FastAPI vs Elysia for Solo Developers

Quick Comparison

FastAPI Overview

FastAPI is Python's answer to the question "can we have great developer experience AND good performance?" The framework uses type hints as the foundation for automatic request validation, response serialization, and Swagger documentation generation. A decorated function with type annotations becomes a fully documented, validated API endpoint.

Async support through Starlette and uvicorn puts FastAPI's performance in the same ballpark as Node.js for I/O-bound workloads. That's remarkable for Python. And because it's Python, you inherit the richest library ecosystem in programming. Data processing, machine learning, automation, scientific computing, all available without leaving your API project.

For solo developers, the auto-generated documentation at /docs is a feature that keeps giving. Every route change, every new parameter, every response model update reflects immediately in the interactive Swagger UI. No documentation drift, no manual updates, no maintenance burden.

Elysia Overview

Elysia is designed to extract maximum performance from Bun. It's the framework that proves JavaScript can be genuinely fast when the runtime and framework are built for each other. End-to-end type safety flows from your validation schemas through your handlers to your responses, catching type mismatches at compile time.

The Eden Treaty feature is Elysia's most unique offering. It generates a type-safe client from your API routes, so your frontend knows exactly what your backend expects and returns. Change a field on the backend, and your frontend shows a TypeScript error immediately. For solo developers building both sides of the stack, this eliminates an entire class of integration bugs.

Elysia ships with plugins for common needs: JWT, CORS, Swagger, GraphQL, and WebSockets. The plugin system is composable and type-safe. Each plugin extends the context types so your handlers know what's available without manual type assertions.

Key Differences

Language and ecosystem. FastAPI gives you Python with access to data science, ML, and automation libraries. Elysia gives you TypeScript with access to npm and frontend code sharing. If your project involves data processing or ML, Python wins. If your project involves a TypeScript frontend with shared types, Elysia wins.

Documentation generation. FastAPI's automatic Swagger docs are the best in any framework. No annotations, no plugins, no configuration. Elysia supports Swagger through a plugin that works well but requires explicit setup. FastAPI's zero-effort documentation is a meaningful advantage for solo developers.

Type safety depth. Elysia's end-to-end type inference and Eden Treaty provide tighter type safety between frontend and backend than FastAPI. FastAPI's Pydantic models provide excellent validation and typing within the API, but connecting those types to a JavaScript frontend requires manual effort or code generation tools. Elysia's approach is more seamless.

Performance. Elysia on Bun is faster than FastAPI on uvicorn. The compiled JavaScript runtime outperforms Python's interpreted nature for both I/O-bound and CPU-bound work. If raw performance per dollar of hosting matters, Elysia has the advantage.

Runtime maturity. Python with uvicorn is a mature, well-understood runtime. Bun is newer and while it's improving rapidly, it's still less battle-tested in production environments. For solo developers who prioritize stability and predictability, Python is the safer bet.

Validation approach. FastAPI uses Pydantic with rich features like custom validators, computed fields, JSON schema generation, and complex nested validation. Elysia uses its own type system (inspired by TypeBox) which is performant but less feature-rich. For complex validation scenarios, Pydantic is more capable.

Full-stack integration. Elysia's Eden Treaty creates a type-safe bridge between backend and frontend. FastAPI requires OpenAPI code generators or manual type definitions to achieve similar integration. If you build both frontend and backend, Elysia reduces integration friction significantly.

When to Choose FastAPI

• You need Python ecosystem access for data science, ML, or automation
• Automatic API documentation without any setup is a priority
• You need Pydantic's advanced validation features
• You prefer a mature runtime with proven production stability
• Your project doesn't have a TypeScript frontend that needs type-safe API access

When to Choose Elysia

• You're building both frontend and backend in TypeScript
• End-to-end type safety with Eden Treaty matters for your workflow
• You want the best performance available in the JavaScript ecosystem
• You're running Bun and want a framework optimized for it
• Frontend-backend type integration is more important than auto-documentation

The Verdict

FastAPI holds the edge for most solo developer scenarios because its automatic documentation and Python ecosystem provide broader utility. The Swagger UI generation alone saves hours over a project's lifetime, and Python's library ecosystem opens doors that JavaScript can't match for data-intensive work.

Elysia is the stronger choice for TypeScript-centric full-stack development. If you're building a React or Svelte frontend alongside your API, Eden Treaty's type-safe client removes a real source of bugs and frustration. The performance advantage also translates to lower hosting bills as your project scales.

The 9/10 vs 8/10 reflects the broader applicability of FastAPI's features. Elysia would match or exceed that rating for TypeScript full-stack projects specifically. Choose based on your language ecosystem and whether frontend-backend type integration or automatic documentation matters more for your project.