DENIS FEJZA Get in touch

Open to full-time backend & full-stack roles · remote or Belgrade

I build systems that hold up when they actually get used.

I'm Denis Fejza, a backend and full-stack engineer in Belgrade. Three years of production work for clients in Europe and the US (APIs, PostgreSQL, authentication, payments), Rust services for an AI trading platform, and enough systems-level work (a RISC-V kernel, a compiler, a CPU in SystemVerilog) to understand what runs beneath the framework.

Email me → See the work LinkedIn GitHub
3+ yrs

Production work for paying clients, across backend, AI and Web3

40%

Faster page loads on a live client platform after query and caching work

100% JSS

Job Success and Top Rated Plus as an Upwork freelancer

M.Sc.

Software Engineering, in progress at the University of Belgrade (B.Sc. 2026)

01 / Selected Work

What I've built.

Client platforms carrying real users and real money, trading and AI infrastructure I built end to end, and the research and systems work underneath it all. Source is public where the client allows it.

Production & client systems

Client platform · Security & access control

ShivWorks Network

Full security & reliability audit, then the repairs

A live SvelteKit membership platform on Cloudflare: paid tiers, gated course video, transactional email and a real member base.

  • Audited authentication, payment provisioning, webhook handling and data retention end to end, and delivered the findings ranked by severity.
  • Repaired broken access control, introduced admin role management and secured video delivery.
  • Traced and fixed email deliverability that was silently dropping member mail.
SvelteKitCloudflare D1ClerkStripeAuth & RBAC

Private client codebase

Client platform · Performance

gradi.rs

40% faster page loads

A live Serbian construction marketplace: a multi-level materials catalog, user-posted listings for contractors and equipment rental, and a quote-request flow instead of online checkout. PHP codebase, working directly with the owner.

  • Profiled the slow paths, rewrote the expensive queries and added a caching layer.
  • Cut page load times by roughly 40% without a rewrite, on the live system.
  • Continue to ship features and handle production issues as they come up.
PHPSQL optimizationCachingFull-stack
Live site →

Trading & AI platform · Core engineer

FUKU

Rust monorepo, 22 services in production

An AI trading platform on Solana: an LLM agent that executes DeFi actions from natural language in Telegram, on a Rust monorepo of roughly 87,000 lines behind Postgres, Redis and HashiCorp Vault.

  • 214 commits between January 2025 and March 2026: largest author of the Telegram bot, second largest of the agent tool layer that exposes 113 callable trading functions to the model.
  • Sole author of the signal service, which reads a Redis stream of Hyperliquid perpetuals activity, classifies each signal by risk, fans it out to filtered Telegram groups, and builds indicator packs on demand (RSI, MACD, Stochastic, ADX, DMI, EMA cross, Ichimoku, ATR, Bollinger width, CMF, MFI).
  • Built the Agent Commerce Protocol adapter end to end, so other autonomous agents can buy trade execution from FUKU: runtime, job orchestration, buyer, and the Solana seller agent.
  • Worked across the execution layer (Jupiter, Raydium CLMM, Meteora DLMM, Pump.fun, Hyperliquid perpetuals), wallet PnL and portfolio analytics, and a Postgres schema spanning 50 migrations.
RustSolanaHyperliquidLLM agentsPostgreSQLRedis

Private team repository

Trading infrastructure · Quant research

BTC Binary Market Trading System

Strategy measured, then shelved on the evidence

A TypeScript and PostgreSQL system for Polymarket's BTC 5-minute up/down markets: discover the active market, capture near-expiry order-book snapshots, place and manage live orders, and settle on chain.

  • Built the whole loop: market discovery, a cron-driven collector, order placement through the Polymarket CLOB, open-position risk management, claim and redeem, and a backtester over captured snapshots.
  • Ran the validation myself: Brier scores, cluster-bootstrap confidence intervals on the residual, and breakeven half-spread against tick size.
  • The market midpoint turned out to be well calibrated and the edge did not survive execution costs, so I wrote that up and stopped trading it rather than shipping a losing strategy.
TypeScriptPostgreSQLPrismaPolymarket CLOBethers

Private repository

Product · AI & SaaS

DoorCall AI

Live product, public demo workspace

Vertical SaaS for independent garage-door repair companies in the US: an AI receptionist that answers forwarded calls so the owner stops losing jobs to missed phone calls.

