fix(snippets): bind the action slot on the 4.5 LTS channelbag path

skills/slotted-actions-animation/SKILL.md

@@ -41,7 +41,7 @@ On 5.0+, code that walks `action.fcurves` directly raises `AttributeError` becau
 
 `bpy_extras.anim_utils.action_ensure_channelbag_for_slot(action, slot)` is **new in Blender 5.0**. It ensures the Action has a Layer, a Strip in that Layer, and a Channelbag for the given Slot, then returns the Channelbag. It does **not** exist on 4.4 / 4.5 LTS — there is no auto-detecting shim, and importing-and-calling it on 4.5 raises `AttributeError`.
 
-On 4.4 / 4.5 LTS, ensure the channelbag yourself with `strip.channelbag(slot, ensure=True)` (the slotted model is present there), or just use the still-present legacy `action.fcurves`. So a genuinely cross-version helper must branch on `bpy.app.version`:
+On 4.4 / 4.5 LTS, ensure the channelbag yourself with `strip.channelbag(slot, ensure=True)` (the slotted model is present there). The still-present legacy `action.fcurves` is a trap on a fresh Action: it creates a slot but does not bind it (see the legacy section below). So a genuinely cross-version helper must branch on `bpy.app.version`:
 
 Import path (5.0+) verified against the Blender 5.1 API reference: `bpy_extras.anim_utils.action_ensure_channelbag_for_slot(action, slot)`. See [`bpy_extras.anim_utils`](https://docs.blender.org/api/current/bpy_extras.anim_utils.html).
 
@@ -156,24 +156,29 @@ Notes:
 
 ## The legacy `action.fcurves` path (4.4 / 4.5 LTS only)
 
-On 4.4 and 4.5 LTS the legacy flat API is still present and is the simplest path if you do not need to target 5.x:
+On 4.4 and 4.5 LTS the legacy flat API is still present, but on a fresh Action it does **not** bind a slot. `action.fcurves.new(...)` creates a slot named `Legacy Slot` and leaves `obj.animation_data.action_slot` as `None`, so the keys exist and the object never animates. Bind the slot yourself after the first legacy write:
 
 ```python
 import bpy
 
 obj = bpy.context.active_object
+if obj is None:
+    raise RuntimeError("no active object")
 if obj.animation_data is None:
     obj.animation_data_create()
 if obj.animation_data.action is None:
     obj.animation_data.action = bpy.data.actions.new(name=f"{obj.name}_Action")
 
 action = obj.animation_data.action
 fcurve = action.fcurves.new(data_path="location", index=0)
+if obj.animation_data.action_slot is None:
+    # The legacy write created the slot but did not assign it.
+    obj.animation_data.action_slot = action.slots[0]
 fcurve.keyframe_points.insert(1, 0.0)
 fcurve.keyframe_points.insert(24, 5.0)
 ```
 
-This works on 4.4 / 4.5 LTS, but on **5.0+** the `action.fcurves` attribute was removed entirely, so `action.fcurves.new(...)` raises `AttributeError`. For 5.x, go through the channelbag (`get_channelbag_for_slot` above).
+Measured on 4.5.11 LTS: without the `action_slot` line, `frame_set(24)` leaves the object at its rest value. Prefer the channelbag path (`get_channelbag_for_slot` above), which binds the slot before writing on every version. On **5.0+** the `action.fcurves` attribute was removed entirely, so `action.fcurves.new(...)` raises `AttributeError`.
 
 ## Detecting the version boundaries
 
@@ -204,7 +209,7 @@ The slotted data model (and `action.slots`) is meaningful on 4.4 and 4.5 too, so
    # Missing: setting obj.animation_data.action_slot
    ```
 
-   Without a slot binding, the Action has no idea which datablock it drives. Symptom: animation plays back as if it isn't there.
+   Without a slot binding, the Action has no idea which datablock it drives. Symptom: animation plays back as if it isn't there. On 4.5 LTS the legacy `action.fcurves.new(...)` path hits this silently: it creates a slot but never assigns it.
 
