VSCodium Extension Not Found? Diagnosing Marketplace Configuration Failures

The recognizable condition

You search the Extensions view in VSCodium for an extension you know exists — say, the C# Dev Kit or a popular linter — and it simply isn't there. Or an update fails with "extension not found." Or the search box returns nothing at all. In most cases the editor is fine; the problem is which marketplace it's talking to.

VSCodium ships configured to use the Open VSX Registry (open-vsx.org) instead of Microsoft's Visual Studio Marketplace. That choice is deliberate — Microsoft's marketplace terms restrict use to official VS Code builds, and VSCodium exists to avoid Microsoft's telemetry and licensing — but it produces a class of failures that look like broken networking or a buggy editor.

Cause and diagnostic table

Ordered checks

1. Confirm which gallery you're using

Open resources/app/product.json inside your VSCodium installation (location varies by OS: on Linux typically /usr/share/codium/resources/app/product.json, on macOS inside the .app bundle). Look at extensionsGallery. A stock VSCodium shows serviceUrl pointing at https://open-vsx.org/vscode/gallery. If it points at Microsoft's endpoints, someone (or the first-run prompt in VSCodium 1.85+) already switched it.

2. Rule out the network

From a terminal on the same machine, run curl -I https://open-vsx.org/api/-/search?text=test. A 200 response means the registry is reachable. A timeout or 403 from a corporate proxy explains silent search failures — VSCodium's UI often shows an empty result list rather than a network error. If open-vsx.org is blocked but Microsoft's marketplace domains are allowed (a common corporate allowlist), that's a policy conversation, not a config bug.

3. Check whether the extension exists on Open VSX at all

Search for the exact extension ID (e.g., ms-dotnettools.csharp) on open-vsx.org in a browser. Many Microsoft-published extensions — C# Dev Kit, the C/C++ extension, Live Share — are absent because their licenses restrict distribution. Absence here confirms the gallery mismatch is your root cause.

Fixes tied to findings

If the extension is on Open VSX but outdated

Wait for the publication lag to catch up, or install the newer version manually as a .vsix (below). Don't assume VSCodium is broken; the delay is in the publishing pipeline.

If the extension is Microsoft-only: install the .vsix directly

Download the .vsix file from the Microsoft Marketplace web page ("Download Extension" link under Resources), then in a terminal run:

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

This bypasses gallery lookup entirely. Two caveats: the extension won't auto-update (you must repeat this for new versions), and some Microsoft extensions check the host product at runtime and disable features or refuse to download components under VSCodium. Check the extension's Output panel after install.

If you decide to switch galleries

You can edit product.json to point extensionsGallery at Microsoft's endpoints. Be aware of the trade-offs before doing this: it reintroduces the telemetry and licensing surface VSCodium avoids, Microsoft's Marketplace Terms of Use restrict access to official VS Code clients, and the edit survives updates but not a full reinstall. This is a decision to make deliberately, not a default fix.

If the network is the blocker

Configure your proxy in VSCodium's settings (http.proxy) or ask your network team to allowlist open-vsx.org. Verify with the curl check above after the change.

Escalation criteria

Escalate beyond local fixes when: (1) a runtime product-identity check disables an extension you depend on — the realistic options are official VS Code or a different extension, not more VSCodium tweaking; (2) mixed-source installations (some extensions from Open VSX, some from .vsix) cause update loops or version conflicts — standardize on one source per extension; (3) your organization can't allowlist open-vsx.org and policy forbids the Microsoft endpoints — at that point .vsix side-loading with a documented manual update process is the supportable path.

Whatever fix you apply, verify it: reload the window, confirm the extension appears in codium --list-extensions, and exercise one of its actual features rather than trusting the install message.