Diagnosing VSCodium Extension Installation Failures on Open VSX

Recognizable condition

When you try to install or update an extension in VSCodium, the operation fails with messages such as Extension not found or Network error. The same extension installs without issue in the official VS Code build.

Cause / diagnostic table

Observed indicator	Likely root cause
settings.json points to https://marketplace.visualstudio.com	Marketplace URL misconfigured – VSCodium cannot authenticate to the Microsoft Marketplace.
Extension exists only on the Microsoft Marketplace (e.g., C#, C++, Azure tools)	Proprietary extension not published to Open VSX.
curl https://open-vsx.org times out or returns a non‑2xx status	Network or proxy blocking Open VSX.
Errors persist after deleting ~/.vscode-oss/extensions	Corrupted local cache or stale metadata.
VSCodium version older than the extension’s engine requirement	Version mismatch – the extension requires a newer editor core.

Ordered checks

1. Verify marketplace configuration – Open the Command Palette (Ctrl+Shift+P), run Preferences: Open Settings (JSON), and locate the extensionsGallery object. It should contain serviceUrl and itemUrl pointing to https://open-vsx.org.
2. Test Open VSX connectivity – In a terminal (no elevated rights required) run:

   curl -s https://open-vsx.org/api/-/health

   Expect a JSON response with {"status":"ok"}. A non‑2xx response or timeout indicates a network block.
3. Confirm extension availability on Open VSX – Browse https://open-vsx.org and search the extension ID. If it is absent, the extension is not published there.
4. Check VSCodium version vs. extension engine – Run codium --version and compare the reported version to the engine field in the extension’s package.json (visible on the Open VSX page).
5. Inspect Developer Tools console – Choose Help → Toggle Developer Tools and watch the Network tab while attempting an install. Look for failed requests to open-vsx.org or unexpected redirects to the Microsoft Marketplace.

Fixes tied to findings

• Wrong marketplace URL – Replace the extensionsGallery block in settings.json with:

  "extensionsGallery": {
    "serviceUrl": "https://open-vsx.org/vscode/gallery",
    "itemUrl": "https://open-vsx.org/vscode/item"
  }

  Save the file; VSCodium will use Open VSX for subsequent installs.
• Extension missing from Open VSX – Download the .vsix file from the publisher’s GitHub releases or the Microsoft Marketplace (if licensing permits) and install locally:

  codium --install-extension /path/to/extension.vsix

  This bypasses the marketplace entirely.
• Network blocks Open VSX – Configure a proxy in settings.json:

  "http.proxy": "http://proxy.example.com:3128",
  "http.proxyStrictSSL": false

  Restart VSCodium. If authentication is required and unsupported, see escalation criteria.
• Corrupted cache – Close VSCodium, then delete the extensions folder:

  rm -rf ~/.vscode-oss/extensions

  Restart and reinstall needed extensions. Risk: all locally installed extensions are removed; you must reinstall them.
• Version mismatch – Update VSCodium to the latest release (via your package manager or the official binaries). If updating is impossible, install an older compatible version of the extension via its .vsix file (see previous fix).

Escalation criteria

Open an issue on the VSCodium GitHub repository when:

• Open VSX returns 5xx errors for valid extension IDs.
• The extension host crashes during installation (visible in Developer Tools console).
• Signature verification fails for a downloaded .vsix from Open VSX.
• Proxy authentication is required but the current network stack does not support it.

Practical engineering decision

Prefer configuring extensionsGallery through a system‑wide policy or distribution‑level settings file (e.g., /etc/vscodium/product.json or a managed settings.json deployed by configuration management). This avoids per‑user manual edits and prevents configuration drift across machines.

Limitations & verification

• Proprietary Microsoft extensions (C#, C++, Azure tools) are not available on Open VSX due to licensing; open‑source alternatives must be used.
• VSCodium does not support Microsoft Marketplace authentication; pointing to Microsoft URLs will always yield 401/403 errors.
• Auto‑update behavior differs from VS Code. Disable it by setting extensions.autoUpdate and extensions.autoCheckUpdates to false in settings.json.

To verify a fix, run the health check again and attempt a CLI install of a known‑good extension:

codium --install-extension ms-python.python

A successful install prints the extension ID and version without network errors.