diff --git a/autofit/mapper/prior_model/abstract.py b/autofit/mapper/prior_model/abstract.py
index 0f3e16dc9..ba9d0ea99 100644
--- a/autofit/mapper/prior_model/abstract.py
+++ b/autofit/mapper/prior_model/abstract.py
@@ -1,5 +1,4 @@
 import copy
-import functools
 import inspect
 import json
 import logging
@@ -1860,12 +1859,26 @@ def order_no(self) -> str:
         ]
         return ":".join(values)
 
-    @functools.cached_property
+    @property
     def parameterization(self) -> str:
         """
         Describes the path to each of the PriorModels, its class
-        and its number of free parameters
-        """
+        and its number of free parameters.
+
+        Cached on first access in ``self.__dict__`` under the
+        ``_`` -prefixed key ``_parameterization_cache`` so that
+        ``Collection._instance_for_arguments`` and
+        ``ModelInstance.dict`` (which iterate ``__dict__`` and filter
+        underscore-prefixed keys) do not propagate the cached string
+        onto the constructed ``ModelInstance``. A plain
+        ``functools.cached_property`` writes to ``__dict__[name]``
+        without a leading underscore, which would leak the string as
+        a non-array JAX pytree leaf and break ``jax.jit(fit_from)``.
+        """
+        cached = self.__dict__.get("_parameterization_cache")
+        if cached is not None:
+            return cached
+
         from .prior_model import Model
 
         formatter = TextFormatter(line_length=info_whitespace())
@@ -1900,6 +1913,7 @@ def parameterization(self) -> str:
         for group in find_groups(paths, limit=0):
             formatter.add(*group)
 
+        self.__dict__["_parameterization_cache"] = formatter.text
         return formatter.text
 
     @property
diff --git a/autofit/non_linear/fitness.py b/autofit/non_linear/fitness.py
index 9b95d48f5..ce49b81a7 100644
--- a/autofit/non_linear/fitness.py
+++ b/autofit/non_linear/fitness.py
@@ -122,6 +122,12 @@ def __init__(
         self.use_jax_vmap = use_jax_vmap
         self.use_jax_jit = use_jax_jit
 
+        if getattr(self.analysis, "_use_jax", False):
+            from autofit.jax.pytrees import enable_pytrees, register_model
+
+            enable_pytrees()
+            register_model(self.model)
+
         self._call = self.call
 
         if self.use_jax_vmap:
diff --git a/test_autofit/mapper/test_parameterization.py b/test_autofit/mapper/test_parameterization.py
index ec3667a8b..416c07313 100644
--- a/test_autofit/mapper/test_parameterization.py
+++ b/test_autofit/mapper/test_parameterization.py
@@ -141,6 +141,41 @@ def test_tuple_instance_model_info(self, mapper):
         assert len(info.split("\n")) == len(mapper.info.split("\n"))
 
 
+def test_parameterization_cache_does_not_leak_into_instance():
+    """Regression: ``parameterization`` is cached in
+    ``self.__dict__["_parameterization_cache"]`` so that
+    ``Collection._instance_for_arguments`` and ``ModelInstance.dict``
+    (which skip underscore-prefixed keys) do not propagate the cached
+    string onto the constructed instance. A plain
+    ``functools.cached_property`` would write to ``__dict__["parameterization"]``
+    without an underscore, leaking the string into ``ModelInstance.dict``
+    and downstream JAX pytree flattening — see commit 4564ae9a1."""
+
+    model = af.Collection(gaussian=af.Model(af.ex.Gaussian))
+
+    # Touch model.info → exercises the same propagation path that every
+    # workspace script hits at construction time.
+    _ = model.info
+    _ = model.parameterization  # second access uses the cache
+
+    # The cache must live behind an underscore key on the model.
+    assert "_parameterization_cache" in model.__dict__
+    assert "parameterization" not in model.__dict__
+
+    instance = model.instance_from_prior_medians()
+
+    # Neither the cached key nor the public name may appear on the
+    # constructed instance.
+    assert "parameterization" not in instance.__dict__
+    assert "_parameterization_cache" not in instance.__dict__
+    assert "parameterization" not in instance.dict
+    assert "_parameterization_cache" not in instance.dict
+
+    # The instance must yield only model components when iterated.
+    for child in instance:
+        assert not isinstance(child, str)
+
+
 def test_integer_attributes():
     model = af.Model(af.ex.Gaussian)