Note they say that most extensions are compatible, and those not listed as compatible might still be.

The API surface covers almost 100% that of vscode, I only see some AI integration API's that are stubbed, and that is because Theia has their own vision here and doesn't want to depend on MS.

The complete API compatibility list is here, the stubbed API's are not core imho:

https://eclipse-theia.github.io/vscode-theia-comparator/stat...