How I built a switchable AI provider system in Next.js (Gemini GPT-4 Claude, one workspace setting)

I had Gemini hardcoded into every AI function in my codebase. Then a beta user told me her clients have data policies about Google. I rebuilt the whole AI layer in two days. Here's the architecture.

The wrong way (what I had)

import { GoogleGenerativeAI } from "@google/generative-ai";

const genAI = new GoogleGenerativeAI(process.env.GEMINI_API_KEY!);

export async function summarizeProject(projectId: string) {