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 # APIs

 Last update: 19.06.2026 

- The main interfaces a plug-in must implement:
    
    
    - [`IHarnessRunner`](apidocs/Phoenix.ComponentAPI.IHarnessRunner.md)
    - [`IHarnessBuilderUI<RUNNER>`](apidocs/Phoenix.ComponentBuilderAPI.IHarnessBuilderUI-1.md)
- However, for the `BuilderUI` most plug-in writers will prefer to extend one of the abstract base class helpers in this SDK:
    
    
    - [`AbstractVariableBasedBuilderUI<RUNNER>`](apidocs/Phoenix.ComponentPlugInSDK.AbstractVariableBasedBuilderUI-1.md)
    - [`AbstractScriptBasedBuilderUI<RUNNER, MODEL>`](apidocs/Phoenix.ComponentPlugInSDK.AbstractScriptBasedBuilderUI-2.md)

Here is a class diagram of the interfaces and abstract base classes relevant for a plug-in developer:

![Class diagram of key plug-in interfaces and abstract base classes](/sites/default/files/git_ansys/component_plug_in_sdk_2026_r1_1/PACZComponentPlugInClassDiagram.drawio.svg)