How I built an Agentic AI UI with React and Gemini API (and handled the 503 Overload Error)

Микола Бондар — Mon, 21 Sep 2026 16:02:09 +0000

Building a wrapper around an AI API is easy. Building an "Agentic UI" where the AI actually controls the interface is much harder.
Recently, I built a full-stack Music Recommendation Engine (FastAPI + React). Instead of just chatting, the Gemini AI can autonomously trigger music searches in the UI based on user prompts.

The Problem: AI Server Overloads
When using the free tier of Google's Gemini 3.6 API, you will inevitably hit the 503 Service Unavailable error during high demand. If you don't handle this, your backend crashes and the frontend shows a blank screen.

The Solution: Graceful Error Handling
I wrapped the Gemini chat call in a try...except block in FastAPI to intercept this specific error and send a clean, user-friendly JSON response to React.

Then, on the frontend, React simply displays this as a normal chat message from the AI, keeping the app alive:

Conclusion & Source Code
Handling third-party API failures gracefully is crucial for any production app. This simple pattern keeps users informed without breaking the experience.

🚀 To save time, you can get the complete, production-ready source code of this project (including the Glassmorphism UI, Docker setup, and hybrid search) on my Gumroad: https://koliabondar.gumroad.com/l/wepfo

DEV Community: Микола Бондар

How I built an Agentic AI UI with React and Gemini API (and handled the 503 Overload Error)