Serving Your AI: The FastAPI Bridge

A Python script running in your terminal is not a "Product." To build a website or a mobile app, your AI must live behind a REST API. FastAPI is the industry-standard choice for AI backends because it is extremely fast and natively supports asynchronous operations (async), which is perfect for LLM calls.

1. Why FastAPI?

1. Concurrency: It can handle multiple users at once without blocking.
2. JSON Validation: It uses Pydantic (Module 11) to ensure the frontend sends the right data.
3. Speed: It is one of the fastest Python frameworks available.

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2. The Basic Endpoint

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class ChatRequest(BaseModel):
    input: str

@app.post("/chat")
async def chat(request: ChatRequest):
    # Here is where your LangChain logic lives
    response = chain.invoke({"input": request.input})
    return {"reply": response}

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3. Streaming Over the Web

One of the best features of FastAPI + LangChain is Streaming. You can send tokens to the browser as they are generated. We use StreamingResponse.

from fastapi.responses import StreamingResponse

@app.get("/stream")
async def stream_chat(query: str):
    async def generate():
        async for chunk in model.astream(query):
            yield chunk.content
            
    return StreamingResponse(generate(), media_type="text/event-stream")

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4. Visualizing the Web Stack

graph LR
    U[User Browser] -->|Post /chat| F[FastAPI Server]
    F -->|Invoke| LC[LangChain Object]
    LC -->|API Request| O[OpenAI/Cloud]
    O -->|Tokens| F
    F -->|JSON| U

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5. Engineering Tip: CORS

When you build your frontend (React/Next.js), it will try to talk to your FastAPI server. By default, browsers block this for security. You must enable CORS (Cross-Origin Resource Sharing) in your FastAPI code.

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Key Takeaways

• FastAPI is the bridge between your AI logic and the world.
• POST requests are preferred for sending large prompt data.
• StreamingResponse enables the "ChatGPT-style" typing effect in your UI.
• Pydantic ensures your API stays stable and well-documented.