  • Built call handling, safety screening, job qualification, service-area validation, calendar availability checks and booking.
  • Built the surrounding product too: transcripts, lead pipeline, analytics and Stripe billing.
  • Made the whole flow runnable with zero external accounts, so the product can be evaluated without telephony or payment credentials.
Next.jsTypeScriptVoice AISupabaseStripe
Live demo →

Product · SaaS

InRooms

Events, payments and onboarding in one build

A networking platform for tech sales professionals, built solo as one coherent React and TypeScript application.

  • Event discovery and registration, connections and messaging, and an admin panel.
  • Stripe-backed subscriptions and LinkedIn OAuth onboarding.
ReactTypeScriptSupabaseStripeOAuth
Source →

Research & systems work

AI research · B.Sc. thesis

Text-to-SQL with LLMs

Abstains instead of guessing

Translates natural-language questions into SQL, explains the result, and declines to answer when the model's confidence is low rather than returning a wrong query. Evaluated on the Spider benchmark and two demo databases.

PythonLLMsSQLEvaluation
Source →

Systems · C++

RISC-V OS Kernel

From scratch, no standard library

A multithreaded kernel for RISC-V running bare metal: threads and context switching in assembly, counting semaphores, a first-fit allocator, and the system-call boundary between user and supervisor mode.

C++RISC-V asmQEMU
Source →

Systems · Java

MicroJava Compiler

Source to bytecode, full pipeline

A complete compiler for a Java-like language: JFlex lexer, LALR(1) grammar building a typed syntax tree, semantic analysis against a symbol table, and bytecode generation with backpatched jumps.

JavaJFlexCUP
Source →

Systems · SystemVerilog

PicoComputer CPU

RTL, UVM verification, synthesis

A CPU implemented in SystemVerilog: register-transfer-level design, a UVM verification environment, and an FPGA-ready synthesis flow.

SystemVerilogUVMFPGA
Source →

02 / Depth

Four layers, not one.

I've built at four levels of the stack: product systems, backend and data, AI and on-chain services, and the kernel underneath. That range is the useful part. When a performance problem or a security hole crosses an abstraction boundary, I can follow it across instead of stopping at the edge of what I know.

Layer 04

AI & on-chain systems

LLM agents that place real trades: a function-calling layer wired to Solana venues (Jupiter, Raydium, Meteora, Pump.fun) and Hyperliquid perpetuals, a concentrated-liquidity strategy engine that simulates fees and impermanent loss before committing a position, and LLM voice agents in production SaaS.

RustSolanaHyperliquidLLM agents

Layer 03

Product systems

Authentication, role-based access control, admin tooling, payment flows and media pipelines: the parts of a platform that decide whether users can trust it.

Auth & RBACStripeMedia delivery

Layer 02

Backend & data

APIs, relational schemas, query optimization and scheduled pipelines, including a market-data collector that snapshots order books on a cron and stores them for offline analysis. Built for throughput, and for the person who has to debug them at 2am.

Node.jsPythonPostgreSQL

Layer 01

Kernel & silicon

A from-scratch RISC-V kernel with threading, semaphores and a custom allocator, a compiler front to back, and a CPU in SystemVerilog with UVM verification. The ground the rest of it stands on.

C / C++RISC-VSystemVerilog

03 / Stack

What I work with.

Comfortable across the stack, strongest on the backend and in systems languages.

Languages

  • TypeScript
  • Rust
  • Python
  • SQL
  • PHP
  • Java
  • C++
  • C
  • Dart
  • SystemVerilog

Backend & data

  • Node.js
  • Express
  • FastAPI
  • REST APIs
  • PostgreSQL
  • Redis
  • Prisma
  • Supabase
  • Firebase
  • Cloudflare D1
  • Microservices

Frontend & mobile

  • React
  • Next.js
  • SvelteKit
  • Vite
  • Tailwind
  • Flutter

Tooling & infra

  • Docker
  • Linux
  • Git
  • CI/CD
  • Cloudflare Workers
  • Vercel
  • HashiCorp Vault
  • Stripe
  • Clerk

04 / Experience

Where I've done it.

