diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
index a9a7e485dd2bac..ee50effd662f12 100644
--- a/Doc/whatsnew/3.13.rst
+++ b/Doc/whatsnew/3.13.rst
@@ -888,7 +888,7 @@ Experimental JIT Compiler
 =========================
 
 When CPython is configured using the ``--enable-experimental-jit`` option,
-a just-in-time compiler is added which can speed up some Python programs.
+a just-in-time compiler is added which may speed up some Python programs.
 
 The internal architecture is roughly as follows.
 
@@ -905,19 +905,35 @@ The internal architecture is roughly as follows.
   before it is interpreted or translated to machine code.
 
 * There is a Tier 2 interpreter, but it is mostly intended for debugging
-  the earlier stages of the optimization pipeline. If the JIT is not
-  enabled, the Tier 2 interpreter can be invoked by passing Python the
-  ``-X uops`` option or by setting the ``PYTHON_UOPS`` environment
-  variable to ``1``.
+  the earlier stages of the optimization pipeline.
+  The Tier 2 interpreter can be enabled by configuring Python
+  with ``--enable-experimental-jit=interpreter``.
 
-* When the ``--enable-experimental-jit`` option is used, the optimized
+* When the JIT is enabled, the optimized
   Tier 2 IR is translated to machine code, which is then executed.
-  This does not require additional runtime options.
 
 * The machine code translation process uses an architecture called
   *copy-and-patch*. It has no runtime dependencies, but there is a new
   build-time dependency on LLVM.
 
+The ``--enable-experimental-jit`` flag has the following optional values:
+
+* ``no`` (default) -- Disable the entire Tier 2 and JIT pipeline.
+
+* ``yes`` (default if the flag is present without optional value)
+  -- Enable the JIT. To disable the JIT at runtime,
+  pass the environment variable ``PYTHON_JIT=0``.
+
+* ``yes-off`` -- Build the JIT but disable it by default.
+  To enable the JIT at runtime, pass the environment variable
+  ``PYTHON_JIT=1``.
+
+* ``interpreter`` -- Enable the Tier 2 interpreter but disable the JIT.
+  The interpreter can be disabled by running with
+  ``PYTHON_JIT=0``.
+
+(On Windows, use ``PCbuild/build.bat --enable-jit`` to enable the JIT.)
+
 See :pep:`744` for more details.
 
 (JIT by Brandt Bucher, inspired by a paper by Haoran Xu and Fredrik Kjolstad.
diff --git a/Include/internal/pycore_opcode_metadata.h b/Include/internal/pycore_opcode_metadata.h
index 2ccc548ca6c5fd..808badea90d8ed 100644
--- a/Include/internal/pycore_opcode_metadata.h
+++ b/Include/internal/pycore_opcode_metadata.h
@@ -981,7 +981,7 @@ const struct opcode_metadata _PyOpcode_opcode_metadata[268] = {
     [BUILD_SLICE] = { true, INSTR_FMT_IB, HAS_ARG_FLAG | HAS_ERROR_FLAG },
     [BUILD_STRING] = { true, INSTR_FMT_IB, HAS_ARG_FLAG | HAS_ERROR_FLAG },
     [BUILD_TUPLE] = { true, INSTR_FMT_IB, HAS_ARG_FLAG | HAS_ERROR_FLAG },
-    [CACHE] = { true, INSTR_FMT_IX, HAS_ESCAPES_FLAG },
+    [CACHE] = { true, INSTR_FMT_IX, 0 },
     [CALL] = { true, INSTR_FMT_IBC00, HAS_ARG_FLAG | HAS_EVAL_BREAK_FLAG | HAS_ERROR_FLAG | HAS_ERROR_NO_POP_FLAG | HAS_ESCAPES_FLAG },
     [CALL_ALLOC_AND_ENTER_INIT] = { true, INSTR_FMT_IBC00, HAS_ARG_FLAG | HAS_DEOPT_FLAG | HAS_ERROR_FLAG | HAS_ERROR_NO_POP_FLAG | HAS_ESCAPES_FLAG },
     [CALL_BOUND_METHOD_EXACT_ARGS] = { true, INSTR_FMT_IBC00, HAS_ARG_FLAG | HAS_DEOPT_FLAG | HAS_EXIT_FLAG },
@@ -1121,7 +1121,7 @@ const struct opcode_metadata _PyOpcode_opcode_metadata[268] = {
     [PUSH_NULL] = { true, INSTR_FMT_IX, HAS_PURE_FLAG },
     [RAISE_VARARGS] = { true, INSTR_FMT_IB, HAS_ARG_FLAG | HAS_ERROR_FLAG | HAS_ERROR_NO_POP_FLAG | HAS_ESCAPES_FLAG },
     [RERAISE] = { true, INSTR_FMT_IB, HAS_ARG_FLAG | HAS_ERROR_FLAG | HAS_ERROR_NO_POP_FLAG | HAS_ESCAPES_FLAG },
-    [RESERVED] = { true, INSTR_FMT_IX, HAS_ESCAPES_FLAG },
+    [RESERVED] = { true, INSTR_FMT_IX, 0 },
     [RESUME] = { true, INSTR_FMT_IB, HAS_ARG_FLAG | HAS_EVAL_BREAK_FLAG | HAS_ERROR_FLAG | HAS_ESCAPES_FLAG },
     [RESUME_CHECK] = { true, INSTR_FMT_IX, HAS_DEOPT_FLAG },
     [RETURN_CONST] = { true, INSTR_FMT_IB, HAS_ARG_FLAG | HAS_CONST_FLAG },
diff --git a/Include/internal/pycore_uop_metadata.h b/Include/internal/pycore_uop_metadata.h
index a84212c1ec0b69..5697b13d1fc51d 100644
--- a/Include/internal/pycore_uop_metadata.h
+++ b/Include/internal/pycore_uop_metadata.h
@@ -249,7 +249,7 @@ const uint16_t _PyUop_Flags[MAX_UOP_ID+1] = {
     [_COLD_EXIT] = HAS_ARG_FLAG | HAS_ESCAPES_FLAG,
     [_DYNAMIC_EXIT] = HAS_ARG_FLAG | HAS_ESCAPES_FLAG,
     [_START_EXECUTOR] = HAS_DEOPT_FLAG,
-    [_FATAL_ERROR] = HAS_ESCAPES_FLAG,
+    [_FATAL_ERROR] = 0,
     [_CHECK_VALIDITY_AND_SET_IP] = HAS_DEOPT_FLAG,
     [_DEOPT] = 0,
     [_SIDE_EXIT] = 0,
diff --git a/Lib/dis.py b/Lib/dis.py
index 111d624fc259c5..76934eb00e63f0 100644
--- a/Lib/dis.py
+++ b/Lib/dis.py
@@ -216,7 +216,7 @@ def _get_code_array(co, adaptive):
             if op == ENTER_EXECUTOR:
                 try:
                     ex = get_executor(co, i)
-                except ValueError:
+                except (ValueError, RuntimeError):
                     ex = None
 
