FastAPI OpenAPI Tags Fix

In this tutorial, you'll learn about FastAPI OpenAPI Tags Fix. We cover key concepts, practical examples, and best practices.

The Problem

FastAPI's auto-generated docs list all endpoints in one flat list. With 50+ endpoints, finding the right one is cumbersome. Tags group endpoints logically.

Quick Fix

Wrong — no tags

@app.get("/users")
async def list_users(): ...

@app.post("/users")
async def create_user(): ...

@app.get("/items")
async def list_items(): ...

@app.post("/items")
async def create_item(): ...

Output: Swagger UI shows all endpoints alphabetically. Users and items are mixed together.

Correct — grouping with tags

from fastapi import APIRouter, FastAPI

users_router = APIRouter(prefix="/users", tags=["Users"])
items_router = APIRouter(prefix="/items", tags=["Items"])

@users_router.get("/")
async def list_users(): ...

@users_router.post("/")
async def create_user(): ...

@items_router.get("/")
async def list_items(): ...

@items_router.post("/")
async def create_item(): ...

app = FastAPI()
app.include_router(users_router)
app.include_router(items_router)

Output: Swagger UI shows two sections: "Users" and "Items". Endpoints are organized and searchable.

Tag metadata

from fastapi import FastAPI
from enum import Enum

app = FastAPI(openapi_tags=[
    {
        "name": "Users",
        "description": "User management endpoints",
        "externalDocs": {
            "description": "User guide",
            "url": "https://docs.dodatech.com/users",
        },
    },
    {
        "name": "Items",
        "description": "Product catalog operations",
    },
])

Multiple tags per endpoint

@app.get("/search", tags=["Items", "Users"])
async def search(q: str):
    ...

Output: Endpoint appears in both "Items" and "Users" sections.

Tags on individual endpoints

@app.get("/health", tags=["Health"])
async def health_check():
    return {"status": "ok"}

@app.get("/metrics", tags=["Monitoring"])
async def metrics():
    ...

Router with multiple tags

admin_router = APIRouter(
    prefix="/admin",
    tags=["Admin", "Users"],  # Appears in both
)

Prevention

• Group endpoints by resource (Users, Items, Orders) using routers with tags.
• Add tag descriptions for documentation context.
• Use multiple tags sparingly — prefer one primary tag per endpoint.

Common Mistakes with openapi tags

1. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors
2. Non-exhaustive pattern matches that compile with warnings then crash at runtime
3. Misunderstanding that String is [Char] with poor performance for large text operations

These mistakes appear frequently in real-world FASTAPI code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