Senior Full-Stack Developer (Next.js / Supabase / Claude API)

We are building RailX — an AI-native B2B web marketplace for block train backhaul capacity on the Poland–Ukraine rail corridor. Built by United Multimodal Services LLC (Szczecin, Poland), a rail and multimodal logistics operator on the US–Poland–Ukraine corridor.

The platform lets block train operators post available return-leg capacity and cargo owners commit to it. The core interface is Rex — an AI agent powered by the Anthropic Claude API. Rex replaces all forms and search bars with expert conversation. 6 agents total at MVP.

We are looking for one senior developer (freelance or small team) to build the MVP. Contract: 6 months, fixed price €20,000–€35,000. Fully remote.

REQUIREMENTS:

— Proven Anthropic Claude API experience — you MUST provide a live product URL. Generic LLM or ChatGPT experience is not sufficient.

— Next.js (App Router)

— Supabase (auth, database, storage)

— Stripe (payments)

— Multi-agent architecture experience strongly preferred

— Senior level: able to own architecture decisions independently

NICE TO HAVE:

— Experience with logistics, marketplace, or B2B SaaS products

— Familiarity with the Vercel AI SDK

PRODUCT LANGUAGES: English, Polish, Ukrainian

HOW TO APPLY:

Apply directly via Djinni. In your application message, please include a live URL of a product you built using the Anthropic Claude API. Applications without this will not be considered.

PM: Sani Ezugbaia | Company: United Multimodal Services LLC, Szczecin, Poland