Next.js API Routes vs FastAPI for Solo Developers

Quick Comparison

Next.js API Routes Overview

Next.js API Routes are the built-in way to handle server-side logic inside a Next.js project. You create files in app/api/, export request handlers, and everything deploys together. For projects where the frontend is the main product and the backend is secondary, this keeps things simple.

I've used API Routes for payment callbacks, data proxies, and form submissions. The workflow is fast. Write a function, push to Vercel, done. No second deployment, no CORS headaches, no infrastructure to manage.

The downside is that API Routes aren't really a framework. They're just functions that respond to HTTP requests. There's no validation, no serialization, no auto-generated docs. Every endpoint is on its own, and as your backend grows, the lack of structure becomes a real problem.

FastAPI Overview

FastAPI is the Python framework that makes building APIs actually enjoyable. It uses Python type hints for everything, automatically validates requests, serializes responses, and generates interactive OpenAPI documentation. You define your data models with Pydantic, write your endpoint, and FastAPI handles the boring parts.

The auto-generated docs alone saved me hours of work on my last API project. Instead of maintaining Postman collections or writing separate documentation, the interactive Swagger UI updates every time I change an endpoint. During development, I test directly from the docs page. It's the kind of DX improvement that compounds over time.

FastAPI is also genuinely fast for Python. Built on Starlette with async support, it handles concurrent I/O operations efficiently. When your endpoint needs to call three external APIs before responding, async FastAPI does that without blocking other requests.

Key Differences

Language ecosystem split. Next.js API Routes keep you in JavaScript/TypeScript. FastAPI puts you in Python. For solo developers, this decision often comes down to what language you already know. But if you're working with data science, ML models, or Python-heavy tools, FastAPI gives you native access to that ecosystem.

Automatic documentation. FastAPI generates interactive OpenAPI/Swagger docs from your code. Type hints become docs. Next.js API Routes have no documentation generation. You'll write API docs manually or not at all. For solo developers who need to share their API with others (or with their future selves), this is a significant advantage.

Validation approach. FastAPI validates every request against your Pydantic models automatically. Invalid data never reaches your handler. With API Routes, you either add manual validation with Zod in every endpoint or risk runtime errors from malformed requests.

Async capabilities. Both support async operations. FastAPI is built on async Python from the ground up. Next.js API Routes support async handlers but within the constraints of serverless execution. FastAPI gives you more control over connection management and long-running operations.

Deployment model. API Routes deploy to Vercel serverless with zero config. FastAPI needs a server running uvicorn, which means a VPS, container, or platform like Railway. Slightly more setup, but no cold start penalties and no execution time limits.

Data science and ML integration. If your API needs to run ML models, process data with pandas, or integrate with Python scientific libraries, FastAPI gives you direct access. With Next.js, you'd call a separate Python service, adding complexity.

When to Choose Next.js API Routes

• You're already in Next.js and the backend is simple
• You want one codebase and one deployment
• JavaScript/TypeScript is your primary language
• The backend has fewer than 15 endpoints
• You're deploying to Vercel and want zero-config hosting

When to Choose FastAPI

• You want auto-generated API documentation
• Request validation should be automatic, not manual
• You need Python ecosystem access (ML, data processing)
• You're building a standalone API that multiple clients consume
• Async performance for I/O-heavy workloads is important
• You value strong typing with runtime validation

The Verdict

FastAPI is the better backend framework. That's not a close call. The auto-generated docs, automatic validation, and type-driven development make it more productive for building real APIs. Next.js API Routes are convenient when you're already in Next.js, but they're not playing in the same league.

The real question is whether you want to run two languages. If your frontend is React/Next.js and your backend is FastAPI, you're context-switching between JavaScript and Python. Some developers handle that fine. Others find it exhausting.

For solo developers who know Python or are willing to learn it, FastAPI with a separate React/Next.js frontend is a strong combination. You get the best frontend framework paired with the best API framework, even if it means two deployment targets. The developer experience on each side is good enough to justify the split.

If you're strictly JavaScript and don't want Python, that's fair. But consider Hono or AdonisJS as your dedicated backend instead of stretching API Routes beyond what they're designed for.