way.

Three things I do best: voice agents that sound human, multi-agent orchestration that actually ships, and RAG systems that hold up in production. Voice agents. People on the other end of the call don't realize they're talking to an AI. That takes a lot of small things working together: VAD endpointing so the agent starts responding before the caller finishes, barge-in so it stops when interrupted, filler words and breathing pauses between sentences, dynamic speaking rate that slows down for complex info, sentiment-driven tone shifts, voice cloning. Sub-300ms latency. Self-hosted models alongside cloud providers. SIP telephony for outbound calls, plus inbound with intent routing and human handoff. Agent orchestration. Built a system with five specialist agents (voice, browser, RAG, MCP tools, scheduler) that coordinate through dependency DAGs and run in parallel. The agents work through real tools: browser automation via Chrome DevTools Protocol, MCP servers managing Google Calendar, Gmail, GitHub, PostgreSQL, Slack. Per-room credential isolation. Human-in-the-loop approval gates for sensitive operations like calls and emails. AI that takes real actions, not just answers questions. Production RAG. I've shipped: Legal RAG (FlawFinder.ai) indexing US police departments, zero-hallucination citations, paying subscribers Scientific document RAG with hybrid search, multi-language support, figure extraction via vision models GraphRAG with 40% better retrieval vs vector search alone I also help teams whose RAG works in demos but struggles with real data. Usually it's chunking strategy, missing evaluation, or grounding gaps. I audit the pipeline and rebuild the parts that need it. Beyond the AI layer, I build the infrastructure that keeps systems running when real users show up: Kubernetes, CI/CD, monitoring, evaluation harnesses. Most recently Infrastructure Lead at an AI startup where I built the full platform from scratch. Day to day: Python (FastAPI, Django), LangChain/LangGraph, PostgreSQL, pgvector, Redis, Kubernetes, Docker, Terraform, AWS. Frontend when needed (React/TypeScript). 15+ years in software, last 3 on LLM applications. Based in Madrid (GMT+2), flexible for EST overlap. Available now, async-first, same-day responses.