diff --git a/Include/internal/pycore_dict.h b/Include/internal/pycore_dict.h
index 74ac8f2148174c..f4c55ca6cf64d2 100644
--- a/Include/internal/pycore_dict.h
+++ b/Include/internal/pycore_dict.h
@@ -347,8 +347,7 @@ PyDictObject *_PyObject_MaterializeManagedDict_LockHeld(PyObject *);
 static inline Py_ssize_t
 _PyDict_UniqueId(PyDictObject *mp)
 {
-    // Offset by one so that _ma_watcher_tag=0 represents an unassigned id
-    return (Py_ssize_t)(mp->_ma_watcher_tag >> DICT_UNIQUE_ID_SHIFT) - 1;
+    return (Py_ssize_t)(mp->_ma_watcher_tag >> DICT_UNIQUE_ID_SHIFT);
 }
 
 static inline void
diff --git a/Include/internal/pycore_object.h b/Include/internal/pycore_object.h
index 322305bc8c664a..19d657070ff221 100644
--- a/Include/internal/pycore_object.h
+++ b/Include/internal/pycore_object.h
@@ -336,20 +336,20 @@ _Py_THREAD_INCREF_OBJECT(PyObject *obj, Py_ssize_t unique_id)
 {
     _PyThreadStateImpl *tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
 
-    // Unsigned comparison so that `unique_id=-1`, which indicates that
-    // per-thread refcounting has been disabled on this object, is handled by
-    // the "else".
-    if ((size_t)unique_id < (size_t)tstate->refcounts.size) {
+    // The table index is `unique_id - 1` because 0 is not a valid unique id.
+    // Unsigned comparison so that `idx=-1` is handled by the "else".
+    size_t idx = (size_t)(unique_id - 1);
+    if (idx < (size_t)tstate->refcounts.size) {
 #  ifdef Py_REF_DEBUG
         _Py_INCREF_IncRefTotal();
 #  endif
         _Py_INCREF_STAT_INC();
-        tstate->refcounts.values[unique_id]++;
+        tstate->refcounts.values[idx]++;
     }
     else {
         // The slow path resizes the per-thread refcount array if necessary.
-        // It handles the unique_id=-1 case to keep the inlinable function smaller.
-        _PyObject_ThreadIncrefSlow(obj, unique_id);
+        // It handles the unique_id=0 case to keep the inlinable function smaller.
+        _PyObject_ThreadIncrefSlow(obj, idx);
     }
 }
 
@@ -386,15 +386,15 @@ _Py_THREAD_DECREF_OBJECT(PyObject *obj, Py_ssize_t unique_id)
 {
     _PyThreadStateImpl *tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
 
-    // Unsigned comparison so that `unique_id=-1`, which indicates that
-    // per-thread refcounting has been disabled on this object, is handled by
-    // the "else".
-    if ((size_t)unique_id < (size_t)tstate->refcounts.size) {
+    // The table index is `unique_id - 1` because 0 is not a valid unique id.
+    // Unsigned comparison so that `idx=-1` is handled by the "else".
+    size_t idx = (size_t)(unique_id - 1);
+    if (idx < (size_t)tstate->refcounts.size) {
 #  ifdef Py_REF_DEBUG
         _Py_DECREF_DecRefTotal();
 #  endif
         _Py_DECREF_STAT_INC();
-        tstate->refcounts.values[unique_id]--;
+        tstate->refcounts.values[idx]--;
     }
     else {
         // Directly decref the object if the id is not assigned or if
diff --git a/Include/internal/pycore_uniqueid.h b/Include/internal/pycore_uniqueid.h
index d3db49ddb78103..9d3c866a704894 100644
--- a/Include/internal/pycore_uniqueid.h
+++ b/Include/internal/pycore_uniqueid.h
@@ -16,7 +16,7 @@ extern "C" {
 // Per-thread reference counting is used along with deferred reference
 // counting to avoid scaling bottlenecks due to reference count contention.
 //
-// An id of -1 is used to indicate that an object doesn't use per-thread
+// An id of 0 is used to indicate that an object doesn't use per-thread
 // refcounting. This value is used when the object is finalized by the GC
 // and during interpreter shutdown to allow the object to be
 // deallocated promptly when the object's refcount reaches zero.
@@ -45,6 +45,8 @@ struct _Py_unique_id_pool {
     Py_ssize_t size;
 };
 
+#define _Py_INVALID_UNIQUE_ID 0
+
 // Assigns the next id from the pool of ids.
 extern Py_ssize_t _PyObject_AssignUniqueId(PyObject *obj);
 
@@ -65,7 +67,7 @@ extern void _PyObject_FinalizePerThreadRefcounts(_PyThreadStateImpl *tstate);
 extern void _PyObject_FinalizeUniqueIdPool(PyInterpreterState *interp);
 
 // Increfs the object, resizing the thread-local refcount array if necessary.
-PyAPI_FUNC(void) _PyObject_ThreadIncrefSlow(PyObject *obj, Py_ssize_t unique_id);
+PyAPI_FUNC(void) _PyObject_ThreadIncrefSlow(PyObject *obj, size_t idx);
 
