FastAPI "orjson"/"ujson" not installed for the response class in CI

FastAPI can serialize responses with orjson or ujson, but those are optional extras. If your app sets default_response_class=ORJSONResponse without installing orjson, importing or serving fails in CI.

What this error means

App import or a request fails with "orjson must be installed to use ORJSONResponse" or "ujson must be installed to use UJSONResponse".

python

AssertionError: orjson must be installed to use ORJSONResponse

Common causes

The optional JSON extra is not installed in CI

orjson and ujson are not FastAPI runtime dependencies; using their response classes requires installing them explicitly.

A local machine had it while CI did not

orjson was present locally as a transitive dependency but never declared, so the fresh CI environment lacks it.

How to fix it

Declare and install the JSON library

Add orjson (or ujson) to your dependencies so the response class can import it on every runner.

Terminal

python -m pip install orjson
# or install the FastAPI extra that bundles it
python -m pip install "fastapi[all]"

Fall back to the default JSON response

If you do not need the faster serializer, drop the custom response class and use the built-in JSONResponse.

app/main.py

from fastapi import FastAPI

app = FastAPI()  # default JSONResponse needs no extra deps

How to prevent it

Declare orjson/ujson explicitly when you use their response classes.
Pin optional extras in requirements so CI matches local.
Do not rely on transitive presence of optional serializers.