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Streamlit Quick Start - Fix 403 WebSocket Error

Quick fix for "You need to enable JavaScript" and 403 WebSocket errors.

The Problem​

Your Streamlit app loads but doesn't work because:

  • WebSocket connections (/_stcore/stream) return 403 Forbidden
  • Streamlit is blocking connections from the proxy domain

Quick Fix​

Production (Render)​

Check that render.yaml includes the proxy domain in STREAMLIT_SERVER_ALLOWED_ORIGINS:

- key: STREAMLIT_SERVER_ALLOWED_ORIGINS
  value: "https://ocapistaine.onrender.com,https://ocapistaine.vaettir.locki.io,https://vaettir.locki.io"

Redeploy after changes.

Development (ngrok)​

# 1. Set up CORS
eval $(python scripts/set_streamlit_env.py)

# 2. Start Streamlit
streamlit run app/front.py

# 3. In another terminal, start ngrok
ngrok http 8502 --domain=$NGROK_DOMAIN

Access via https://ocapistaine-dev.vaettir.locki.io

Verify It Works​

Open browser DevTools (F12) → Network tab:

  • Look for /_stcore/stream WebSocket
  • Should show status 101 Switching Protocols
  • NOT 403 Forbidden

If Still Not Working​

See full troubleshooting guide: STREAMLIT_SETUP.md

Common Issues​

Still 403?

# Check what origins are configured
echo $STREAMLIT_SERVER_ALLOWED_ORIGINS

# Try wildcard (dev only)
export STREAMLIT_SERVER_ALLOWED_ORIGINS="*"
streamlit run app/front.py

App not responding?

# Check health
curl http://localhost:8502/_stcore/health

# Production
curl https://ocapistaine.vaettir.locki.io/_stcore/health

Render cold start?

Standard plan instances may take 30-60s to wake up. Wait and retry.