 #endif   /* Py_GIL_DISABLED */
 
diff --git a/Lib/test/test_capi/test_import.py b/Lib/test/test_capi/test_import.py
new file mode 100644
index 00000000000000..94f96728d9174b
--- /dev/null
+++ b/Lib/test/test_capi/test_import.py
@@ -0,0 +1,327 @@
+import importlib.util
+import os.path
+import sys
+import types
+import unittest
+from test.support import os_helper
+from test.support import import_helper
+from test.support.warnings_helper import check_warnings
+
+_testlimitedcapi = import_helper.import_module('_testlimitedcapi')
+NULL = None
+
+
+class ImportTests(unittest.TestCase):
+    def test_getmagicnumber(self):
+        # Test PyImport_GetMagicNumber()
+        magic = _testlimitedcapi.PyImport_GetMagicNumber()
+        self.assertEqual(magic,
+                         int.from_bytes(importlib.util.MAGIC_NUMBER, 'little'))
+
+    def test_getmagictag(self):
+        # Test PyImport_GetMagicTag()
+        tag = _testlimitedcapi.PyImport_GetMagicTag()
+        self.assertEqual(tag, sys.implementation.cache_tag)
+
+    def test_getmoduledict(self):
+        # Test PyImport_GetModuleDict()
+        modules = _testlimitedcapi.PyImport_GetModuleDict()
+        self.assertIs(modules, sys.modules)
+
+    def check_import_loaded_module(self, import_module):
+        for name in ('os', 'sys', 'test', 'unittest'):
+            with self.subTest(name=name):
+                self.assertIn(name, sys.modules)
+                old_module = sys.modules[name]
+                module = import_module(name)
+                self.assertIsInstance(module, types.ModuleType)
+                self.assertIs(module, old_module)
+
+    def check_import_fresh_module(self, import_module):
+        old_modules = dict(sys.modules)
+        try:
+            for name in ('colorsys', 'math'):
+                with self.subTest(name=name):
+                    sys.modules.pop(name, None)
+                    module = import_module(name)
+                    self.assertIsInstance(module, types.ModuleType)
+                    self.assertIs(module, sys.modules[name])
+                    self.assertEqual(module.__name__, name)
+        finally:
+            sys.modules.clear()
+            sys.modules.update(old_modules)
+
+    def test_getmodule(self):
+        # Test PyImport_GetModule()
+        getmodule = _testlimitedcapi.PyImport_GetModule
+        self.check_import_loaded_module(getmodule)
+
+        nonexistent = 'nonexistent'
+        self.assertNotIn(nonexistent, sys.modules)
+        self.assertIs(getmodule(nonexistent), KeyError)
+        self.assertIs(getmodule(''), KeyError)
+        self.assertIs(getmodule(object()), KeyError)
+
+        self.assertRaises(TypeError, getmodule, [])  # unhashable
+        # CRASHES getmodule(NULL)
+
+    def check_addmodule(self, add_module, accept_nonstr=False):
+        # create a new module
+        names = ['nonexistent']
+        if accept_nonstr:
+            names.append(b'\xff')  # non-UTF-8
+            # PyImport_AddModuleObject() accepts non-string names
+            names.append(tuple(['hashable non-string']))
+        for name in names:
+            with self.subTest(name=name):
+                self.assertNotIn(name, sys.modules)
+                try:
+                    module = add_module(name)
+                    self.assertIsInstance(module, types.ModuleType)
+                    self.assertEqual(module.__name__, name)
+                    self.assertIs(module, sys.modules[name])
+                finally:
+                    sys.modules.pop(name, None)
+
+        # get an existing module
+        self.check_import_loaded_module(add_module)
+
+    def test_addmoduleobject(self):
+        # Test PyImport_AddModuleObject()
+        addmoduleobject = _testlimitedcapi.PyImport_AddModuleObject
+        self.check_addmodule(addmoduleobject, accept_nonstr=True)
+
+        self.assertRaises(TypeError, addmoduleobject, [])  # unhashable
+        # CRASHES addmoduleobject(NULL)
+
+    def test_addmodule(self):
+        # Test PyImport_AddModule()
+        addmodule = _testlimitedcapi.PyImport_AddModule
+        self.check_addmodule(addmodule)
+
+        self.assertRaises(UnicodeDecodeError, addmodule, b'\xff')
+        # CRASHES addmodule(NULL)
+
+    def test_addmoduleref(self):
+        # Test PyImport_AddModuleRef()
+        addmoduleref = _testlimitedcapi.PyImport_AddModuleRef
+        self.check_addmodule(addmoduleref)
+
+        self.assertRaises(UnicodeDecodeError, addmoduleref, b'\xff')
+        # CRASHES addmoduleref(NULL)
+
+    def check_import_func(self, import_module):
+        self.check_import_loaded_module(import_module)
+        self.check_import_fresh_module(import_module)
+        self.assertRaises(ModuleNotFoundError, import_module, 'nonexistent')
+        self.assertRaises(ValueError, import_module, '')
+
+    def test_import(self):
+        # Test PyImport_Import()
+        import_ = _testlimitedcapi.PyImport_Import
+        self.check_import_func(import_)
+
+        self.assertRaises(TypeError, import_, b'os')
+        self.assertRaises(SystemError, import_, NULL)
+
+    def test_importmodule(self):
+        # Test PyImport_ImportModule()
+        importmodule = _testlimitedcapi.PyImport_ImportModule
+        self.check_import_func(importmodule)
+
+        self.assertRaises(UnicodeDecodeError, importmodule, b'\xff')
+        # CRASHES importmodule(NULL)
+
+    def test_importmodulenoblock(self):
+        # Test deprecated PyImport_ImportModuleNoBlock()
+        importmodulenoblock = _testlimitedcapi.PyImport_ImportModuleNoBlock
+        with check_warnings(('', DeprecationWarning)):
+            self.check_import_func(importmodulenoblock)
+            self.assertRaises(UnicodeDecodeError, importmodulenoblock, b'\xff')
+
+        # CRASHES importmodulenoblock(NULL)
+
+    def check_frozen_import(self, import_frozen_module):
+        # Importing a frozen module executes its code, so start by unloading
+        # the module to execute the code in a new (temporary) module.
+        old_zipimport = sys.modules.pop('zipimport')
+        try:
+            self.assertEqual(import_frozen_module('zipimport'), 1)
+
+            # import zipimport again
+            self.assertEqual(import_frozen_module('zipimport'), 1)
+        finally:
+            sys.modules['zipimport'] = old_zipimport
+
+        # not a frozen module
+        self.assertEqual(import_frozen_module('sys'), 0)
+        self.assertEqual(import_frozen_module('nonexistent'), 0)
+        self.assertEqual(import_frozen_module(''), 0)
+
+    def test_importfrozenmodule(self):
+        # Test PyImport_ImportFrozenModule()
+        importfrozenmodule = _testlimitedcapi.PyImport_ImportFrozenModule
+        self.check_frozen_import(importfrozenmodule)
+
+        self.assertRaises(UnicodeDecodeError, importfrozenmodule, b'\xff')
+        # CRASHES importfrozenmodule(NULL)
+
+    def test_importfrozenmoduleobject(self):
+        # Test PyImport_ImportFrozenModuleObject()
+        importfrozenmoduleobject = _testlimitedcapi.PyImport_ImportFrozenModuleObject
+        self.check_frozen_import(importfrozenmoduleobject)
+        self.assertEqual(importfrozenmoduleobject(b'zipimport'), 0)
+        self.assertEqual(importfrozenmoduleobject(NULL), 0)
+
+    def test_importmoduleex(self):
+        # Test PyImport_ImportModuleEx()
+        importmoduleex = _testlimitedcapi.PyImport_ImportModuleEx
+        self.check_import_func(lambda name: importmoduleex(name, NULL, NULL, NULL))
+
+        self.assertRaises(ModuleNotFoundError, importmoduleex, 'nonexistent', NULL, NULL, NULL)
+        self.assertRaises(ValueError, importmoduleex, '', NULL, NULL, NULL)
+        self.assertRaises(UnicodeDecodeError, importmoduleex, b'\xff', NULL, NULL, NULL)
+        # CRASHES importmoduleex(NULL, NULL, NULL, NULL)
+
+    def check_importmodulelevel(self, importmodulelevel):
+        self.check_import_func(lambda name: importmodulelevel(name, NULL, NULL, NULL, 0))
+
+        self.assertRaises(ModuleNotFoundError, importmodulelevel, 'nonexistent', NULL, NULL, NULL, 0)
+        self.assertRaises(ValueError, importmodulelevel, '', NULL, NULL, NULL, 0)
+
+        if __package__:
+            self.assertIs(importmodulelevel('test_import', globals(), NULL, NULL, 1),
+                          sys.modules['test.test_capi.test_import'])
+            self.assertIs(importmodulelevel('test_capi', globals(), NULL, NULL, 2),
+                          sys.modules['test.test_capi'])
+        self.assertRaises(ValueError, importmodulelevel, 'os', NULL, NULL, NULL, -1)
+        with self.assertWarns(ImportWarning):
+            self.assertRaises(KeyError, importmodulelevel, 'test_import', {}, NULL, NULL, 1)
+        self.assertRaises(TypeError, importmodulelevel, 'test_import', [], NULL, NULL, 1)
+
+    def test_importmodulelevel(self):
+        # Test PyImport_ImportModuleLevel()
+        importmodulelevel = _testlimitedcapi.PyImport_ImportModuleLevel
+        self.check_importmodulelevel(importmodulelevel)
+
+        self.assertRaises(UnicodeDecodeError, importmodulelevel, b'\xff', NULL, NULL, NULL, 0)
+        # CRASHES importmodulelevel(NULL, NULL, NULL, NULL, 0)
+
+    def test_importmodulelevelobject(self):
+        # Test PyImport_ImportModuleLevelObject()
+        importmodulelevel = _testlimitedcapi.PyImport_ImportModuleLevelObject
+        self.check_importmodulelevel(importmodulelevel)
+
+        self.assertRaises(TypeError, importmodulelevel, b'os', NULL, NULL, NULL, 0)
+        self.assertRaises(ValueError, importmodulelevel, NULL, NULL, NULL, NULL, 0)
+
+    def check_executecodemodule(self, execute_code, *args):
+        name = 'test_import_executecode'
+        try:
+            # Create a temporary module where the code will be executed
+            self.assertNotIn(name, sys.modules)
+            module = _testlimitedcapi.PyImport_AddModuleRef(name)
+            self.assertNotHasAttr(module, 'attr')
+
+            # Execute the code
+            code = compile('attr = 1', '<test>', 'exec')
+            module2 = execute_code(name, code, *args)
+            self.assertIs(module2, module)
+
+            # Check the function side effects
+            self.assertEqual(module.attr, 1)
+        finally:
+            sys.modules.pop(name, None)
+        return module.__spec__.origin
+
+    def test_executecodemodule(self):
+        # Test PyImport_ExecCodeModule()
+        execcodemodule = _testlimitedcapi.PyImport_ExecCodeModule
+        self.check_executecodemodule(execcodemodule)
+
+        code = compile('attr = 1', '<test>', 'exec')
+        self.assertRaises(UnicodeDecodeError, execcodemodule, b'\xff', code)
+        # CRASHES execcodemodule(NULL, code)
+        # CRASHES execcodemodule(name, NULL)
+
+    def test_executecodemoduleex(self):
+        # Test PyImport_ExecCodeModuleEx()
+        execcodemoduleex = _testlimitedcapi.PyImport_ExecCodeModuleEx
+
+        # Test NULL path (it should not crash)
+        self.check_executecodemodule(execcodemoduleex, NULL)
+
+        # Test non-NULL path
+        pathname = b'pathname'
+        origin = self.check_executecodemodule(execcodemoduleex, pathname)
+        self.assertEqual(origin, os.path.abspath(os.fsdecode(pathname)))
+
+        pathname = os_helper.TESTFN_UNDECODABLE
+        if pathname:
+            origin = self.check_executecodemodule(execcodemoduleex, pathname)
+            self.assertEqual(origin, os.path.abspath(os.fsdecode(pathname)))
+
+        code = compile('attr = 1', '<test>', 'exec')
+        self.assertRaises(UnicodeDecodeError, execcodemoduleex, b'\xff', code, NULL)
+        # CRASHES execcodemoduleex(NULL, code, NULL)
+        # CRASHES execcodemoduleex(name, NULL, NULL)
+
+    def check_executecode_pathnames(self, execute_code_func, object=False):
+        # Test non-NULL pathname and NULL cpathname
+
+        # Test NULL paths (it should not crash)
+        self.check_executecodemodule(execute_code_func, NULL, NULL)
+
+        pathname = 'pathname'
+        origin = self.check_executecodemodule(execute_code_func, pathname, NULL)
+        self.assertEqual(origin, os.path.abspath(os.fsdecode(pathname)))
+        origin = self.check_executecodemodule(execute_code_func, NULL, pathname)
+        if not object:
+            self.assertEqual(origin, os.path.abspath(os.fsdecode(pathname)))
+
+        pathname = os_helper.TESTFN_UNDECODABLE
+        if pathname:
+            if object:
+                pathname = os.fsdecode(pathname)
+            origin = self.check_executecodemodule(execute_code_func, pathname, NULL)
+            self.assertEqual(origin, os.path.abspath(os.fsdecode(pathname)))
+            self.check_executecodemodule(execute_code_func, NULL, pathname)
+
+        # Test NULL pathname and non-NULL cpathname
+        pyc_filename = importlib.util.cache_from_source(__file__)
+        py_filename = importlib.util.source_from_cache(pyc_filename)
+        origin = self.check_executecodemodule(execute_code_func, NULL, pyc_filename)
+        if not object:
+            self.assertEqual(origin, py_filename)
+
+    def test_executecodemodulewithpathnames(self):
+        # Test PyImport_ExecCodeModuleWithPathnames()
+        execute_code_func = _testlimitedcapi.PyImport_ExecCodeModuleWithPathnames
+        self.check_executecode_pathnames(execute_code_func)
+
+        code = compile('attr = 1', '<test>', 'exec')
+        self.assertRaises(UnicodeDecodeError, execute_code_func, b'\xff', code, NULL, NULL)
+        # CRASHES execute_code_func(NULL, code, NULL, NULL)
+        # CRASHES execute_code_func(name, NULL, NULL, NULL)
+
+    def test_executecodemoduleobject(self):
+        # Test PyImport_ExecCodeModuleObject()
+        execute_code_func = _testlimitedcapi.PyImport_ExecCodeModuleObject
+        self.check_executecode_pathnames(execute_code_func, object=True)
+
+        code = compile('attr = 1', '<test>', 'exec')
+        self.assertRaises(TypeError, execute_code_func, [], code, NULL, NULL)
+        nonstring = tuple(['hashable non-string'])
+        self.assertRaises(AttributeError, execute_code_func, nonstring, code, NULL, NULL)
+        sys.modules.pop(nonstring, None)
+        # CRASHES execute_code_func(NULL, code, NULL, NULL)
+        # CRASHES execute_code_func(name, NULL, NULL, NULL)
+
+    # TODO: test PyImport_GetImporter()
+    # TODO: test PyImport_ReloadModule()
+    # TODO: test PyImport_ExtendInittab()
+    # PyImport_AppendInittab() is tested by test_embed
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Lib/test/test_capi/test_type.py b/Lib/test/test_capi/test_type.py
index 92d056e802eeed..ffcaae73bca236 100644
--- a/Lib/test/test_capi/test_type.py
+++ b/Lib/test/test_capi/test_type.py
@@ -67,3 +67,10 @@ class FreezeThis(metaclass=Meta):
             Base.value = 3
         type_freeze(FreezeThis)
         self.assertEqual(FreezeThis.value, 2)
+
+    def test_manual_heap_type(self):
+        # gh-128923: test that a manually allocated and initailized heap type
+        # works correctly
+        ManualHeapType = _testcapi.ManualHeapType
+        for i in range(100):
+            self.assertIsInstance(ManualHeapType(), ManualHeapType)
diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index c2cec6444cb43a..1e706023c795b6 100644
--- a/Lib/test/test_import/__init__.py
+++ b/Lib/test/test_import/__init__.py
@@ -3311,30 +3311,6 @@ def test_basic_multiple_interpreters_reset_each(self):
         #  * module's global state was initialized, not reset
 
