# Enable RunCell server extension (JupyterLab restart required)

> Fix local backend not running (local ping failed) by restarting JupyterLab and enabling the RunCell Jupyter Server extension when needed

Canonical HTML: https://www.runcell.dev/docs/faq/enable-runcell-server-extension

## 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 using one of these methods:
   * Run `jupyter lab stop` in a new terminal window, or
   * Press `Ctrl + C` in the terminal where JupyterLab is running
3. Start it again:

```bash
jupyter lab
```

Alternatively, you can select **Restart Server** from the JupyterLab menu if available.

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

***

## Step 2 — Verify the server extension is enabled

Run:

```bash
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:

```bash
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:

```bash
jupyter server extension enable runcell
```

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

```bash
# 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:

```bash
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:

```bash
jupyter lab --log-level=DEBUG
```

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

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

and contact support.