ChatKit Python Backend Skill
Build custom chat API backends that work with OpenAI ChatKit frontend.
Architecture
┌─────────────────────────────────────────────────────────────────────────┐
│ Frontend (ChatKit-JS) │
│ useChatKit({ api: { url, domainKey } }) │
└─────────────────────────────────────────────────────────────────────────┘
│
│ HTTP POST /api/chat
│ (SSE streaming response)
▼
┌─────────────────────────────────────────────────────────────────────────┐
│ FastAPI Backend │
│ ┌─────────────────────────────────────────────────────────────────┐ │
│ │ /api/chat Endpoint │ │
│ │ • Parse ChatKit request │ │
│ │ • Run Agent with MCP tools │ │
│ │ • Stream response via SSE │ │
│ │ • Persist conversation to DB │ │
│ └─────────────────────────────────────────────────────────────────┘ │
│ │ │
│ ▼ │
│ ┌─────────────────┐ ┌─────────────────┐ ┌─────────────────────┐ │
│ │ OpenAI Agents │ │ MCP Server │ │ Neon Database │ │
│ │ SDK + Gemini │ │ (Todo Tools) │ │ (Conversations) │ │
│ └─────────────────┘ └─────────────────┘ └─────────────────────┘ │
└─────────────────────────────────────────────────────────────────────────┘
Quick Start
Installation
pip install fastapi uvicorn sse-starlette "openai-agents[litellm]"
# Or with uv
uv add fastapi uvicorn sse-starlette "openai-agents[litellm]"
Environment Variables
# Database
DATABASE_URL=postgresql://user:password@host/database
# Gemini API
GOOGLE_API_KEY=your-gemini-api-key
# CORS (frontend URL)
CORS_ORIGINS=http://localhost:3000,https://your-app.vercel.app
Reference
Examples
Templates
Basic Chat Endpoint
from fastapi import FastAPI, Request
from fastapi.middleware.cors import CORSMiddleware
from sse_starlette.sse import EventSourceResponse
from pydantic import BaseModel
from typing import Optional, List, AsyncGenerator
import json
app = FastAPI()
# CORS for ChatKit frontend
app.add_middleware(
CORSMiddleware,
allow_origins=["http://localhost:3000"],
allow_credentials=True,
allow_methods=["*"],
allow_headers=["*"],
)
class ChatMessage(BaseModel):
role: str # "user" or "assistant"
content: str
class ChatRequest(BaseModel):
messages: List[ChatMessage]
thread_id: Optional[str] = None
user_id: Optional[str] = None
async def generate_response(messages: List[ChatMessage]) -> AsyncGenerator[str, None]:
"""Generate streaming response from agent."""
# Your agent logic here
response = "Hello! How can I help you?"
# Stream response word by word
for word in response.split():
yield f"data: {json.dumps({'content': word + ' '})}\n\n"
yield f"data: {json.dumps({'done': True})}\n\n"
@app.post("/api/chat")
async def chat_endpoint(request: ChatRequest):
"""Chat endpoint with SSE streaming."""
return EventSourceResponse(
generate_response(request.messages),
media_type="text/event-stream",
)
Chat Endpoint with Agent
import os
from fastapi import FastAPI, Request, HTTPException
from sse_starlette.sse import EventSourceResponse
from pydantic import BaseModel
from typing import Optional, List, AsyncGenerator
import json
from agents import Agent, Runner, set_tracing_disabled
from agents.mcp import MCPServerStreamableHttp
from agents.extensions.models.litellm_model import LitellmModel
set_tracing_disabled(disabled=True)
app = FastAPI()
class ChatMessage(BaseModel):
role: str
content: str
class ChatRequest(BaseModel):
messages: List[ChatMessage]
thread_id: Optional[str] = None
user_id: str
# Global MCP server connection (managed via lifespan)
mcp_server = None
@app.on_event("startup")
async def startup():
global mcp_server
mcp_server = MCPServerStreamableHttp(
name="Todo MCP",
params={"url": "http://localhost:8000/api/mcp", "timeout": 30},
cache_tools_list=True,
)
await mcp_server.__aenter__()
@app.on_event("shutdown")
async def shutdown():
global mcp_server
if mcp_server:
await mcp_server.__aexit__(None, None, None)
def create_agent(user_id: str):
"""Create agent with user context."""
return Agent(
name="Todo Assistant",
instructions=f"""You are a task management assistant for user {user_id}.