 
-@cpython_only
-class CAPITests(unittest.TestCase):
-    def test_pyimport_addmodule(self):
-        # gh-105922: Test PyImport_AddModuleRef(), PyImport_AddModule()
-        # and PyImport_AddModuleObject()
-        _testcapi = import_module("_testcapi")
-        for name in (
-            'sys',     # frozen module
-            'test',    # package
-            __name__,  # package.module
-        ):
-            _testcapi.check_pyimport_addmodule(name)
-
-    def test_pyimport_addmodule_create(self):
-        # gh-105922: Test PyImport_AddModuleRef(), create a new module
-        _testcapi = import_module("_testcapi")
-        name = 'dontexist'
-        self.assertNotIn(name, sys.modules)
-        self.addCleanup(unload, name)
-
-        mod = _testcapi.check_pyimport_addmodule(name)
-        self.assertIs(mod, sys.modules[name])
-
-
 @cpython_only
 class TestMagicNumber(unittest.TestCase):
     def test_magic_number_endianness(self):
diff --git a/Modules/Setup.stdlib.in b/Modules/Setup.stdlib.in
index b7357f41768a2f..6b6a8ae57a5119 100644
--- a/Modules/Setup.stdlib.in
+++ b/Modules/Setup.stdlib.in
@@ -163,7 +163,7 @@
 @MODULE__TESTBUFFER_TRUE@_testbuffer _testbuffer.c
 @MODULE__TESTINTERNALCAPI_TRUE@_testinternalcapi _testinternalcapi.c _testinternalcapi/test_lock.c _testinternalcapi/pytime.c _testinternalcapi/set.c _testinternalcapi/test_critical_sections.c
 @MODULE__TESTCAPI_TRUE@_testcapi _testcapimodule.c _testcapi/vectorcall.c _testcapi/heaptype.c _testcapi/abstract.c _testcapi/unicode.c _testcapi/dict.c _testcapi/set.c _testcapi/list.c _testcapi/tuple.c _testcapi/getargs.c _testcapi/datetime.c _testcapi/docstring.c _testcapi/mem.c _testcapi/watchers.c _testcapi/long.c _testcapi/float.c _testcapi/complex.c _testcapi/numbers.c _testcapi/structmember.c _testcapi/exceptions.c _testcapi/code.c _testcapi/buffer.c _testcapi/pyatomic.c _testcapi/run.c _testcapi/file.c _testcapi/codec.c _testcapi/immortal.c _testcapi/gc.c _testcapi/hash.c _testcapi/time.c _testcapi/bytes.c _testcapi/object.c _testcapi/monitoring.c  _testcapi/config.c
-@MODULE__TESTLIMITEDCAPI_TRUE@_testlimitedcapi _testlimitedcapi.c _testlimitedcapi/abstract.c _testlimitedcapi/bytearray.c _testlimitedcapi/bytes.c _testlimitedcapi/codec.c _testlimitedcapi/complex.c _testlimitedcapi/dict.c _testlimitedcapi/eval.c _testlimitedcapi/float.c _testlimitedcapi/heaptype_relative.c _testlimitedcapi/list.c _testlimitedcapi/long.c _testlimitedcapi/object.c _testlimitedcapi/pyos.c _testlimitedcapi/set.c _testlimitedcapi/sys.c _testlimitedcapi/tuple.c _testlimitedcapi/unicode.c _testlimitedcapi/vectorcall_limited.c _testlimitedcapi/version.c
+@MODULE__TESTLIMITEDCAPI_TRUE@_testlimitedcapi _testlimitedcapi.c _testlimitedcapi/abstract.c _testlimitedcapi/bytearray.c _testlimitedcapi/bytes.c _testlimitedcapi/codec.c _testlimitedcapi/complex.c _testlimitedcapi/dict.c _testlimitedcapi/eval.c _testlimitedcapi/float.c _testlimitedcapi/heaptype_relative.c _testlimitedcapi/import.c _testlimitedcapi/list.c _testlimitedcapi/long.c _testlimitedcapi/object.c _testlimitedcapi/pyos.c _testlimitedcapi/set.c _testlimitedcapi/sys.c _testlimitedcapi/tuple.c _testlimitedcapi/unicode.c _testlimitedcapi/vectorcall_limited.c _testlimitedcapi/version.c
 @MODULE__TESTCLINIC_TRUE@_testclinic _testclinic.c
 @MODULE__TESTCLINIC_LIMITED_TRUE@_testclinic_limited _testclinic_limited.c
 
