diff --git a/pytensor/compile/mode.py b/pytensor/compile/mode.py
index 152ad3554d..60d65bbde1 100644
--- a/pytensor/compile/mode.py
+++ b/pytensor/compile/mode.py
@@ -503,46 +503,45 @@ def get_mode(orig_string):
     if not isinstance(string, str):
         return string  # it is hopefully already a mode...
 
+    if string in predefined_modes:
+        return predefined_modes[string]
+
+    if string not in ("Mode", "DebugMode", "DEBUG_MODE", "NanGuardMode"):
+        raise ValueError(f"No predefined mode exist for string: {string}")
+
     global instantiated_default_mode
     # The default mode is cached. However, config.mode can change
     # If instantiated_default_mode has the right class, use it.
+
     if orig_string is None and instantiated_default_mode:
-        if string in predefined_modes:
-            default_mode_class = predefined_modes[string].__class__.__name__
-        else:
-            default_mode_class = string
+        default_mode_class = string
+        # FIXME: This is flawed, we should use proper object comparison.
         if instantiated_default_mode.__class__.__name__ == default_mode_class:
             return instantiated_default_mode
 
-    if string in ("Mode", "DebugMode", "NanGuardMode"):
-        if string == "DebugMode":
-            # need to import later to break circular dependency.
-            from .debugmode import DebugMode
+    if string in ("DebugMode", "DEBUG_MODE"):
+        # need to import later to break circular dependency.
+        from .debugmode import DebugMode
 
-            # DebugMode use its own linker.
-            ret = DebugMode(optimizer=config.optimizer)
-        elif string == "NanGuardMode":
-            # need to import later to break circular dependency.
-            from .nanguardmode import NanGuardMode
+        # DebugMode use its own linker.
+        ret = DebugMode(optimizer=config.optimizer)
+    elif string == "NanGuardMode":
+        # need to import later to break circular dependency.
+        from .nanguardmode import NanGuardMode
 
-            # NanGuardMode use its own linker.
-            ret = NanGuardMode(True, True, True, optimizer=config.optimizer)
-        else:
-            # TODO: Can't we look up the name and invoke it rather than using eval here?
-            ret = eval(string + "(linker=config.linker, optimizer=config.optimizer)")
-    elif string in predefined_modes:
-        ret = predefined_modes[string]
+        # NanGuardMode use its own linker.
+        ret = NanGuardMode(True, True, True, optimizer=config.optimizer)
     else:
-        raise Exception(f"No predefined mode exist for string: {string}")
+        ret = Mode(linker=config.linker, optimizer=config.optimizer)
 
     if orig_string is None:
-        # Build and cache the default mode
         if config.optimizer_excluding:
             ret = ret.excluding(*config.optimizer_excluding.split(":"))
         if config.optimizer_including:
             ret = ret.including(*config.optimizer_including.split(":"))
         if config.optimizer_requiring:
             ret = ret.requiring(*config.optimizer_requiring.split(":"))
+        # Override the cache with the new class mode
         instantiated_default_mode = ret
 
     return ret
diff --git a/pytensor/configdefaults.py b/pytensor/configdefaults.py
index a81fd63905..144b5daef2 100644
--- a/pytensor/configdefaults.py
+++ b/pytensor/configdefaults.py
@@ -388,7 +388,8 @@ def add_compile_configvars():
     config.add(
         "linker",
         "Default linker used if the pytensor flags mode is Mode",
-        EnumStr("cvm", linker_options),
+        # Not mutable because the default mode is cached after the first use.
+        EnumStr("cvm", linker_options, mutable=False),
         in_c_key=False,
     )
 
@@ -411,6 +412,7 @@ def add_compile_configvars():
         EnumStr(
             "o4",
             ["o3", "o2", "o1", "unsafe", "fast_run", "fast_compile", "merge", "None"],
+            mutable=False,  # Not mutable because the default mode is cached after the first use.
         ),
         in_c_key=False,
     )
diff --git a/tests/compile/function/test_types.py b/tests/compile/function/test_types.py
index bef3ae25bf..0990dbeca0 100644
--- a/tests/compile/function/test_types.py
+++ b/tests/compile/function/test_types.py
@@ -1105,14 +1105,10 @@ def test_optimizations_preserved(self):
             ((a.T.T) * (dot(xm, (sm.T.T.T)) + x).T * (x / x) + s),
         )
         old_default_mode = config.mode
-        old_default_opt = config.optimizer
-        old_default_link = config.linker
         try:
             try:
                 str_f = pickle.dumps(f, protocol=-1)
-                config.mode = "Mode"
-                config.linker = "py"
-                config.optimizer = "None"
+                config.mode = "NUMBA"
                 g = pickle.loads(str_f)
                 # print g.maker.mode
                 # print compile.mode.default_mode
@@ -1121,8 +1117,6 @@ def test_optimizations_preserved(self):
                     g = "ok"
         finally:
             config.mode = old_default_mode
-            config.optimizer = old_default_opt
-            config.linker = old_default_link
 
         if g == "ok":
             return
diff --git a/tests/compile/test_mode.py b/tests/compile/test_mode.py
index c965087ea2..291eac0782 100644
--- a/tests/compile/test_mode.py
+++ b/tests/compile/test_mode.py
@@ -13,6 +13,7 @@
 from pytensor.graph.features import NoOutputFromInplace
 from pytensor.graph.rewriting.db import RewriteDatabaseQuery, SequenceDB
 from pytensor.link.basic import LocalLinker
+from pytensor.link.jax import JAXLinker
 from pytensor.tensor.math import dot, tanh
 from pytensor.tensor.type import matrix, vector
 
@@ -142,3 +143,15 @@ class MyLinker(LocalLinker):
     test_mode = Mode(linker=MyLinker())
     with pytest.raises(Exception):
         get_target_language(test_mode)
+
+
+def test_predefined_modes_respected():
+    default_mode = get_default_mode()
+    assert not isinstance(default_mode.linker, JAXLinker)
+
+    with config.change_flags(mode="JAX"):
+        jax_mode = get_default_mode()
+        assert isinstance(jax_mode.linker, JAXLinker)
+
+    default_mode_again = get_default_mode()
+    assert not isinstance(default_mode_again.linker, JAXLinker)