Use the MCP tools to help manage tasks:
- add_task: Create new tasks
- list_tasks: View tasks
- complete_task: Mark done
- delete_task: Remove tasks
Always pass user_id="{user_id}" to tools.""",
model=LitellmModel(
model="gemini/gemini-2.0-flash",
api_key=os.getenv("GOOGLE_API_KEY"),
),
mcp_servers=[mcp_server] if mcp_server else [],
)
async def stream_agent_response(
agent: Agent,
user_message: str,
) -> AsyncGenerator[str, None]:
"""Stream agent response as SSE events."""
try:
result = Runner.run_streamed(agent, user_message)
async for event in result.stream_events():
if hasattr(event, 'item') and event.item:
yield f"data: {json.dumps({'content': str(event.item)})}\n\n"
# Final response
final = result.final_output
yield f"data: {json.dumps({'content': final, 'done': True})}\n\n"
except Exception as e:
yield f"data: {json.dumps({'error': str(e)})}\n\n"
@app.post("/api/chat")
async def chat(request: ChatRequest):
"""Chat endpoint with streaming response."""
if not request.messages:
raise HTTPException(400, "No messages provided")
# Get last user message
user_message = request.messages[-1].content
# Create agent for this user
agent = create_agent(request.user_id)
return EventSourceResponse(
stream_agent_response(agent, user_message),
media_type="text/event-stream",
)
SSE Event Format
ChatKit expects Server-Sent Events in this format:
# Content chunk
yield f"data: {json.dumps({'content': 'partial response'})}\n\n"
# Done signal
yield f"data: {json.dumps({'done': True})}\n\n"
# Error
yield f"data: {json.dumps({'error': 'error message'})}\n\n"
# Tool call (optional)
yield f"data: {json.dumps({'tool_call': {'name': 'add_task', 'result': {...}}})}\n\n"
Conversation Persistence
from sqlmodel import SQLModel, Field, Session
from datetime import datetime
from typing import Optional
class Conversation(SQLModel, table=True):
id: Optional[int] = Field(default=None, primary_key=True)
thread_id: str = Field(index=True, unique=True)
user_id: str = Field(index=True)
created_at: datetime = Field(default_factory=datetime.utcnow)
updated_at: datetime = Field(default_factory=datetime.utcnow)
class Message(SQLModel, table=True):
id: Optional[int] = Field(default=None, primary_key=True)
thread_id: str = Field(index=True)
role: str # "user" or "assistant"
content: str
created_at: datetime = Field(default_factory=datetime.utcnow)
async def save_message(thread_id: str, role: str, content: str):
"""Save message to database."""
with Session(engine) as session:
message = Message(thread_id=thread_id, role=role, content=content)
session.add(message)
session.commit()
async def load_conversation(thread_id: str) -> List[dict]:
"""Load conversation history."""
with Session(engine) as session:
messages = session.exec(
select(Message)
.where(Message.thread_id == thread_id)
.order_by(Message.created_at)
).all()
return [{"role": m.role, "content": m.content} for m in messages]
Authentication Integration
from fastapi import Depends, HTTPException, Header
async def get_current_user(authorization: str = Header(...)) -> dict:
"""Verify JWT and extract user."""
if not authorization.startswith("Bearer "):
raise HTTPException(401, "Invalid authorization header")
token = authorization[7:]
user = await verify_jwt_token(token) # Your JWT verification
if not user:
raise HTTPException(401, "Invalid token")
return user
@app.post("/api/chat")
async def chat(
request: ChatRequest,
user: dict = Depends(get_current_user),
):
"""Authenticated chat endpoint."""
# Use user["id"] as user_id
agent = create_agent(user["id"])
# ...
CORS Configuration
from fastapi.middleware.cors import CORSMiddleware
# Get from environment
CORS_ORIGINS = os.getenv("CORS_ORIGINS", "http://localhost:3000").split(",")
app.add_middleware(
CORSMiddleware,
allow_origins=CORS_ORIGINS,
allow_credentials=True,
allow_methods=["*"],
allow_headers=["*"],
)
Error Handling
@app.exception_handler(Exception)
async def global_exception_handler(request: Request, exc: Exception):
return JSONResponse(
status_code=500,
content={"error": str(exc)},
)
@app.post("/api/chat")
async def chat(request: ChatRequest):
try:
# ... chat logic
pass
except ConnectionError:
raise HTTPException(503, "MCP server unavailable")
except TimeoutError:
raise HTTPException(504, "Request timed out")
except Exception as e:
raise HTTPException(500, f"Internal error: {e}")
Testing
import pytest
from httpx import AsyncClient
@pytest.mark.asyncio
async def test_chat_endpoint():
async with AsyncClient(app=app, base_url="http://test") as client:
response = await client.post(
"/api/chat",
json={
"messages": [{"role": "user", "content": "Hello"}],
"user_id": "test-user",
},
)
assert response.status_code == 200
assert "text/event-stream" in response.headers["content-type"]
Best Practices
- Stream responses - Better UX with SSE streaming
- Persist conversations - Enable conversation continuity
- Authenticate requests - Verify user before processing
- Handle errors gracefully - Return structured error responses
- Configure CORS - Allow frontend domain
- Use connection pooling - For database and MCP connections
- Log conversations - For debugging and analytics