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 7d304add5999d1..996b96bc000450 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -3059,52 +3059,6 @@ function_set_closure(PyObject *self, PyObject *args)
     Py_RETURN_NONE;
 }
 
-static PyObject *
-check_pyimport_addmodule(PyObject *self, PyObject *args)
-{
-    const char *name;
-    if (!PyArg_ParseTuple(args, "s", &name)) {
-        return NULL;
-    }
-
-    // test PyImport_AddModuleRef()
-    PyObject *module = PyImport_AddModuleRef(name);
-    if (module == NULL) {
-        return NULL;
-    }
-    assert(PyModule_Check(module));
-    // module is a strong reference
-
-    // test PyImport_AddModule()
-    PyObject *module2 = PyImport_AddModule(name);
-    if (module2 == NULL) {
-        goto error;
-    }
-    assert(PyModule_Check(module2));
-    assert(module2 == module);
-    // module2 is a borrowed ref
-
-    // test PyImport_AddModuleObject()
-    PyObject *name_obj = PyUnicode_FromString(name);
-    if (name_obj == NULL) {
-        goto error;
-    }
-    PyObject *module3 = PyImport_AddModuleObject(name_obj);
-    Py_DECREF(name_obj);
-    if (module3 == NULL) {
-        goto error;
-    }
-    assert(PyModule_Check(module3));
-    assert(module3 == module);
-    // module3 is a borrowed ref
-
-    return module;
-
-error:
-    Py_DECREF(module);
-    return NULL;
-}
-
 
 static PyObject *
 test_weakref_capi(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
@@ -3668,7 +3622,6 @@ static PyMethodDef TestMethods[] = {
     {"function_set_kw_defaults", function_set_kw_defaults, METH_VARARGS, NULL},
     {"function_get_closure", function_get_closure, METH_O, NULL},
     {"function_set_closure", function_set_closure, METH_VARARGS, NULL},
-    {"check_pyimport_addmodule", check_pyimport_addmodule, METH_VARARGS},
     {"test_weakref_capi", test_weakref_capi, METH_NOARGS},
     {"function_set_warning", function_set_warning, METH_NOARGS},
     {"test_critical_sections", test_critical_sections, METH_NOARGS},
@@ -4149,6 +4102,61 @@ static PyTypeObject ContainerNoGC_type = {
     .tp_new = ContainerNoGC_new,
 };
 
