diff --git a/scripts/jax_assertions/pytree_leaf_registration.py b/scripts/jax_assertions/pytree_leaf_registration.py
new file mode 100644
index 0000000..eb9eb3a
--- /dev/null
+++ b/scripts/jax_assertions/pytree_leaf_registration.py
@@ -0,0 +1,78 @@
+"""
+Jax Assertions: Pytree Leaf-Type Registration Guard
+===================================================
+
+Regression guard for PyAutoFit #1365 (2026-07-13 release-validation): JAX-fitting
+a shapelet model crashed in ``jax.vmap(jax.jit(...))`` because ``register_model``
+registered a *leaf type* as a pytree node.
+
+``af.Model(ShapeletPolar)`` auto-wraps its ``int``-typed constructor args
+(``n``/``m``) as ``af.Model(int)`` sub-models. ``register_model._walk`` then
+registered ``builtins.int`` as an instance pytree node, so JAX called the
+instance-flatten ``_partition(5)`` on every ``int`` leaf, and ``vars(5)`` raised
+``TypeError: vars() argument must have __dict__ attribute``.
+
+``int`` (like ``float``/``str``/``bool``) is a native JAX leaf and must never be
+registered. The fix gates registration on ``_instances_carry_dict(cls)``. This
+script reproduces the exact structure without autogalaxy: a model whose
+component carries an ``af.Model(int)`` sub-model.
+
+Previously: N/A (new guard).
+"""
+
+import jax
+
+import autofit as af
+from autofit.jax import pytrees
+
+
+class WithIntArg:
+    """Stand-in for a shapelet-like component: an int arg autofit wraps as Model(int)."""
+
+    def __init__(self, n: int = 2, x: float = 1.0):
+        self.n = n
+        self.x = x
+
+
+"""
+__Model with an af.Model(int) sub-model (mirrors ShapeletPolar n/m)__
+"""
+model = af.Model(
+    WithIntArg, x=af.UniformPrior(lower_limit=0.0, upper_limit=1.0)
+)
+model.n = af.Model(int)
+
+collection = af.Collection(component=model)
+instance = collection.instance_from_prior_medians()
+
+registered = pytrees.register_model(collection)
+assert registered, "register_model returned False — is JAX installed?"
+
+
+"""
+__Leaf types are NOT registered as pytree nodes__
+"""
+for leaf_type in (int, float, str, bool, type(None), tuple):
+    assert leaf_type not in pytrees._REGISTERED_INSTANCE_CLASSES, (
+        f"{leaf_type.__name__} was registered as an instance pytree node — "
+        "native JAX leaves must never be registered (the flatten does vars())."
+    )
+
+# The real component class (has a __dict__) still IS registered.
+assert WithIntArg in pytrees._REGISTERED_INSTANCE_CLASSES, (
+    "WithIntArg should be registered — its instances carry a __dict__."
+)
+
+
+"""
+__tree_flatten / tree_unflatten round-trips (the crash site)__
+"""
+leaves, treedef = jax.tree_util.tree_flatten(instance)
+rebuilt = jax.tree_util.tree_unflatten(treedef, leaves)
+
+assert isinstance(rebuilt, af.ModelInstance)
+assert isinstance(rebuilt.component, WithIntArg)
+assert rebuilt.component.n == instance.component.n
+
+
+print("pytree_leaf_registration: all assertions passed")