[Bug] Python Azure Function worker incorrectly URL encodes cookie value

[mabeshark]

Expected Behavior

Azure Function worker using Python FastAPI should not URL encode cookie values

Expected behavior on HTTP Response Headers:
set-cookie: test="hello//world"; Path=/; SameSite=lax

e.g. when using Python FastAPI without Azure Functions, it correctly does not URL encode.

import uvicorn
from fastapi import FastAPI
from fastapi.responses import PlainTextResponse

app = FastAPI()

@app.get("/")
def http_trigger():
    response = PlainTextResponse("", status_code=200)
    response.set_cookie(key='test', value='hello//world')

    return response

if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=7071)

Run curl -v http://localhost:7071/ and response is correct as set-cookie: test="hello//world"; Path=/; SameSite=lax

Actual Behavior

Actual incorrect behavior on HTTP Response Headers, cookie value is incorrect and unexpectedly URL encoded

Set-Cookie: test=hello%2F%2Fworld; domain=; path=/; samesite=lax

Tested bug with

• Python 3.12 with FastAPI
• Azure Functions Core tools 4.3.0 locally
• Deployed to production Azure Function

Steps to Reproduce

1. Run func start or deploy to Azure using sample code provided below
2. Send a test HTTP request e.g. curl -v http://localhost:7071/
3. Azure Functions worker incorrectly URL encodes the response cookie value in the Set-Cookie header

Relevant code being tried

import azure.functions as func
from fastapi import FastAPI
from fastapi.responses import PlainTextResponse

app = FastAPI()
func_app = func.AsgiFunctionApp(app=app, http_auth_level=func.AuthLevel.ANONYMOUS)

@app.get("/")
def http_trigger():

    response = PlainTextResponse("", status_code=200)
    response.set_cookie(key='test', value='hello//world')

    return response

Relevant log output

N/A

requirements.txt file

azure-functions
fastapi

Where are you facing this problem?

Production Environment (explain below)

Function app name

Private

Additional Information

No response