WebSocket closed FastAPI

Why This Happens

In FastAPI, the websocket connection was unexpectedly closed. This is one of the most common FastAPI errors developers encounter. Understanding the root cause helps you fix it quickly and prevent it from recurring.

The Problem

@app.websocket('/ws')
async def ws(websocket: WebSocket):
    data = await websocket.receive_text()

The Fix

@app.websocket('/ws')
async def ws(websocket: WebSocket):
    await websocket.accept()
    try:
        data = await websocket.receive_text()
    except WebSocketDisconnect:
        pass

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: WebSocket connection closed. This tells you exactly what went wrong.

2. 2

   Find the cause

   Check the stack trace to find which line of your FastAPI code triggered this error.

3. 3

   Apply the fix

   Use the corrected code pattern shown above. Test to confirm the error is resolved.