+/* Manually allocated heap type */
+
+typedef struct {
+    PyObject_HEAD
+    PyObject *dict;
+} ManualHeapType;
+
+static int
+ManualHeapType_traverse(PyObject *self, visitproc visit, void *arg)
+{
+    ManualHeapType *mht = (ManualHeapType *)self;
+    Py_VISIT(mht->dict);
+    return 0;
+}
+
+static void
+ManualHeapType_dealloc(PyObject *self)
+{
+    ManualHeapType *mht = (ManualHeapType *)self;
+    PyObject_GC_UnTrack(self);
+    Py_XDECREF(mht->dict);
+    PyTypeObject *type = Py_TYPE(self);
+    Py_TYPE(self)->tp_free(self);
+    Py_DECREF(type);
+}
+
+static PyObject *
+create_manual_heap_type(void)
+{
+    // gh-128923: Ensure that a heap type allocated through PyType_Type.tp_alloc
+    // with minimal initialization works correctly.
+    PyHeapTypeObject *heap_type = (PyHeapTypeObject *)PyType_Type.tp_alloc(&PyType_Type, 0);
+    if (heap_type == NULL) {
+        return NULL;
+    }
+    PyTypeObject* type = &heap_type->ht_type;
+    type->tp_basicsize = sizeof(ManualHeapType);
+    type->tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HEAPTYPE | Py_TPFLAGS_HAVE_GC;
+    type->tp_new = PyType_GenericNew;
+    type->tp_name = "ManualHeapType";
+    type->tp_dictoffset = offsetof(ManualHeapType, dict);
+    type->tp_traverse = ManualHeapType_traverse;
+    type->tp_dealloc = ManualHeapType_dealloc;
+    heap_type->ht_name = PyUnicode_FromString(type->tp_name);
+    if (!heap_type->ht_name) {
+        Py_DECREF(type);
+        return NULL;
+    }
+    heap_type->ht_qualname = Py_NewRef(heap_type->ht_name);
+    if (PyType_Ready(type) < 0) {
+        Py_DECREF(type);
+        return NULL;
+    }
+    return (PyObject *)type;
+}
 
 static struct PyModuleDef _testcapimodule = {
     PyModuleDef_HEAD_INIT,
@@ -4283,6 +4291,15 @@ PyInit__testcapi(void)
                            (PyObject *) &ContainerNoGC_type) < 0)
         return NULL;
 
