Blender Agent: Version Migrator

Systematic migration process for Blender Python code across major versions. This agent skill activates when migrating, porting, updating, or upgrading Blender Python scripts or addons between versions 3.x, 4.x, and 5.x.

Dependencies:

• blender-core-versions — version matrix, detection API, breaking changes
• blender-errors-version — error diagnosis for version-related failures

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1. Quick Reference — Migration Paths

Supported Migration Paths

From	To	Complexity	Key Changes
3.x	4.0	HIGH	Context overrides, mesh attributes, node interface, bone collections
4.0	4.1	MEDIUM	Auto-smooth removal, light probe renames, displacement method move
4.1	4.2	MEDIUM	Extension manifest, EEVEE rename, static IDProperties
4.2	4.3	HIGH	Grease Pencil rewrite, AttributeGroup split, EEVEE property removal
4.3	4.4	MEDIUM	super().init required, Sequence→Strip, Slotted Actions
4.4	4.5	LOW	Minor deprecations only
4.x	5.0	HIGH	BGL removal, compositor change, slotted actions mandatory
5.0	5.1	LOW	Python 3.13, VSE property renames (deprecated)
3.x	5.x	CRITICAL	ALL of the above — execute sequentially

Migration Execution Order

For a 3.x → 5.x migration, ALWAYS apply changes in this exact order:

1. 3.x → 4.0 changes
2. 4.0 → 4.1 changes
3. 4.1 → 4.2 changes
4. 4.2 → 4.3 changes
5. 4.3 → 4.4 changes
6. 4.4 → 5.0 changes
7. 5.0 → 5.1 changes

NEVER skip intermediate versions. Each version removes APIs deprecated in prior versions.

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2. Migration Agent Process

When migrating Blender Python code, follow these steps in order:

Step 1: Identify Source and Target Versions

# Determine current version compatibility from code signals
# Check for these indicators:
# - bl_info dict → 3.x / 4.0 / 4.1 addon
# - blender_manifest.toml → 4.2+ extension
# - import bgl → pre-5.0 code
# - context override dicts on bpy.ops → pre-4.0 code
# - mesh.use_auto_smooth → pre-4.1 code
# - NodeTree.inputs.new() → pre-4.0 code

Step 2: Scan for Affected APIs

Search the codebase for ALL patterns listed in the Find-and-Replace Tables (Section 3). Mark each occurrence with the version transition that affects it.

Step 3: Apply Changes Per Version Transition

Apply changes from the relevant migration checklists below. Process ONE version transition at a time.

Step 4: Update Metadata

• Replace bl_info with blender_manifest.toml if targeting 4.2+
• Update minimum version requirements
• Update any version checks in the code

Step 5: Validate

• Search for ANY remaining references to removed APIs
• Verify all import statements reference available modules
• Check that gpu.state is restored at end of draw callbacks
• Confirm super().__init__(*args, **kwargs) in all Blender type subclasses (4.4+)

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3. Find-and-Replace Tables

3.x → 4.0 Replacements

Find	Replace With	Category
bpy.ops.*.call(override_dict, (dict as first arg)	with bpy.context.temp_override(**kwargs):	Context
mesh.edges[i].bevel_weight	mesh.attributes.get("bevel_weight_edge").data[i].value	Mesh
mesh.edges[i].crease	mesh.attributes.get("crease_edge").data[i].value	Mesh
obj.face_maps	Integer face attributes	Mesh
mesh.calc_normals()	Remove call (auto-calculated)	Mesh
bone.layers[i]	bone.collections	Armature
pose.bone_groups	Bone collections with colors	Armature
NodeTree.inputs.new(type, name)	NodeTree.interface.new_socket(name=name, in_out='INPUT', socket_type=type)	Nodes
NodeTree.outputs.new(type, name)	NodeTree.interface.new_socket(name=name, in_out='OUTPUT', socket_type=type)	Nodes
node.inputs["Subsurface"]	node.inputs["Subsurface Weight"]	Shader
node.inputs["Specular"]	node.inputs["Specular IOR Level"]	Shader
node.inputs["Transmission"]	node.inputs["Transmission Weight"]	Shader
gpu.shader.from_builtin('3D_UNIFORM_COLOR')	gpu.shader.from_builtin('POLYLINE_UNIFORM_COLOR')	GPU
gpu.shader.from_builtin('3D_FLAT_COLOR')	gpu.shader.from_builtin('POLYLINE_FLAT_COLOR')	GPU
bpy.ops.import_scene.obj(	bpy.ops.wm.obj_import(	IO
bpy.ops.export_scene.obj(	bpy.ops.wm.obj_export(	IO
filename= (in IO operators)	filepath=	IO

