Diagnose extension installation failures in 3D Slicer

TL;DR

For "loadSourceAsModule - Failed to load file "C:/.../Extensions-32448/OpenDose3D/lib/Slicer-5.6/qt-scripted-modules/OpenDose3D.py" as module "Op": restart Slicer, then check the Extensions Manager's Installed tab. (If you installed from a file per section 1 and the module still doesn't appear after a restart, you are in section 2 territory, verify the package revision.) In Slicer's Extensions Manager, use Install extension from file (wrench/toolbox menu) and select the downloaded package. OpenDose3D packages that predate a Slicer 5.8 update and no longer load under it.

loadSourceAsModule - Failed to load file "C:/.../Extensions-32448/OpenDose3D/lib/Slicer-5.6/qt-scripted-modules/OpenDose3D.py" as module "OpenDose3D" !
Fail to instantiate module "OpenDose3D"
The following modules failed to be instantiated: Calibration OpenDose3D Gamma

When to use

You are seeing this: Extensions are built for a specific Slicer revision (e.g. Use this skill when you run into "Diagnose extension installation failures in 3D Slicer".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

Versions mentioned in the source: Slicer 5.4.0, Slicer 5.6.2. If you are on something much newer or older, the details may have shifted.

Use this when an extension won't download, installs but its modules never appear, or the Extensions Manager itself misbehaves. Extensions are built for a specific Slicer revision (e.g. Extensions-31938 under Slicer 5.4.0, Extensions-32448 under Slicer 5.6.2), most failures are a mismatch between the extension package and your Slicer build, or a network problem. Find your revision in Help / About; the catalog uses it in the URL (extensions.slicer.org/catalog/All/32448/win).

1. Failed downloading: https://slicer-packages.kitware.com/...

The Extensions Manager could not fetch the package, this is a network problem, not a broken extension. It shows up on locked-down machines, proxies, and cloud VMs (firewalled govcloud instances hit it for every extension). Fix:

1. Download the extension package from the catalog website on a machine that has network access. Pick the entry matching your OS and your Slicer revision.

2. In Slicer's Extensions Manager, use Install extension from file (wrench/toolbox menu) and select the downloaded package.

3. Restart Slicer.

The same offline path works on air-gapped machines; the documented section is "Install extensions without network connection" in the user guide.

2. Installed, but the module never appears / loadSourceAsModule - Failed to load

Real symptom, from the Error Log:

loadSourceAsModule - Failed to load file "C:/.../Extensions-32448/OpenDose3D/lib/Slicer-5.6/qt-scripted-modules/OpenDose3D.py" as module "OpenDose3D" !
Fail to instantiate module "OpenDose3D"
The following modules failed to be instantiated: Calibration OpenDose3D Gamma

This means the extension package was built against a different Slicer version than the one running, e.g. OpenDose3D packages that predate a Slicer 5.8 update and no longer load under it. Check, in order:

1. Does your Slicer's revision match the revision the package was built for? Extension folders are named Extensions-[revision]; a folder whose revision differs from Help / About is the wrong package.

2. If the versions genuinely differ, either install the Slicer version the extension targets, or check the extension's own issue tracker, extension authors announce compatibility updates there.

Note the distinction from the next case: here the load is attempted and fails. If no load is attempted at all, see below.

3. Extension shows "installed" but nothing new in the module list

Extensions are not activated until Slicer restarts. This is the most common "it installed but I can't find it" report: the install completed, the module only registers on the next launch. Restart Slicer, then check the Extensions Manager's Installed tab. (If you installed from a file per section 1 and the module still doesn't appear after a restart, you are in section 2 territory verify the package revision.)

4. Extensions Manager crashes or shows a blank/no-connection page

On Linux, opening the Extensions Manager has crashed Slicer outright (QtWebEngine + OpenGL stack traces ending in QQuickWidget/RenderWidgetHostViewQtDelegateWidget segfaults, after [Qt] An OpenGL Core Profile was requested, but it is not supported warnings). This is a rendering-environment problem, not a catalog problem:

• Update the graphics driver / enable a supported OpenGL profile if you can.

• Otherwise skip the in-app browser entirely: use the offline install-from-file flow from section 1.

5. Extension missing from the catalog for your revision

Some extensions are only built for certain platforms or for preview (nightly) builds. Search the catalog for your exact revision and OS; if the extension is absent there but exists for the nightly, install a Slicer preview release and retry. If it is absent for your OS entirely, the extension does not support that platform (check its repository for a platform-support statement before assuming the catalog is broken).

Quick triage checklist

1. Capture the exact symptom: download error, load error in the Error Log, missing-after-restart, or manager crash.

2. Network → offline install from file (section 1).
3. loadSourceAsModule failure → revision mismatch (section 2).
4. Installed-but-missing → restart Slicer (section 3).
5. Manager crash → graphics environment; use offline install (section 4).
6. Not in catalog → platform/nightly coverage (section 5).

Built from 3D Slicer Discourse threads: 'Extension download issues in govcloud' (Failed downloading + offline install-from-file procedure), 'OpenDose3D module is not loaded' (loadSourceAsModule / failed-to-instantiate on a version mismatch, Extensions-32448 under Slicer 5.6.2), 'Opening extension manager makes Slicer crash on Linux' (QtWebEngine OpenGL crash), and the Extensions Manager user guide's 'Install extensions without network connection' section.