 3. **Hardcoding the import path** to where it was in some older 5.x. Verify against current docs:
    - 5.1+: `from bpy_extras.anim_utils import action_ensure_channelbag_for_slot`
@@ -220,7 +225,7 @@ The slotted data model (and `action.slots`) is meaningful on 4.4 and 4.5 too, so
 | Operation | 4.4 / 4.5 LTS | 5.x | Cross-version |
 | --- | --- | --- | --- |
 | Insert a single keyframe | `obj.keyframe_insert("location", frame=1)` | Same | High-level API works on both |
-| Get the F-curves for an Action+Slot | `strip.channelbag(slot, ensure=True)`, or legacy `action.fcurves` | `action_ensure_channelbag_for_slot(action, slot)` | `get_channelbag_for_slot(action, slot)` (branches on version) |
+| Get the F-curves for an Action+Slot | `strip.channelbag(slot, ensure=True)` (legacy `action.fcurves` only with an explicit `action_slot` bind) | `action_ensure_channelbag_for_slot(action, slot)` | `get_channelbag_for_slot(action, slot)` (branches on version) |
 | Create an Action | `bpy.data.actions.new(...)` | Same | Same |
 | Bind Action to ID | `obj.animation_data.action = action` + `action_slot` | Same | `action_slot` is settable on 4.4+; set both |
 

snippets/action-ensure-channelbag-for-slot.py

@@ -5,10 +5,14 @@
 # in Blender 4.4. Two version paths, branched on bpy.app.version below:
 #   - 5.0+: legacy action.fcurves was removed; use the new
 #     bpy_extras.anim_utils.action_ensure_channelbag_for_slot(action, slot).
-#   - 4.4 / 4.5 LTS: that ensure-helper does NOT exist (no shim). The legacy
-#     action.fcurves API is still present, so use it directly.
+#   - 4.4 / 4.5 LTS: that ensure-helper does NOT exist (no shim). Build the
+#     layer / strip / channelbag path explicitly with strip.channelbag(...).
 #
-# Verified on Blender 4.5.10 LTS and 5.1.1. Import path (5.0+):
+# Bind the slot on BOTH versions. On 4.5, legacy action.fcurves.new() on a
+# slotless Action creates a "Legacy Slot" but never assigns it, so the keys
+# exist and the object never animates (action_slot stays None).
+#
+# Verified on Blender 4.5.11 LTS and 5.2.1 LTS. Import path (5.0+):
 #   from bpy_extras import anim_utils
 #
 # Reference:
@@ -28,19 +32,21 @@ def add_z_keyframe(obj, frame=1, value=0.0):
         action = bpy.data.actions.new(name=f"{obj.name}_Action")
         obj.animation_data.action = action
 
+    slot = obj.animation_data.action_slot
+    if slot is None:
+        slot = action.slots.new(id_type='OBJECT', name=obj.name)
+        obj.animation_data.action_slot = slot
+
     if bpy.app.version >= (5, 0, 0):
-        slot = obj.animation_data.action_slot
-        if slot is None:
-            slot = action.slots.new(id_type='OBJECT', name=obj.name)
-            obj.animation_data.action_slot = slot
         channelbag = anim_utils.action_ensure_channelbag_for_slot(action, slot)
-        fcurve = channelbag.fcurves.find(data_path="location", index=2)
-        if fcurve is None:
-            fcurve = channelbag.fcurves.new(data_path="location", index=2)
     else:
-        fcurve = action.fcurves.find(data_path="location", index=2)
-        if fcurve is None:
-            fcurve = action.fcurves.new(data_path="location", index=2)
+        layer = action.layers[0] if action.layers else action.layers.new("Layer")
+        strip = layer.strips[0] if layer.strips else layer.strips.new(type='KEYFRAME')
+        channelbag = strip.channelbag(slot, ensure=True)
+
+    fcurve = channelbag.fcurves.find(data_path="location", index=2)
+    if fcurve is None:
+        fcurve = channelbag.fcurves.new(data_path="location", index=2)
 
     fcurve.keyframe_points.insert(frame, value)
     fcurve.update()