4.0 → 4.1 Replacements

Find	Replace With	Category
mesh.use_auto_smooth = True	Remove (always active)	Mesh
mesh.auto_smooth_angle	Use "Smooth by Angle" modifier	Mesh
mesh.calc_normals_split()	Remove (use mesh.corner_normals)	Mesh
mesh.create_normals_split()	Remove	Mesh
mesh.free_normals_split()	Remove	Mesh
probe.type == 'CUBEMAP'	probe.type == 'SPHERE'	Light Probe
probe.type == 'PLANAR'	probe.type == 'PLANE'	Light Probe
probe.type == 'GRID'	probe.type == 'VOLUME'	Light Probe
mat.cycles.displacement_method	mat.displacement_method	Material
SUBSAMPLING_3x3	BOX	Sequencer

4.1 → 4.2 Replacements

Find	Replace With	Category
bl_info = { (if targeting 4.2+)	blender_manifest.toml file	Extension
BLENDER_EEVEE (render engine)	BLENDER_EEVEE_NEXT	Render
scene.cycles.motion_blur_position	scene.render.motion_blur_position	Render
scene.eevee.use_motion_blur	scene.render.use_motion_blur	Render
object.load_reference_image	object.empty_image_add	Object

4.2 → 4.3 Replacements

Find	Replace With	Category
bpy.types.AttributeGroup	AttributeGroupMesh / AttributeGroupPointCloud / etc.	Attributes
scene.eevee.use_ssr	Remove (engine-internal)	EEVEE
scene.eevee.use_bloom	Remove (engine-internal)	EEVEE
scene.eevee.* (40+ legacy props)	Remove or check 4.3 release notes	EEVEE
ALL Grease Pencil API calls	Complete rewrite required	Grease Pencil

4.3 → 4.4 Replacements

Find	Replace With	Category
class MyOp(bpy.types.Operator): (no super init)	Add super().__init__(*args, **kwargs)	Classes
bpy.types.Sequence	bpy.types.Strip	VSE
action.fcurves.new( (for new code)	Slotted Actions API	Animation
paint.brush (write access)	Read-only; use brush asset system	Paint

4.x → 5.0 Replacements

Find	Replace With	Category
import bgl	import gpu	Drawing
bgl.glEnable(bgl.GL_BLEND)	gpu.state.blend_set('ALPHA')	Drawing
bgl.glDisable(bgl.GL_BLEND)	gpu.state.blend_set('NONE')	Drawing
bgl.glLineWidth(w)	gpu.state.line_width_set(w)	Drawing
bgl.glEnable(bgl.GL_DEPTH_TEST)	gpu.state.depth_test_set('LESS_EQUAL')	Drawing
bgl.glDisable(bgl.GL_DEPTH_TEST)	gpu.state.depth_test_set('NONE')	Drawing
bgl.glPointSize(s)	gpu.state.point_size_set(s)	Drawing
bgl.glDepthMask(GL_TRUE)	gpu.state.depth_mask_set(True)	Drawing
image.gl_load() / image.bindcode	gpu.texture.from_image(image)	Drawing
gpu.types.GPUShader()	gpu.shader.create_from_info()	Drawing
scene['cycles']	scene.cycles.property_name (attribute access)	Properties
scene.node_tree (compositor)	scene.compositing_node_group	Compositor
scene.use_nodes (compositor)	Remove (always active)	Compositor
brush.sculpt_tool	brush.sculpt_brush_type	Sculpt
strip.end_frame (VSE)	strip.length	VSE
action.fcurves (legacy)	Slotted Actions channelbag API	Animation
BLENDER_EEVEE_NEXT	BLENDER_EEVEE (reverted)	Render

5.0 → 5.1 Replacements

Find	Replace With	Category
sculpt.sample_color	paint.sample_color	Sculpt
frame_final_duration	duration (deprecated, removed in 6.0)	VSE
frame_final_start	left_handle (deprecated, removed in 6.0)	VSE
frame_final_end	right_handle (deprecated, removed in 6.0)	VSE

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4. Decision Tree — Compatibility Shims vs Clean Migration

MIGRATION STRATEGY DECISION
│
├── Target SINGLE Blender version?
│   └── YES → Clean migration: remove ALL legacy code, use target API only
│
├── Target VERSION RANGE (e.g., 3.x + 4.x)?
│   ├── Range spans 3.x and 4.0+ ?
│   │   └── YES → Use version-safe wrappers with bpy.app.version checks
│   ├── Range spans 4.x and 5.0+ ?
│   │   └── YES → Use version-safe wrappers; NEVER import bgl at module level
│   └── Range within single major (e.g., 4.0–4.4)?
│       └── Use hasattr() feature detection for minor version differences
│
└── Target LATEST only (drop legacy)?
    └── Clean migration: target version API only, no compatibility code

