Blender Agents Code Validator

Blender Code Validator Agent

Systematic validation checklist for reviewing Blender Python code. Run this checklist against any Blender Python code to identify errors, deprecations, and anti-patterns.

Quick Reference: When to Activate

Activate this validator when:

• Reviewing, auditing, or validating Blender Python scripts
• Checking addon/extension code before distribution
• Migrating code between Blender versions (3.x → 4.x → 5.x)
• Investigating runtime crashes, context errors, or stale references
• Generating new Blender Python code (run validator on output)

Validation Checklist

Run each check in order. Each check has a severity level:

• BLOCKER: Code will crash, produce data corruption, or cause undefined behavior
• WARNING: Code has a bug, performance issue, or deprecated usage that will break in a future version
• INFO: Code works but does not follow best practices

CHECK 1: Determine Target Version

Severity if missing: WARNING

Every Blender Python file MUST declare or imply a target Blender version. Without this, validation cannot determine which APIs are valid.

Detection patterns:

1. Look for bpy.app.version checks
2. Look for bl_info["blender"] tuple in addon code
3. Look for blender_manifest.toml in extension code (implies 4.2+)
4. If no version indicator exists → flag WARNING: "No target Blender version declared"

Decision tree:

Has bl_info with "blender" key? → Target = bl_info["blender"] tuple
Has blender_manifest.toml?      → Target = 4.2+
Has bpy.app.version checks?     → Target = checked range
None of the above?               → WARNING: Undeclared target version

CHECK 2: Context Safety

Severity: BLOCKER

Scan for context-related errors — the #1 source of Blender Python crashes.

2.1 Restricted Context Access

Flag BLOCKER if code calls bpy.ops.* or modifies bpy.data.* inside:

• draw() method of any Panel, Header, Menu, UIList, or Gizmo
• Application handler callbacks (frame_change_pre/post, depsgraph_update_post, load_post, render_*)
• Timer callbacks registered via bpy.app.timers.register()
• Draw callbacks registered via SpaceView3D.draw_handler_add()
• Any background thread (threading.Thread, concurrent.futures)

Detection: Identify all function definitions. Check parent class. If parent inherits from bpy.types.Panel/Header/Menu/UIList/Gizmo, flag any bpy.ops call or data mutation inside draw().

2.2 Operator poll() Not Checked

Flag WARNING if code calls bpy.ops.* without verifying prerequisites:

• bpy.ops.object.mode_set(mode='EDIT') without checking context.active_object is not None
• bpy.ops.mesh.* without verifying context.mode == 'EDIT_MESH'
• bpy.ops.object.modifier_apply() without checking object mode and modifier existence
• Any bpy.ops.view3d.* without verifying area type is 'VIEW_3D'

2.3 Context Override Errors (Version-Critical)

Flag BLOCKER if code uses dict-style context overrides on Blender 4.0+:

# BLOCKER on 4.0+: Dict overrides removed
override = bpy.context.copy()
bpy.ops.something(override, ...)

Flag WARNING if code uses dict overrides without version gating (no bpy.app.version check).

Version matrix:

CHECK 3: API Correctness

Severity: BLOCKER for removed APIs, WARNING for deprecated APIs

3.1 Removed API Usage

Scan for APIs removed in each version. Flag BLOCKER if used without version guard:

Removed in 4.0:

• mesh.calc_normals() → normals are auto-calculated
• bone.layers → use bone.collections
• armature.bone_groups → removed
• obj.face_maps → use integer face attributes
• MeshEdge.bevel_weight → use mesh.attributes with float layer
• MeshEdge.crease → use mesh.attributes with float layer
• NodeTree.inputs.new() / NodeTree.outputs.new() → use NodeTree.interface.new_socket()
• Dict-style context overrides → use context.temp_override()

Removed in 5.0:

• bgl module → use gpu module
• del obj["custom_prop"] for RNA properties → use obj.property_unset("prop")
• scene["cycles"] dict access → use scene.cycles attribute access
• image.bindcode → use gpu.texture.from_image()