+    PyObject *manual_heap_type = create_manual_heap_type();
+    if (manual_heap_type == NULL) {
+        return NULL;
+    }
+    if (PyModule_Add(m, "ManualHeapType", manual_heap_type) < 0) {
+        return NULL;
+    }
+
+
     /* Include tests from the _testcapi/ directory */
     if (_PyTestCapi_Init_Vectorcall(m) < 0) {
         return NULL;
diff --git a/Modules/_testlimitedcapi.c b/Modules/_testlimitedcapi.c
index bcc69a339ec5c4..82dac1c999470f 100644
--- a/Modules/_testlimitedcapi.c
+++ b/Modules/_testlimitedcapi.c
@@ -56,6 +56,9 @@ PyInit__testlimitedcapi(void)
     if (_PyTestLimitedCAPI_Init_HeaptypeRelative(mod) < 0) {
         return NULL;
     }
+    if (_PyTestLimitedCAPI_Init_Import(mod) < 0) {
+        return NULL;
+    }
     if (_PyTestLimitedCAPI_Init_List(mod) < 0) {
         return NULL;
     }
diff --git a/Modules/_testlimitedcapi/import.c b/Modules/_testlimitedcapi/import.c
new file mode 100644
index 00000000000000..3707dbedeea0d9
--- /dev/null
+++ b/Modules/_testlimitedcapi/import.c
@@ -0,0 +1,306 @@
+// Need limited C API version 3.13 for PyImport_AddModuleRef()
+#include "pyconfig.h"   // Py_GIL_DISABLED
+#if !defined(Py_GIL_DISABLED) && !defined(Py_LIMITED_API)
+#  define Py_LIMITED_API 0x030d0000
+#endif
+
+#include "parts.h"
+#include "util.h"
+
+
+/* Test PyImport_GetMagicNumber() */
+static PyObject *
+pyimport_getmagicnumber(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
+{
+    long magic = PyImport_GetMagicNumber();
+    return PyLong_FromLong(magic);
+}
+
+
+/* Test PyImport_GetMagicTag() */
+static PyObject *
+pyimport_getmagictag(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
+{
+    const char *tag = PyImport_GetMagicTag();
+    return PyUnicode_FromString(tag);
+}
+
+
+/* Test PyImport_GetModuleDict() */
+static PyObject *
+pyimport_getmoduledict(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
+{
+    return Py_XNewRef(PyImport_GetModuleDict());
+}
+
+
+/* Test PyImport_GetModule() */
+static PyObject *
+pyimport_getmodule(PyObject *Py_UNUSED(module), PyObject *name)
+{
+    assert(!PyErr_Occurred());
+    NULLABLE(name);
+    PyObject *module = PyImport_GetModule(name);
+    if (module == NULL && !PyErr_Occurred()) {
+        return Py_NewRef(PyExc_KeyError);
+    }
+    return module;
+}
+
+
+/* Test PyImport_AddModuleObject() */
+static PyObject *
+pyimport_addmoduleobject(PyObject *Py_UNUSED(module), PyObject *name)
+{
+    NULLABLE(name);
+    return Py_XNewRef(PyImport_AddModuleObject(name));
+}
+
+
+/* Test PyImport_AddModule() */
+static PyObject *
+pyimport_addmodule(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    if (!PyArg_ParseTuple(args, "z#", &name, &size)) {
+        return NULL;
+    }
+
+    return Py_XNewRef(PyImport_AddModule(name));
+}
+
+
+/* Test PyImport_AddModuleRef() */
+static PyObject *
+pyimport_addmoduleref(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    if (!PyArg_ParseTuple(args, "z#", &name, &size)) {
+        return NULL;
+    }
+
+    return PyImport_AddModuleRef(name);
+}
+
+
+/* Test PyImport_Import() */
+static PyObject *
+pyimport_import(PyObject *Py_UNUSED(module), PyObject *name)
+{
+    NULLABLE(name);
+    return PyImport_Import(name);
+}
+
+
+/* Test PyImport_ImportModule() */
+static PyObject *
+pyimport_importmodule(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    if (!PyArg_ParseTuple(args, "z#", &name, &size)) {
+        return NULL;
+    }
+
+    return PyImport_ImportModule(name);
+}
+
+
+/* Test PyImport_ImportModuleNoBlock() */
+static PyObject *
+pyimport_importmodulenoblock(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    if (!PyArg_ParseTuple(args, "z#", &name, &size)) {
+        return NULL;
+    }
+
+    _Py_COMP_DIAG_PUSH
+    _Py_COMP_DIAG_IGNORE_DEPR_DECLS
+    return PyImport_ImportModuleNoBlock(name);
+    _Py_COMP_DIAG_POP
+}
+
+
+/* Test PyImport_ImportModuleEx() */
+static PyObject *
+pyimport_importmoduleex(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    PyObject *globals, *locals, *fromlist;
+    if (!PyArg_ParseTuple(args, "z#OOO",
+                          &name, &size, &globals, &locals, &fromlist)) {
+        return NULL;
+    }
+    NULLABLE(globals);
+    NULLABLE(locals);
+    NULLABLE(fromlist);
+
+    return PyImport_ImportModuleEx(name, globals, locals, fromlist);
+}
+
+
+/* Test PyImport_ImportModuleLevel() */
+static PyObject *
+pyimport_importmodulelevel(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    PyObject *globals, *locals, *fromlist;
+    int level;
+    if (!PyArg_ParseTuple(args, "z#OOOi",
+                          &name, &size, &globals, &locals, &fromlist, &level)) {
+        return NULL;
+    }
+    NULLABLE(globals);
+    NULLABLE(locals);
+    NULLABLE(fromlist);
+
+    return PyImport_ImportModuleLevel(name, globals, locals, fromlist, level);
+}
+
+
+/* Test PyImport_ImportModuleLevelObject() */
+static PyObject *
+pyimport_importmodulelevelobject(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    PyObject *name, *globals, *locals, *fromlist;
+    int level;
+    if (!PyArg_ParseTuple(args, "OOOOi",
+                          &name, &globals, &locals, &fromlist, &level)) {
+        return NULL;
+    }
+    NULLABLE(name);
+    NULLABLE(globals);
+    NULLABLE(locals);
+    NULLABLE(fromlist);
+
+    return PyImport_ImportModuleLevelObject(name, globals, locals, fromlist, level);
+}
+
+
+/* Test PyImport_ImportFrozenModule() */
+static PyObject *
+pyimport_importfrozenmodule(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    if (!PyArg_ParseTuple(args, "z#", &name, &size)) {
+        return NULL;
+    }
+
+    RETURN_INT(PyImport_ImportFrozenModule(name));
+}
+
+
+/* Test PyImport_ImportFrozenModuleObject() */
+static PyObject *
+pyimport_importfrozenmoduleobject(PyObject *Py_UNUSED(module), PyObject *name)
+{
+    NULLABLE(name);
+    RETURN_INT(PyImport_ImportFrozenModuleObject(name));
+}
+
+
+/* Test PyImport_ExecCodeModule() */
+static PyObject *
+pyimport_executecodemodule(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    PyObject *code;
+    if (!PyArg_ParseTuple(args, "z#O", &name, &size, &code)) {
+        return NULL;
+    }
+    NULLABLE(code);
+
+    return PyImport_ExecCodeModule(name, code);
+}
+
+
+/* Test PyImport_ExecCodeModuleEx() */
+static PyObject *
+pyimport_executecodemoduleex(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    PyObject *code;
+    const char *pathname;
+    if (!PyArg_ParseTuple(args, "z#Oz#", &name, &size, &code, &pathname, &size)) {
+        return NULL;
+    }
+    NULLABLE(code);
+
+    return PyImport_ExecCodeModuleEx(name, code, pathname);
+}
+
+
+/* Test PyImport_ExecCodeModuleWithPathnames() */
+static PyObject *
+pyimport_executecodemodulewithpathnames(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    const char *name;
+    Py_ssize_t size;
+    PyObject *code;
+    const char *pathname;
+    const char *cpathname;
+    if (!PyArg_ParseTuple(args, "z#Oz#z#", &name, &size, &code, &pathname, &size, &cpathname, &size)) {
+        return NULL;
+    }
+    NULLABLE(code);
+
+    return PyImport_ExecCodeModuleWithPathnames(name, code,
+                                                pathname, cpathname);
+}
+
+
+/* Test PyImport_ExecCodeModuleObject() */
+static PyObject *
+pyimport_executecodemoduleobject(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    PyObject *name, *code, *pathname, *cpathname;
+    if (!PyArg_ParseTuple(args, "OOOO", &name, &code, &pathname, &cpathname)) {
+        return NULL;
+    }
+    NULLABLE(name);
+    NULLABLE(code);
+    NULLABLE(pathname);
+    NULLABLE(cpathname);
+
+    return PyImport_ExecCodeModuleObject(name, code, pathname, cpathname);
+}
+
+
+static PyMethodDef test_methods[] = {
+    {"PyImport_GetMagicNumber", pyimport_getmagicnumber, METH_NOARGS},
+    {"PyImport_GetMagicTag", pyimport_getmagictag, METH_NOARGS},
+    {"PyImport_GetModuleDict", pyimport_getmoduledict, METH_NOARGS},
+    {"PyImport_GetModule", pyimport_getmodule, METH_O},
+    {"PyImport_AddModuleObject", pyimport_addmoduleobject, METH_O},
+    {"PyImport_AddModule", pyimport_addmodule, METH_VARARGS},
+    {"PyImport_AddModuleRef", pyimport_addmoduleref, METH_VARARGS},
+    {"PyImport_Import", pyimport_import, METH_O},
+    {"PyImport_ImportModule", pyimport_importmodule, METH_VARARGS},
+    {"PyImport_ImportModuleNoBlock", pyimport_importmodulenoblock, METH_VARARGS},
+    {"PyImport_ImportModuleEx", pyimport_importmoduleex, METH_VARARGS},
+    {"PyImport_ImportModuleLevel", pyimport_importmodulelevel, METH_VARARGS},
+    {"PyImport_ImportModuleLevelObject", pyimport_importmodulelevelobject, METH_VARARGS},
+    {"PyImport_ImportFrozenModule", pyimport_importfrozenmodule, METH_VARARGS},
+    {"PyImport_ImportFrozenModuleObject", pyimport_importfrozenmoduleobject, METH_O},
+    {"PyImport_ExecCodeModule", pyimport_executecodemodule, METH_VARARGS},
+    {"PyImport_ExecCodeModuleEx", pyimport_executecodemoduleex, METH_VARARGS},
+    {"PyImport_ExecCodeModuleWithPathnames", pyimport_executecodemodulewithpathnames, METH_VARARGS},
+    {"PyImport_ExecCodeModuleObject", pyimport_executecodemoduleobject, METH_VARARGS},
+    {NULL},
+};
+
+
+int
+_PyTestLimitedCAPI_Init_Import(PyObject *module)
+{
+    return PyModule_AddFunctions(module, test_methods);
+}
diff --git a/Modules/_testlimitedcapi/parts.h b/Modules/_testlimitedcapi/parts.h
index 56d566b66565a3..9efcd8dcb71e5b 100644
--- a/Modules/_testlimitedcapi/parts.h
+++ b/Modules/_testlimitedcapi/parts.h
@@ -31,6 +31,7 @@ int _PyTestLimitedCAPI_Init_Dict(PyObject *module);
 int _PyTestLimitedCAPI_Init_Eval(PyObject *module);
 int _PyTestLimitedCAPI_Init_Float(PyObject *module);
 int _PyTestLimitedCAPI_Init_HeaptypeRelative(PyObject *module);
+int _PyTestLimitedCAPI_Init_Import(PyObject *module);
 int _PyTestLimitedCAPI_Init_Object(PyObject *module);
 int _PyTestLimitedCAPI_Init_List(PyObject *module);
 int _PyTestLimitedCAPI_Init_Long(PyObject *module);
diff --git a/Objects/codeobject.c b/Objects/codeobject.c
index 15b36a868a47df..539200c97c1206 100644
--- a/Objects/codeobject.c
+++ b/Objects/codeobject.c
@@ -1911,7 +1911,7 @@ code_dealloc(PyObject *self)
     Py_XDECREF(co->co_linetable);
     Py_XDECREF(co->co_exceptiontable);
 #ifdef Py_GIL_DISABLED
-    assert(co->_co_unique_id == -1);
+    assert(co->_co_unique_id == _Py_INVALID_UNIQUE_ID);
 #endif
     if (co->_co_cached != NULL) {
         Py_XDECREF(co->_co_cached->_co_code);
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 82789d5e56f523..504e65b01ca959 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1659,6 +1659,9 @@ _PyDict_EnablePerThreadRefcounting(PyObject *op)
     assert(PyDict_Check(op));
 #ifdef Py_GIL_DISABLED
     Py_ssize_t id = _PyObject_AssignUniqueId(op);
+    if (id == _Py_INVALID_UNIQUE_ID) {
+        return;
+    }
     if ((uint64_t)id >= (uint64_t)DICT_UNIQUE_ID_MAX) {
         _PyObject_ReleaseUniqueId(id);
         return;
@@ -1666,8 +1669,7 @@ _PyDict_EnablePerThreadRefcounting(PyObject *op)
 
