FastAPI VAC Routes Implementation - Key Fixes Summary

Overview

This document summarizes the critical fixes applied to make the FastAPI VAC Routes implementation fully functional with proper SSE streaming support.

Critical Fixes Applied

1. Archive QA Function - Async/Sync Mismatch

Problem: archive_qa() is a synchronous function but was being called with await Error: TypeError: object NoneType can't be used in 'await' expression Fix: Removed await from all archive_qa() calls

# Before (incorrect)
await archive_qa(chunk, vector_name)

# After (correct)
archive_qa(chunk, vector_name)  # This is a sync function, not async

2. Streaming Loop Exit Timing

Problem: The streaming loop exited when stream_finished was set, before the final result could be yielded Fix: Modified loop condition to continue until task is complete

# Changed the loop to check task completion status
while not stop_event.is_set():
    if chat_callback_handler.stream_finished.is_set() and chat_task.done():
        break

3. Empty String Yields

Problem: Empty strings were being yielded, causing issues with SSE format Fix: Added check to not yield empty strings

if final_yield:  # Only yield if we have actual content
    yield final_yield

4. Mock Document Format in Demo

Problem: Demo interpreters returned plain dicts but parse_output expected document objects with page_content attribute Fix: Created MockDocument class in demo

class MockDocument:
    def __init__(self, page_content, metadata):
        self.page_content = page_content
        self.metadata = metadata

5. JavaScript String Escaping in Test Page

Problem: Literal newline characters in JavaScript strings caused syntax errors Error: SyntaxError: '' string literal contains an unescaped line break Fix: Properly escaped newlines in JavaScript strings within Python triple-quoted strings

# Before (incorrect)
const lines = buffer.split('\n');

# After (correct)
const lines = buffer.split('\\n');

6. Error Handler Logging

Problem: log.error() doesn't accept exc_info parameter in StandardLoggerWrapper Fix: Used traceback.format_exc() instead

import traceback
log.error(f"Error in SSE generator: {e}\n{traceback.format_exc()}")

Testing Commands

Basic SSE Test

curl -X POST http://localhost:8000/vac/streaming/demo/sse \
    -H "Content-Type: application/json" \
    -d '{"user_input": "Tell me a story"}'

Expected output:

data: {"chunk": "Hello! I'm a demo async streaming interpreter. \n\n"}
data: {"chunk": "You asked: \"Tell me a story\". \n\n"}
data: {"chunk": "Here's my streaming response..."}
data: {"answer": "...", "source_documents": [...]}
data: [DONE]

Interactive Test Page

Visit http://localhost:8000/test for an interactive test interface with both Plain Text and SSE streaming options.

Key Implementation Details

The Callback Bridge Architecture

LLM → callback.on_llm_new_token() → ContentBuffer → async generator → StreamingResponse

Async/Sync Interpreter Detection

• Async interpreters: Run directly with await
• Sync interpreters: Run in thread executor with queue-based communication

Common Pitfalls to Avoid

1. Don't await sync functions - Check if a function is async before using await
2. Escape strings in triple-quoted strings - Use \\n not \n for JavaScript strings
3. Check for empty yields - Don't yield empty strings in streaming responses
4. Match document format - Ensure mock data matches expected format for parsers
5. Use proper error handling - StandardLoggerWrapper has limited parameters

Verification Steps

1. Start the demo server:

python examples/fastapi_vac_demo.py

2. Test SSE streaming with curl (should complete with [DONE])
3. Test interactive page at /test (both buttons should work)
4. Verify sources are included in final response
5. Check that connection properly terminates