Removed in 5.1:

• Sequence.frame_final_start → use Sequence.frame_start
• Sequence.frame_final_end → use Sequence.frame_end

3.2 Deprecated API Usage

Flag WARNING for deprecated APIs with known removal schedule:

• bpy.utils.register_module(__name__) → removed in 2.80, register classes individually
• obj.select = True → use obj.select_set(True) (removed in 2.80)
• bpy.data.lamps → use bpy.data.lights (removed in 2.80)
• mesh.uv_textures → use mesh.uv_layers (removed in 2.80)
• bl_info dict in addon → use blender_manifest.toml for Blender 4.2+ extensions

3.3 Incorrect EEVEE Identifier

Flag BLOCKER if EEVEE identifier does not match target version:

CHECK 4: Data Safety

Severity: BLOCKER

4.1 Stale bpy.data References

Flag BLOCKER if code stores bpy.data.* references and uses them after:

• Any bpy.ops.* call (operators can trigger undo)
• Any bpy.data.*.remove() call
• Any bpy.ops.wm.open_mainfile() / bpy.ops.wm.read_homefile()
• Mode switches via bpy.ops.object.mode_set()
• Collection property .add() / .remove() (re-allocates C arrays)

Detection: Track variable assignments from bpy.data.*. If the variable is used after any of the above operations without re-fetching → BLOCKER.

4.2 Name Collision on Data Creation

Flag WARNING if code does:

bpy.data.meshes.new("Name")
mesh = bpy.data.meshes["Name"]  # May be "Name.001"

Correct pattern: mesh = bpy.data.meshes.new("Name") — capture the return value.

4.3 Missing Object-Collection Link

Flag BLOCKER if code creates a data block and object but never calls collection.objects.link(obj):

mesh = bpy.data.meshes.new("Mesh")
obj = bpy.data.objects.new("Object", mesh)
# MISSING: bpy.context.collection.objects.link(obj)

4.4 BMesh Lifecycle Errors

Flag BLOCKER if:

• bm.verts[i] accessed without prior bm.verts.ensure_lookup_table() call
• bmesh.new() used without corresponding bm.free() (except edit-mode BMesh from bmesh.from_edit_mesh())
• bm.to_mesh(mesh) called without mesh.update() afterward

4.5 Missing mesh.update() After from_pydata()

Flag BLOCKER if mesh.from_pydata() is called without mesh.update() afterward.

CHECK 5: Threading Safety

Severity: BLOCKER

5.1 bpy Access from Background Threads

Flag BLOCKER if any bpy.* call occurs inside:

• threading.Thread target functions
• concurrent.futures.ThreadPoolExecutor / ProcessPoolExecutor submitted callables
• Any function passed to thread creation

Correct pattern: Use queue.Queue + bpy.app.timers.register() to relay results to main thread.

5.2 Unsafe Timer Callbacks

Flag WARNING if a timer callback performs heavy computation (loops over large data sets, file I/O). Timers run on the main thread and block the UI.

CHECK 6: Addon/Extension Structure

Severity: WARNING for structural issues, BLOCKER for registration errors

6.1 Registration Order

Flag BLOCKER if register() function:

• Registers operators/panels before their dependent PropertyGroup classes
• Creates PointerProperty referencing a type not yet registered
• Does not delete PointerProperty in unregister() before unregistering the type

Correct order: Register dependencies first (PropertyGroup → Operator → Panel). Unregister in reverse order.

6.2 Handler Cleanup

Flag WARNING if addon registers handlers in register() but does not remove them in unregister().

6.3 Missing @persistent on Handlers

Flag BLOCKER if a handler callback is appended to bpy.app.handlers.* without the @bpy.app.handlers.persistent decorator. Without it, the handler is removed on file load.

6.4 Dynamic EnumProperty Items GC

Flag BLOCKER if EnumProperty(items=callback_function) where the callback returns a freshly created list without caching it in a persistent variable. The list gets garbage-collected → crash or empty dropdown.

