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).

Rules (extension-era, 5.x)

• Relative imports only (from . import ops). The package name is decided by Blender, so preferences use bl_idname = __package__.
• No bl_info (it's ignored for extensions). Everything goes in the manifest.
• No network or file access at import/register time. Declare [permissions] (files, network, clipboard, camera, microphone) with a reason. Check bpy.app.online_access before any network call.
• Third-party Python packages: ship .whl files in ./wheels/ and list them under wheels = [...]. Never pip install at runtime. Platform-specific wheels require platforms = [...].
• Properties (5.0+): bpy.props values are attributes, not dict entries (scene["my_prop"] no longer reaches them). For read-only props use options={'READ_ONLY'}. get_transform/set_transform adapt values while keeping storage. Register with PointerProperty(type=PG) on an ID type, and del it on unregister.
• Operators: bl_options={"REGISTER","UNDO"} for undo plus a redo panel. poll() guards the context. Return {"FINISHED"}/{"CANCELLED"}. self.report({"WARNING"}, msg) for user feedback. Don't call other operators in loops if a data API exists.
• UI: layout.use_property_split = True gives modern alignment. Panels in the N-panel use bl_region_type="UI" + bl_category. Append menu entries with VIEW3D_MT_*.append(func) and remove them on unregister.
• Unregister everything in reverse order (classes, props, menus, handlers, timers, keymaps, draw handlers). Leaks break reloading.
• Drawing (overlays): gpu module + gpu_extras.batch. bgl was removed in 5.0. Textures come from gpu.texture.from_image(img). For scene-linear images use draw_texture_2d(..., is_scene_linear_with_rec709_srgb_target=True).
• Long tasks: a modal operator with a timer (wm.event_timer_add), or bpy.app.timers.register with chunked work. Never block the UI for seconds.
• Handlers: decorate persistent ones with @bpy.app.handlers.persistent. Keep them cheap (they run every frame/depsgraph update).
• Keymaps: add them to wm.keyconfigs.addon, store (km, kmi) pairs, and remove them on unregister.
• Translations / i18n: bpy.app.translations.register(__package__, dict).

Publishing (extensions.blender.org)

The Blender ID account uploads the built zip. Requirements: a valid manifest, a GPL-compatible license, no obfuscated code, no bundled binaries without source, and permissions declared. Screenshots/README go on the website listing. Update with a higher version.

Gotchas

• context.scene inside a draw function can be None in some editors. Guard it.
• Storing Blender data references (bpy.types.ID) in Python globals crashes after undo or file load. Store names or session_uid, and look them up again.
• bpy.utils.register_class fails if annotations use = instead of : for properties.
• 5.0 made several bundled modules private (rna_info, bl_ui_utils...). Don't import them.
• 5.1 no longer loads the user site-packages by default. Bundle wheels instead of relying on user installs.
• 5.2: PreferencesFilePaths.active_asset_library indices are offset (the "All"/"Essentials" entries come first). UILayout.template_palette(color=) is removed. New: UILayout.link, UILayout.textbox, bpy.data.all_ids, Window.screenshot.