     PyDictObject *mp = (PyDictObject *)op;
     assert((mp->_ma_watcher_tag >> DICT_UNIQUE_ID_SHIFT) == 0);
-    // Plus 1 so that _ma_watcher_tag=0 represents an unassigned id
-    mp->_ma_watcher_tag += ((uint64_t)id + 1) << DICT_UNIQUE_ID_SHIFT;
+    mp->_ma_watcher_tag += (uint64_t)id << DICT_UNIQUE_ID_SHIFT;
 #endif
 }
 
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index d8f5f6d9cb2366..93920341a179e8 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -6160,7 +6160,7 @@ type_dealloc(PyObject *self)
     Py_XDECREF(et->ht_module);
     PyMem_Free(et->_ht_tpname);
 #ifdef Py_GIL_DISABLED
-    assert(et->unique_id == -1);
+    assert(et->unique_id == _Py_INVALID_UNIQUE_ID);
 #endif
     et->ht_token = NULL;
     Py_TYPE(type)->tp_free((PyObject *)type);
diff --git a/PCbuild/_testlimitedcapi.vcxproj b/PCbuild/_testlimitedcapi.vcxproj
index 0ea5edba3aa9a7..87abff52493098 100644
--- a/PCbuild/_testlimitedcapi.vcxproj
+++ b/PCbuild/_testlimitedcapi.vcxproj
@@ -103,6 +103,7 @@
     <ClCompile Include="..\Modules\_testlimitedcapi\eval.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\float.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\heaptype_relative.c" />
+    <ClCompile Include="..\Modules\_testlimitedcapi\import.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\list.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\long.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\object.c" />
diff --git a/PCbuild/_testlimitedcapi.vcxproj.filters b/PCbuild/_testlimitedcapi.vcxproj.filters
index b379090eb599f5..a975a508506905 100644
--- a/PCbuild/_testlimitedcapi.vcxproj.filters
+++ b/PCbuild/_testlimitedcapi.vcxproj.filters
@@ -18,6 +18,7 @@
     <ClCompile Include="..\Modules\_testlimitedcapi\eval.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\float.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\heaptype_relative.c" />
+    <ClCompile Include="..\Modules\_testlimitedcapi\import.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\list.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\long.c" />
     <ClCompile Include="..\Modules\_testlimitedcapi\object.c" />
diff --git a/Python/ceval.c b/Python/ceval.c
index e0362c3c89fe6a..28b0b4c6de39a7 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -846,7 +846,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
             _Py_CODEUNIT *bytecode =
                 _PyEval_GetExecutableCode(tstate, _PyFrame_GetCode(frame));
             if (bytecode == NULL) {
-                goto error;
+                goto exit_unwind;
             }
             ptrdiff_t off = frame->instr_ptr - _PyFrame_GetBytecode(frame);
             frame->tlbc_index = ((_PyThreadStateImpl *)tstate)->tlbc_index;
diff --git a/Python/uniqueid.c b/Python/uniqueid.c
index b9f30713feeb57..64c3e6cfbbe825 100644
--- a/Python/uniqueid.c
+++ b/Python/uniqueid.c
@@ -86,7 +86,7 @@ _PyObject_AssignUniqueId(PyObject *obj)
     if (pool->freelist == NULL) {
         if (resize_interp_type_id_pool(pool) < 0) {
             UNLOCK_POOL(pool);
-            return -1;
+            return _Py_INVALID_UNIQUE_ID;
         }
     }
 
@@ -94,7 +94,9 @@ _PyObject_AssignUniqueId(PyObject *obj)
     pool->freelist = entry->next;
     entry->obj = obj;
     _PyObject_SetDeferredRefcount(obj);
-    Py_ssize_t unique_id = (entry - pool->table);
+    // The unique id is one plus the index of the entry in the table.
+    Py_ssize_t unique_id = (entry - pool->table) + 1;
+    assert(unique_id > 0);
     UNLOCK_POOL(pool);
     return unique_id;
 }
@@ -106,8 +108,9 @@ _PyObject_ReleaseUniqueId(Py_ssize_t unique_id)
     struct _Py_unique_id_pool *pool = &interp->unique_ids;
 
     LOCK_POOL(pool);
-    assert(unique_id >= 0 && unique_id < pool->size);
-    _Py_unique_id_entry *entry = &pool->table[unique_id];
+    assert(unique_id > 0 && unique_id <= pool->size);
+    Py_ssize_t idx = unique_id - 1;
+    _Py_unique_id_entry *entry = &pool->table[idx];
     entry->next = pool->freelist;
     pool->freelist = entry;
     UNLOCK_POOL(pool);
@@ -116,18 +119,18 @@ _PyObject_ReleaseUniqueId(Py_ssize_t unique_id)
 static Py_ssize_t
 clear_unique_id(PyObject *obj)
 {
-    Py_ssize_t id = -1;
+    Py_ssize_t id = _Py_INVALID_UNIQUE_ID;
     if (PyType_Check(obj)) {
         if (PyType_HasFeature((PyTypeObject *)obj, Py_TPFLAGS_HEAPTYPE)) {
             PyHeapTypeObject *ht = (PyHeapTypeObject *)obj;
             id = ht->unique_id;
-            ht->unique_id = -1;
+            ht->unique_id = _Py_INVALID_UNIQUE_ID;
         }
     }
     else if (PyCode_Check(obj)) {
         PyCodeObject *co = (PyCodeObject *)obj;
         id = co->_co_unique_id;
-        co->_co_unique_id = -1;
+        co->_co_unique_id = _Py_INVALID_UNIQUE_ID;
     }
     else if (PyDict_Check(obj)) {
         PyDictObject *mp = (PyDictObject *)obj;
@@ -141,23 +144,23 @@ void
 _PyObject_DisablePerThreadRefcounting(PyObject *obj)
 {
     Py_ssize_t id = clear_unique_id(obj);
-    if (id >= 0) {
+    if (id != _Py_INVALID_UNIQUE_ID) {
         _PyObject_ReleaseUniqueId(id);
     }
 }
 
 void
-_PyObject_ThreadIncrefSlow(PyObject *obj, Py_ssize_t unique_id)
+_PyObject_ThreadIncrefSlow(PyObject *obj, size_t idx)
 {
     _PyThreadStateImpl *tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
-    if (unique_id < 0 || resize_local_refcounts(tstate) < 0) {
+    if (((Py_ssize_t)idx) < 0 || resize_local_refcounts(tstate) < 0) {
         // just incref the object directly.
         Py_INCREF(obj);
         return;
     }
 