When to Use Compatibility Shims

ALWAYS use compatibility shims when:

• The addon targets 2+ major Blender versions simultaneously
• The addon is distributed to users on different Blender versions

NEVER use compatibility shims when:

• The migration targets a single Blender version
• The code is an internal tool pinned to one Blender version

Compatibility Module Pattern

# compat.py — Version-safe compatibility layer
import bpy

BLENDER_4 = bpy.app.version >= (4, 0, 0)
BLENDER_41 = bpy.app.version >= (4, 1, 0)
BLENDER_42 = bpy.app.version >= (4, 2, 0)
BLENDER_43 = bpy.app.version >= (4, 3, 0)
BLENDER_44 = bpy.app.version >= (4, 4, 0)
BLENDER_5 = bpy.app.version >= (5, 0, 0)
BLENDER_51 = bpy.app.version >= (5, 1, 0)

def apply_modifier(obj, modifier_name):
    if BLENDER_4:
        with bpy.context.temp_override(object=obj, active_object=obj):
            bpy.ops.object.modifier_apply(modifier=modifier_name)
    else:
        override = {"object": obj, "active_object": obj}
        bpy.ops.object.modifier_apply(override, modifier=modifier_name)

Conditional Import Pattern for BGL → gpu

# NEVER import bgl at module level in multi-version addons
import gpu

# For addons targeting 3.x–4.x (before 5.0 removal):
try:
    import bgl
    HAS_BGL = True
except ImportError:
    HAS_BGL = False  # Blender 5.0+

# ALWAYS use gpu module for new code regardless of version

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5. EEVEE Identifier Migration

The EEVEE render engine identifier changed across versions. ALWAYS use this lookup:

Blender Version	EEVEE Identifier
3.x	BLENDER_EEVEE
4.0 – 4.1	BLENDER_EEVEE
4.2 – 4.x	BLENDER_EEVEE_NEXT
5.0+	BLENDER_EEVEE

def get_eevee_identifier():
    if bpy.app.version >= (5, 0, 0):
        return 'BLENDER_EEVEE'
    elif bpy.app.version >= (4, 2, 0):
        return 'BLENDER_EEVEE_NEXT'
    else:
        return 'BLENDER_EEVEE'

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6. Extension System Migration (4.2+)

When migrating from legacy addon to extension system:

1. Create blender_manifest.toml in addon root
2. Remove bl_info dict from __init__.py
3. Set blender_version_min = "4.2.0" (or target minimum)
4. Add id matching the package directory name
5. Add tagline (max 64 chars, no trailing punctuation)
6. Add type = "add-on" or type = "theme"
7. Add network permissions if addon accesses internet: [permissions] → network = "Reason for access"
8. Add license = ["SPDX:GPL-3.0-or-later"] (or appropriate SPDX)

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7. Critical Migration Rules

1. NEVER use import bgl in code targeting Blender 5.0+ — the module is REMOVED.
2. ALWAYS restore gpu.state to defaults at the end of every draw callback.
3. NEVER pass a dict as the first argument to bpy.ops in Blender 4.0+ — use context.temp_override().
4. ALWAYS add super().__init__(*args, **kwargs) to Blender type subclasses in 4.4+.
5. NEVER suppress DeprecationWarning — it hides migration signals.
6. NEVER use try/except AttributeError as a version check — use bpy.app.version or hasattr().
7. ALWAYS set POLYLINE_UNIFORM_COLOR shader uniforms: viewportSize and lineWidth.
8. ALWAYS migrate Grease Pencil code completely when targeting 4.3+ — partial migration breaks.
9. NEVER assign embedded IDs to PointerProperty in 4.3+.
10. ALWAYS check the deprecation timeline in blender-errors-version before using any API.

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8. Reference Links

• references/methods.md — Complete API migration mappings organized by version transition
• references/examples.md — Before/after code examples for each migration path
• references/anti-patterns.md — Common migration mistakes with explanations

Official Documentation

• Blender 4.0 Python API Changes
• Blender 4.1 Python API Changes
• Blender 4.2 Python API Changes
• Blender 4.3 Python API Changes
• Blender 4.4 Python API Changes
• Blender 4.5 Python API Changes
• Blender 5.0 Python API Changes
• Blender 5.1 Python API Changes
• Blender API Compatibility Notes
• Grease Pencil 4.3 Migration Guide