Full-time, part-time and contract roles since 2023: shipping to deadlines, owning production incidents, and explaining technical trade-offs to the person paying for the outcome.

May 2026 to present

Full-Stack Engineer

Macks Studios · Contract · US studio serving service businesses

  • Own security, reliability and integrations across client platforms, including a live SvelteKit membership platform on Cloudflare Workers with Stripe billing, role-based access and gated video for about 640 members.
  • Audited authentication, payment provisioning, webhook handling and data retention end to end, ranked the findings by severity, then repaired access control and admin role management.
  • Fixed email deliverability (SPF/DKIM, list hygiene, WooCommerce transactional email) and now delivering a full-funnel event tracking and attribution rebuild.

May 2024 to present

Full-Stack Engineer

gradi.rs · Part-time · construction marketplace

  • Development of a live Serbian marketplace for construction materials, contractors and equipment rental on a PHP codebase, working directly with the owner.
  • Profiled the slow paths, rewrote the expensive queries and added caching, cutting page load times by roughly 40% without a rewrite.

Jan 2025 to Mar 2026

Backend Engineer (Rust)

FUKU AI · Full-time · AI trading platform

  • Built on a Rust monorepo running roughly 22 containerised services in production: an LLM trading agent, Solana execution, Hyperliquid perpetuals and real-time market streams.
  • Owned the Telegram signal service and the Agent Commerce Protocol adapter end to end, and worked across the shared agent, database and Jupiter SDK crates.
  • Shipped into a codebase with fifteen contributors, behind Traefik, with HashiCorp Vault for key custody and 50 forward migrations on Postgres. Contributed to the token migration from Solana to Base.

Feb 2023 to Oct 2025

Freelance Backend & Full-Stack Engineer

Upwork and direct clients · Remote

  • Delivered backend and full-stack builds end to end for EU and US clients: requirements, architecture, implementation, deployment and handover.
  • Took over existing production codebases and made them faster and safer without rewrites, through query optimization, caching and access-control repair.
  • Reached Top Rated Plus with a 100% Job Success Score.

May 2023 to Mar 2024

Junior Software Engineer

SEOTEAM.IO

  • Built Python automation tools, scheduled jobs and API integrations that replaced manual SEO data collection and reporting.
  • Turned a recurring human task into a scheduled, reviewable pipeline.

Product work

Independent Products

InRooms, DoorCall AI, Zentrix, AskyPrint

  • Built and shipped complete products solo: a SaaS networking platform, an AI receptionist for a US service vertical, a five-platform Flutter media player, and a Windows desktop client for thermal receipt printers.
  • Each one covers the full path: data model, backend integration, interface, packaging and release.

05 / Education

Where the fundamentals came from.

M.Sc. Computer Software Engineering

University of Belgrade, School of Electrical Engineering (ETF), Software Engineering module. Started October 2026, alongside work.

In progress

B.Sc. Electrical Engineering and Computing

University of Belgrade, School of Electrical Engineering (ETF), Computer Engineering and Informatics, 2020 to 2026. A mathematics and systems curriculum, assessed on implementation projects: the kernel, the compiler, the CPU and the Text-to-SQL thesis in Selected Work all started here.

  • Operating systems
  • Compilers
  • Computer architecture
  • Algorithms & data structures
  • Digital design / VLSI
  • Databases
Completed 2026
  • Serbian · Native
  • English · Full professional
  • German · Limited working

Let's talk about the work.

A full-time backend or full-stack role is the priority, remote across EU and US time zones or on-site in Belgrade. I also take on contract work alongside it. Send a short note about the team and the problem, and I'll reply within a day.

Discuss a role → Discuss a project LinkedIn

Or write directly to fejza.denis@gmail.com