Enable RunCell server extension (JupyterLab restart required)

When you need this

You might see RunCell installed in JupyterLab, but it doesn’t work or shows a setup/connection issue.

This usually means the RunCell Jupyter Server extension (the backend handler) is not running yet.

The most common cause: you installed RunCell via JupyterLab Plugin Manager, and JupyterLab needs a full restart (not a browser refresh) for the server-side part to load.

Step 1 — Fully restart JupyterLab (not refresh)

1. Close the JupyterLab browser tab.
2. Stop the running JupyterLab process in your terminal (usually Ctrl + C).
3. Start it again:

jupyter lab

If you installed via Plugin Manager, this step alone fixes most cases.

Step 2 — Verify the server extension is enabled

Run:

jupyter server extension list

Look for something like runcell in the output.

If you’re using a virtualenv/conda environment, make sure you run the command in the same environment as jupyter lab.

Quick sanity check:

which jupyter
python -c "import sys; print(sys.executable)"
python -m pip show runcell

Step 3 — Enable the server extension (only if needed)

If the extension is missing/disabled, enable it and restart JupyterLab.

Try:

jupyter server extension enable runcell

If that doesn’t work (common with conda/brew edge cases), try one of these variants and then restart:

# Enable for the current environment (recommended for venv/conda)
jupyter server extension enable --sys-prefix runcell

# Enable only for the current OS user
jupyter server extension enable --user runcell

Then restart:

jupyter lab

Common edge cases (conda / Homebrew / multiple Pythons)

• Environment mismatch: jupyter lab is running from one Python env, but runcell is installed in another.
  • Fix by installing RunCell into the same environment that provides jupyter.
• Homebrew / system-wide Jupyter: you may need to ensure your terminal PATH points to the expected jupyter.
• Still stuck?
  • Restart JupyterLab with debug logs:

jupyter lab --log-level=DEBUG

If you still can’t get it working, collect the output of:

python --version
jupyter lab --version
which python
which jupyter
jupyter server extension list
python -m pip show runcell

and contact support.