# OpenAPI in Other Frameworks — OpenAPI / Swagger

Source: https://www.skillbyai.com/en/openapi/c-frameworks

> See how FastAPI, NestJS and ASP.NET Core produce or consume OpenAPI documents.

## Most modern frameworks speak OpenAPI

Many frameworks generate OpenAPI from code. **FastAPI** (Python) builds an OpenAPI 3.1 document from type hints and **Pydantic** models automatically, serving it at `/openapi.json` with Swagger UI at `/docs` and Redoc at `/redoc`; response models, status codes and examples come from decorators. **NestJS** (TypeScript) uses the `@nestjs/swagger` module with decorators such as `@ApiProperty` and `@ApiResponse`. **ASP.NET Core** includes built-in OpenAPI document generation in recent versions through `Microsoft.AspNetCore.OpenApi` (`AddOpenApi()` and `MapOpenApi()`), with Swashbuckle and NSwag as long-standing alternatives. **Go** has generators such as `oapi-codegen` that go the other way, generating server interfaces and types from a design-first document. The same principles apply everywhere: annotate thoughtfully (descriptions, examples, error responses), export the document in CI, lint it and check for breaking changes, rather than treating the generated document as an afterthought.

## FastAPI generating OpenAPI from type hints

Pydantic models become schemas; decorators add responses and metadata.

```python
from enum import Enum
from fastapi import FastAPI, HTTPException, Path
from pydantic import BaseModel, Field

app = FastAPI(title="Orders API", version="2.3.0")

class OrderStatus(str, Enum):
    PLACED = "PLACED"
    PAID = "PAID"
    SHIPPED = "SHIPPED"

class Order(BaseModel):
    id: str = Field(examples=["ord_7Fq2LmX0aB9c"])
    status: OrderStatus
    total: str = Field(pattern=r"^\d+\.\d{2}$", examples=["1499.00"])

@app.get(
    "/orders/{order_id}",
    response_model=Order,
    operation_id="getOrder",
    tags=["Orders"],
    responses={404: {"description": "Order not found"}},
)
def get_order(order_id: str = Path(description="Order id")) -> Order:
    order = repository.find(order_id)
    if order is None:
        raise HTTPException(status_code=404, detail="Order not found")
    return order

# document: /openapi.json   Swagger UI: /docs   Redoc: /redoc
```

## Export and commit the generated document

Even in code-first projects, export the OpenAPI document during the build and keep it under review. Diffing it in pull requests shows API changes that a code diff hides.

**Quiz:** In FastAPI, where is the interactive Swagger UI served by default?

- [ ] /swagger
- [ ] /api-docs
- [x] /docs
- [ ] /ui

*Answer:* /docs. FastAPI serves Swagger UI at /docs and Redoc at /redoc by default.