-    assert(unique_id < tstate->refcounts.size);
-    tstate->refcounts.values[unique_id]++;
+    assert(idx < (size_t)tstate->refcounts.size);
+    tstate->refcounts.values[idx]++;
 #ifdef Py_REF_DEBUG
     _Py_IncRefTotal((PyThreadState *)tstate);
 #endif
@@ -217,7 +220,7 @@ _PyObject_FinalizeUniqueIdPool(PyInterpreterState *interp)
         if (obj != NULL) {
             Py_ssize_t id = clear_unique_id(obj);
             (void)id;
-            assert(id == i);
+            assert(id == i + 1);
         }
     }
     PyMem_Free(pool->table);
diff --git a/Tools/cases_generator/optimizer_generator.py b/Tools/cases_generator/optimizer_generator.py
index d08b621aed552b..2928440fecca0c 100644
--- a/Tools/cases_generator/optimizer_generator.py
+++ b/Tools/cases_generator/optimizer_generator.py
@@ -126,7 +126,7 @@ def write_uop(
     try:
         out.start_line()
         if override:
-            code_list, storage = Storage.for_uop(stack, prototype, extract_bits=False)
+            code_list, storage = Storage.for_uop(stack, prototype)
             for code in code_list:
                 out.emit(code)
         if debug:
@@ -151,11 +151,11 @@ def write_uop(
                 var.defined = False
             storage = emitter.emit_tokens(override, storage, None)
             out.start_line()
-            storage.flush(out, cast_type="_Py_UopsSymbol *", extract_bits=False)
+            storage.flush(out, cast_type="_Py_UopsSymbol *")
         else:
             emit_default(out, uop, stack)
             out.start_line()
-            stack.flush(out, cast_type="_Py_UopsSymbol *", extract_bits=False)
+            stack.flush(out, cast_type="_Py_UopsSymbol *")
     except StackError as ex:
         raise analysis_error(ex.args[0], prototype.body[0]) # from None
 
@@ -198,7 +198,7 @@ def generate_abstract_interpreter(
             declare_variables(override, out, skip_inputs=False)
         else:
             declare_variables(uop, out, skip_inputs=True)
-        stack = Stack()
+        stack = Stack(False)
         write_uop(override, uop, out, stack, debug, skip_inputs=(override is None))
         out.start_line()
         out.emit("break;\n")
diff --git a/Tools/cases_generator/stack.py b/Tools/cases_generator/stack.py
index 286f47d0cfb11b..5121837ed8334b 100644
--- a/Tools/cases_generator/stack.py
+++ b/Tools/cases_generator/stack.py
@@ -224,13 +224,14 @@ def array_or_scalar(var: StackItem | Local) -> str:
     return "array" if var.is_array() else "scalar"
 
 class Stack:
-    def __init__(self) -> None:
+    def __init__(self, extract_bits: bool=True) -> None:
         self.top_offset = StackOffset.empty()
         self.base_offset = StackOffset.empty()
         self.variables: list[Local] = []
         self.defined: set[str] = set()
+        self.extract_bits = extract_bits
 
-    def pop(self, var: StackItem, extract_bits: bool = True) -> tuple[str, Local]:
+    def pop(self, var: StackItem) -> tuple[str, Local]:
         self.top_offset.pop(var)
         indirect = "&" if var.is_array() else ""
         if self.variables:
@@ -272,7 +273,7 @@ def pop(self, var: StackItem, extract_bits: bool = True) -> tuple[str, Local]:
             return "", Local.unused(var)
         self.defined.add(var.name)
         cast = f"({var.type})" if (not indirect and var.type) else ""
-        bits = ".bits" if cast and extract_bits else ""
+        bits = ".bits" if cast and self.extract_bits else ""
         assign = f"{var.name} = {cast}{indirect}stack_pointer[{self.base_offset.to_c()}]{bits};"
         if var.condition:
             if var.condition == "1":
@@ -315,7 +316,7 @@ def _adjust_stack_pointer(self, out: CWriter, number: str) -> None:
             out.emit("assert(WITHIN_STACK_BOUNDS());\n")
 
     def flush(
-        self, out: CWriter, cast_type: str = "uintptr_t", extract_bits: bool = True
+        self, out: CWriter, cast_type: str = "uintptr_t"
     ) -> None:
         out.start_line()
         var_offset = self.base_offset.copy()
@@ -324,7 +325,7 @@ def flush(
                 var.defined and
                 not var.in_memory
             ):
-                Stack._do_emit(out, var.item, var_offset, cast_type, extract_bits)
+                Stack._do_emit(out, var.item, var_offset, cast_type, self.extract_bits)
                 var.in_memory = True
             var_offset.push(var.item)
         number = self.top_offset.to_c()
@@ -346,7 +347,7 @@ def as_comment(self) -> str:
         )
 
     def copy(self) -> "Stack":
-        other = Stack()
+        other = Stack(self.extract_bits)
         other.top_offset = self.top_offset.copy()
         other.base_offset = self.base_offset.copy()
         other.variables = [var.copy() for var in self.variables]
@@ -507,10 +508,10 @@ def locals_cached(self) -> bool:
                 return True
         return False
 
-    def flush(self, out: CWriter, cast_type: str = "uintptr_t", extract_bits: bool = True) -> None:
+    def flush(self, out: CWriter, cast_type: str = "uintptr_t") -> None:
         self.clear_dead_inputs()
         self._push_defined_outputs()
-        self.stack.flush(out, cast_type, extract_bits)
+        self.stack.flush(out, cast_type)
 
     def save(self, out: CWriter) -> None:
         assert self.spilled >= 0
@@ -530,12 +531,12 @@ def reload(self, out: CWriter) -> None:
             out.emit("stack_pointer = _PyFrame_GetStackPointer(frame);\n")
 
     @staticmethod
-    def for_uop(stack: Stack, uop: Uop, extract_bits: bool = True) -> tuple[list[str], "Storage"]:
+    def for_uop(stack: Stack, uop: Uop) -> tuple[list[str], "Storage"]:
         code_list: list[str] = []
         inputs: list[Local] = []
         peeks: list[Local] = []
         for input in reversed(uop.stack.inputs):
-            code, local = stack.pop(input, extract_bits)
+            code, local = stack.pop(input)
             code_list.append(code)
             if input.peek:
                 peeks.append(local)