Fixing FastAPI StreamingResponse Not Streaming

Problem

When using FastAPI's StreamingResponse with a generator function to stream responses from services like OpenAI's API, you may encounter an issue where the entire response gets sent at once instead of streaming incrementally. This occurs despite:

1. The generator function correctly yielding chunks of data
2. OpenAI API sending streamed responses
3. Server-side logging confirming chunks are being processed

The client receives the complete response only after all processing finishes, missing the real-time streaming experience. This problem typically stems from a combination of generator implementation issues, client handling, and browser behavior.

Key Solutions

1. Use Synchronous Generators for Blocking Operations

For generators containing blocking operations (like time.sleep() or synchronous OpenAI calls):

def ask_statesman(query: str):  # Use def instead of async def
    completion_reason = None
    while not completion_reason or completion_reason == "length":
        openai_stream = openai.ChatCompletion.create(...)  # With stream=True
        for line in openai_stream:
            if "content" in line["choices"][0].delta:
                current_response = line["choices"][0].delta.content
                yield current_response  # FastAPI runs this in a thread pool

Why this works

FastAPI runs synchronous generators in a separate thread using iterate_in_threadpool, preventing blocking of the main async event loop.

2. Adjust Media Type or Headers

Browser buffering behavior interferes with text/plain streaming. Use either:

# Option 1: Change media type for event streaming
return StreamingResponse(ask_statesman(query), media_type='text/event-stream')

# Option 2: Add header to disable MIME sniffing
headers = {'X-Content-Type-Options': 'nosniff'}
return StreamingResponse(ask_statesman(query), headers=headers, media_type='text/plain')

3. Handle Client-Side Streaming Correctly

Use appropriate chunk iteration methods in your client:

Python Requests Client

import requests

with requests.post(url, params=params, stream=True) as r:
    # For raw chunks
    for chunk in r.iter_content(chunk_size=1024):
        print(chunk.decode('utf-8'), end='')
    
    # Or for lines (if chunks contain \n)
    # for line in r.iter_lines():
    #   print(line.decode('utf-8'))

HTTPX Client (recommended for async)

import httpx

with httpx.stream('POST', url, params=params) as r:
    for chunk in r.iter_text():
        print(chunk, end='')

Testing with cURL

curl -N -X POST "http://localhost:8000?auth_key=123&query=Your+query"
# -N flag disables response buffering

Complete Working Example

FastAPI Server (app.py)

from fastapi import FastAPI, HTTPException, status
from fastapi.responses import StreamingResponse
import os
import openai
import time

app = FastAPI()
openai.api_key = os.environ["OPENAI_API_KEY"]

def ask_statesman(query: str):  # Synchronous generator
    stream = openai.ChatCompletion.create(
        model="gpt-3.5-turbo",
        messages=[{"role": "user", "content": query}],
        temperature=0.0,
        stream=True
    )
    for chunk in stream:
        if content := chunk.choices[0].delta.get("content"):
            yield content
            time.sleep(0.25)  # Simulate delay if needed

@app.post("/")
def main(auth_key: str, query: str):
    if auth_key != "123":
        raise HTTPException(status.HTTP_401_UNAUTHORIZED, detail="Invalid key")
    return StreamingResponse(ask_statesman(query), media_type="text/event-stream")

Python Test Client (test.py)

import httpx

url = "http://localhost:8000"
params = {"auth_key": "123", "query": "Explain streaming responses"}

with httpx.stream('POST', url, params=params) as response:
    for chunk in response.iter_text():
        print(chunk, end='', flush=True)  # Prints chunks in real-time

Security Best Practices

1. Use Proper HTTP Methods:

   • Prefer GET over POST for data retrieval
   • If using POST, send credentials in headers/cookies, not URL parameters

   from fastapi.security import OAuth2PasswordBearer
   oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")
   
   @app.get("/")
   async def secure_endpoint(token: str = Depends(oauth2_scheme)):
       ...

2. Always Use HTTPS:

   uvicorn app:app --host 0.0.0.0 --port 443 --ssl-keyfile key.pem --ssl-certfile cert.pem

Alternative Approach: Server-Sent Events (SSE)

For more robust event streaming:

# Install: pip install sse-starlette
from sse_starlette.sse import EventSourceResponse

@app.get('/sse')
async def sse_endpoint():
    return EventSourceResponse(ask_statesman(query))

SSE Note

SSE requires special client handling and uses text-based formatting. Use only when needed for browser compatibility.

Conclusion

To ensure proper streaming with FastAPI:

1. Use synchronous (def) generators for blocking operations
2. Set media_type='text/event-stream' or X-Content-Type-Options: nosniff header
3. Verify client logic uses proper streaming methods (iter_content()/iter_text())
4. Test with curl -N or httpx to validate real-time streaming

Following these patterns guarantees true chunk-by-chunk streaming of your API responses while maintaining security and performance best practices.