Add-on (Extension) Development

The execution model is in blender-core. Start from assets/addon_template/ (validated, built, and registered in CI): blender_manifest.toml, __init__.py, operators.py, ui.py.

Procedure

1. Copy the template, then set id (lowercase_underscores, permanent), name, tagline (no trailing period, ≤ 64 chars), maintainer, version (semver), license (SPDX, GPL-3.0-or-later for code using bpy), and blender_version_min = "4.2.0" or higher if you use newer APIs (e.g. "5.2.0" for the GN properties.inputs API).
2. Write classes with the prefix convention MYTOOL_OT_x (operator), _PT_ (panel), _MT_ (menu), _PG_ (property group), _AP_ (preferences), _UL_ (UI list). bl_idname for operators is my_tool.verb_noun.
3. Validate and build:

   blender --command extension validate ./my_tool
   blender --command extension build --source-dir ./my_tool --output-dir ./dist
   blender --command extension install-file -r user_default -e ./dist/my_tool-1.0.0.zip
   

4. Test headless: register the package in-process, call operators with temp_override, and assert on data (see tests/test_addon.py in this repo).