Fix Jupyter CORS connection errors

Orion runs in your browser and talks to Jupyter over HTTP. If Jupyter does not allow cross-origin requests from Orion's URL, the connection fails.

Fix

Start Jupyter with an explicit allow origin:

macOS / Linux (bash/zsh):

jupyter server --ServerApp.allow_origin='*'

Windows (PowerShell):

jupyter server --ServerApp.allow_origin='*'

Copy the URL with token from the terminal and paste it into Orion's kernel selector.

When you need this

• Connection dialog succeeds briefly then errors mention CORS or blocked
• Browser developer console shows cross-origin failures to 127.0.0.1:8888 (or your Jupyter host)
• Manual Jupyter works in the terminal but Orion cannot attach

Persistent configuration

Add to Jupyter config so you do not pass the flag every time:

macOS / Linux: ~/.jupyter/jupyter_server_config.py

Windows: %USERPROFILE%\.jupyter\jupyter_server_config.py

c.ServerApp.allow_origin = "*"

Restart Jupyter after editing.

Related

• Connect to an external Jupyter server
• Your first session

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Last updated May 2026.