Addon Scaffolding (Extensions Platform)

Trigger

Use this skill when the user:

• Wants to start a new Blender add-on
• Mentions bl_info, blender_manifest.toml, "extension", "register_classes_factory"
• Is migrating an existing add-on from the legacy bl_info format to the Extensions Platform
• Is unsure which file layout or registration pattern to use for Blender 4.5+ or 5.x

Required inputs

• Add-on name and short id (snake_case for the manifest id, human-readable name)
• Target Blender versions (defaults to blender_version_min = "4.5.0" so the add-on works on 4.5 LTS and 5.1)
• Maintainer string (Name <email@example.com> form)
• License SPDX identifier (SPDX:MIT, SPDX:GPL-2.0-or-later, etc.)

The Extensions Platform format (5.x default)

Since 4.2 and required in 5.x for new submissions to the official extensions site, add-ons ship as Extensions. An Extension is a directory containing a blender_manifest.toml plus Python modules.

blender_manifest.toml required fields

schema_version = "1.0.0"

id = "my_addon"
version = "0.1.0"
name = "My Add-on"
tagline = "Short description, max 64 chars, no trailing period"
maintainer = "TMHSDigital <contact@example.com>"
type = "add-on"

blender_version_min = "4.5.0"

license = ["SPDX:MIT"]
copyright = ["2026 TMHSDigital"]

Field rules that AIs commonly get wrong:

• tagline is hard-capped at 64 characters and must not end with a period. Blender's manifest validator rejects both.
• id must be a valid Python identifier ([A-Za-z_][A-Za-z0-9_]*). The directory name does not need to match the id but conventionally does.
• license and copyright are arrays of strings, not bare strings.
• blender_version_min is the floor. Setting it to "4.5.0" lets the same code run on 4.5 LTS and any 5.x. Setting it to "5.0.0" cuts off LTS users with no warning.
• schema_version is "1.0.0" for the current manifest schema. Do not bump it.

Optional but commonly wanted:

website = "https://github.com/TMHSDigital/your-repo"
tags = ["Mesh", "Geometry Nodes"]
permissions = { network = "Optional, only used to fetch updates" }

tags must come from Blender's known tag list. permissions is a dict of granted-permission to human-readable reason; if omitted, the extension is sandboxed against network, files, clipboard, and camera.

File layout

The conventional layout for a non-trivial add-on:

my_addon/
  blender_manifest.toml
  __init__.py            # registration entry point only
  operators.py           # Operator subclasses
  panels.py              # Panel subclasses
  properties.py          # PropertyGroup subclasses, PointerProperty bindings
  utils.py               # Pure helpers, no bpy imports beyond types

Keep __init__.py small. It registers and unregisters classes; everything else lives in submodules.

The register_classes_factory pattern

This is the canonical 5.x registration pattern. It eliminates the loop boilerplate that AIs frequently bug.

import bpy
from bpy.utils import register_classes_factory

from . import operators, panels, properties

classes = (
    properties.MyAddonProperties,
    operators.MESH_OT_my_addon_action,
    panels.VIEW3D_PT_my_addon,
)

_register_classes, _unregister_classes = register_classes_factory(classes)


def register():
    _register_classes()
    bpy.types.Scene.my_addon = bpy.props.PointerProperty(type=properties.MyAddonProperties)


def unregister():
    del bpy.types.Scene.my_addon
    _unregister_classes()

Why this pattern:

• register_classes_factory returns a (register, unregister) pair that loops over classes in the right direction (forward for register, reverse for unregister) and skips the wrong-order foot-gun.
• Properties bound to types (bpy.types.Scene.my_addon = ...) must be deleted before the corresponding PropertyGroup class is unregistered. Hence del bpy.types.Scene.my_addon precedes _unregister_classes().
• Class registration order matters: PropertyGroup subclasses must come before any class that references them via PointerProperty.

Working example: minimal init.py

import bpy
from bpy.utils import register_classes_factory


class MY_ADDON_PG_settings(bpy.types.PropertyGroup):
    intensity: bpy.props.FloatProperty(name="Intensity", default=1.0, min=0.0, max=10.0)


class MESH_OT_my_addon_action(bpy.types.Operator):
    bl_idname = "mesh.my_addon_action"
    bl_label = "My Addon Action"
    bl_options = {'REGISTER', 'UNDO'}

    @classmethod
    def poll(cls, context):
        return context.active_object is not None and context.active_object.type == 'MESH'

    def execute(self, context):
        obj = context.active_object
        settings = context.scene.my_addon
        self.report({'INFO'}, f"Ran on {obj.name} with intensity {settings.intensity:.2f}")
        return {'FINISHED'}


class VIEW3D_PT_my_addon(bpy.types.Panel):
    bl_label = "My Addon"
    bl_idname = "VIEW3D_PT_my_addon"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = "My Addon"

    def draw(self, context):
        layout = self.layout
        settings = context.scene.my_addon
        layout.prop(settings, "intensity")
        layout.operator("mesh.my_addon_action")


classes = (
    MY_ADDON_PG_settings,
    MESH_OT_my_addon_action,
    VIEW3D_PT_my_addon,
)

_register_classes, _unregister_classes = register_classes_factory(classes)


def register():
    _register_classes()
    bpy.types.Scene.my_addon = bpy.props.PointerProperty(type=MY_ADDON_PG_settings)


def unregister():
    del bpy.types.Scene.my_addon
    _unregister_classes()

Common AI mistakes

1. Emitting only bl_info and skipping blender_manifest.toml. New add-ons must ship as Extensions for the official extensions platform. bl_info may appear alongside the manifest as a fallback for backward compatibility, but the manifest is the source of truth.

2. Manual register/unregister loops with the wrong direction:

   # WRONG: same direction in unregister, classes referencing each other will fail
   for cls in classes:
       bpy.utils.register_class(cls)
   
   def unregister():
       for cls in classes:
           bpy.utils.unregister_class(cls)  # missing reversed()

   Use register_classes_factory and let it own the order.

3. Forgetting to delete bound properties before unregister. Trying to unregister MY_ADDON_PG_settings while bpy.types.Scene.my_addon still points to it will hard-fail on Blender restart and leak.

4. Overlong tagline or trailing period in tagline. The validator rejects both.

5. Hardcoding blender_version_min = "5.0.0" without reason. Cuts off the LTS user base. Default to "4.5.0" unless you genuinely require a 5.x-only API.

Compatibility paths

For libraries you want to be installable on both 4.5 LTS and 5.1, also keep bl_info for 4.5 fallback even though new submissions to the extensions platform require the manifest. Blender prefers the manifest when both are present.

bl_info = {
    "name": "My Addon",
    "blender": (4, 5, 0),
    "version": (0, 1, 0),
    "category": "Mesh",
}

Related

• See the extension-addon-template template under templates/ for a ready-to-copy version of this layout.
• See the operators, ui-panels, and custom-properties skills for the contents of each submodule.
• See rules target-extensions-platform-format and type-annotate-props-and-defend-context.

References

• Extensions Platform manual: https://docs.blender.org/manual/en/latest/advanced/extensions/index.html
• bpy.utils.register_classes_factory: https://docs.blender.org/api/current/bpy.utils.html
• Manifest reference: https://docs.blender.org/manual/en/latest/advanced/extensions/getting_started.html