6.5 Extension vs Legacy Addon Format

Flag INFO if target is Blender 4.2+ and code uses bl_info dict instead of blender_manifest.toml.

CHECK 7: Operator Best Practices

Severity: WARNING

7.1 Unnecessary bpy.ops Usage

Flag WARNING if bpy.ops.* is used where direct data access achieves the same result:

• bpy.ops.object.location_clear() → obj.location = (0, 0, 0)
• bpy.ops.object.select_all(action='DESELECT') then individual select → obj.select_set()
• bpy.ops.transform.translate(value=...) → obj.location += Vector(...)

7.2 Missing Undo Support in Custom Operators

Flag WARNING if a custom Operator modifies data but does not set bl_options = {'REGISTER', 'UNDO'}.

CHECK 8: GPU / Drawing Code (Blender 5.0+)

Severity: BLOCKER

Flag BLOCKER if code imports bgl on Blender 5.0+. The bgl module is removed.

Flag WARNING if code uses bgl on Blender 4.x without version guard — bgl is deprecated since 3.x.

All drawing code MUST use the gpu module (gpu.shader, gpu.types.GPUBatch, gpu.state).

Severity Summary Decision Tree

Code will crash or corrupt data?
├── YES → BLOCKER
│   ├── Context access in restricted callback
│   ├── bpy access from thread
│   ├── Stale reference after undo/remove/mode-switch
│   ├── Removed API without version guard
│   ├── Missing ensure_lookup_table() on BMesh
│   ├── Missing bm.free() on standalone BMesh
│   ├── Missing collection.objects.link()
│   ├── Wrong registration order (PointerProperty before type)
│   ├── Missing @persistent on handler
│   ├── EnumProperty items GC (no cache)
│   ├── bgl import on Blender 5.0+
│   └── Missing mesh.update() after from_pydata()
├── NO → Code uses deprecated API?
│   ├── YES → WARNING
│   │   ├── Deprecated API with known removal version
│   │   ├── Dict context override on 3.2–3.6
│   │   ├── Operator without poll() pre-check
│   │   ├── Unnecessary bpy.ops usage
│   │   ├── Missing handler cleanup in unregister()
│   │   ├── Heavy computation in timer callback
│   │   └── Missing undo support in operator
│   └── NO → Best practice issue?
│       ├── YES → INFO
│       │   ├── No target version declared
│       │   ├── bl_info on 4.2+ (use blender_manifest.toml)
│       │   └── Style/naming conventions
│       └── NO → PASS

Auto-Fix Patterns

When a check fails, apply these fixes automatically where safe:

Validation Report Format

After running all checks, produce a report:

## Validation Report
Target Version: Blender X.Y
Total Issues: N

### BLOCKERS (N)
- [CHECK X.Y] Description — file:line

### WARNINGS (N)
- [CHECK X.Y] Description — file:line

### INFO (N)
- [CHECK X.Y] Description — file:line

### Auto-Fixes Applied (N)
- Description — file:line

Reference Links

• Validation Rules: references/methods.md
• Before/After Examples: references/examples.md
• Anti-Patterns Catalog: references/anti-patterns.md

Dependency Skills

• blender-core-api — bpy module structure and data access patterns
• blender-core-versions — Version matrix and breaking changes
• blender-core-runtime — Threading, handlers, timers, mathutils
• blender-errors-context — Context error diagnosis and resolution
• blender-errors-data — Data reference errors and safe access patterns
• blender-errors-version — Version compatibility error diagnosis

Official Documentation

• Blender Python API: https://docs.blender.org/api/current/
• Blender Gotchas: https://docs.blender.org/api/current/info_gotcha.html
• Blender 4.0 Release Notes: https://wiki.blender.org/wiki/Reference/Release_Notes/4.0
• Blender 5.0 Release Notes: https://wiki.blender.org/wiki/Reference/Release_Notes/5.0