                 if ex:
diff --git a/Lib/test/support/__init__.py b/Lib/test/support/__init__.py
index 15f654302b1793..52573e665a1273 100644
--- a/Lib/test/support/__init__.py
+++ b/Lib/test/support/__init__.py
@@ -2539,17 +2539,17 @@ def exceeds_recursion_limit():
 # Decorator to disable optimizer while a function run
 def without_optimizer(func):
     try:
-        import _testinternalcapi
+        from _testinternalcapi import get_optimizer, set_optimizer
     except ImportError:
         return func
     @functools.wraps(func)
     def wrapper(*args, **kwargs):
-        save_opt = _testinternalcapi.get_optimizer()
+        save_opt = get_optimizer()
         try:
-            _testinternalcapi.set_optimizer(None)
+            set_optimizer(None)
             return func(*args, **kwargs)
         finally:
-            _testinternalcapi.set_optimizer(save_opt)
+            set_optimizer(save_opt)
     return wrapper
 
 
diff --git a/Lib/test/test_capi/test_opt.py b/Lib/test/test_capi/test_opt.py
index c798b343626677..6e5b626e93291a 100644
--- a/Lib/test/test_capi/test_opt.py
+++ b/Lib/test/test_capi/test_opt.py
@@ -34,6 +34,8 @@ def clear_executors(func):
 
 
 @requires_specialization
+@unittest.skipUnless(hasattr(_testinternalcapi, "get_optimizer"),
+                     "Requires optimizer infrastructure")
 class TestOptimizerAPI(unittest.TestCase):
 
     def test_new_counter_optimizer_dealloc(self):
@@ -136,6 +138,8 @@ def get_opnames(ex):
 
 
 @requires_specialization
+@unittest.skipUnless(hasattr(_testinternalcapi, "get_optimizer"),
+                     "Requires optimizer infrastructure")
 class TestExecutorInvalidation(unittest.TestCase):
 
     def setUp(self):
@@ -215,6 +219,8 @@ def f():
 
 
 @requires_specialization
+@unittest.skipUnless(hasattr(_testinternalcapi, "get_optimizer"),
+                     "Requires optimizer infrastructure")
 @unittest.skipIf(os.getenv("PYTHON_UOPS_OPTIMIZE") == "0", "Needs uop optimizer to run.")
 class TestUops(unittest.TestCase):
 
@@ -579,6 +585,8 @@ def testfunc(n):
 
 
 @requires_specialization
+@unittest.skipUnless(hasattr(_testinternalcapi, "get_optimizer"),
+                     "Requires optimizer infrastructure")
 @unittest.skipIf(os.getenv("PYTHON_UOPS_OPTIMIZE") == "0", "Needs uop optimizer to run.")
 class TestUopsOptimization(unittest.TestCase):
 
diff --git a/Lib/test/test_monitoring.py b/Lib/test/test_monitoring.py
index 11c61bc2e0688d..a9140d4d3dd743 100644
--- a/Lib/test/test_monitoring.py
+++ b/Lib/test/test_monitoring.py
@@ -1831,15 +1831,17 @@ class TestOptimizer(MonitoringTestBase, unittest.TestCase):
 
     def setUp(self):
         _testinternalcapi = import_module("_testinternalcapi")
-        self.old_opt = _testinternalcapi.get_optimizer()
-        opt = _testinternalcapi.new_counter_optimizer()
-        _testinternalcapi.set_optimizer(opt)
+        if hasattr(_testinternalcapi, "get_optimizer"):
+            self.old_opt = _testinternalcapi.get_optimizer()
+            opt = _testinternalcapi.new_counter_optimizer()
+            _testinternalcapi.set_optimizer(opt)
         super(TestOptimizer, self).setUp()
 
     def tearDown(self):
         super(TestOptimizer, self).tearDown()
         import _testinternalcapi
-        _testinternalcapi.set_optimizer(self.old_opt)
+        if hasattr(_testinternalcapi, "get_optimizer"):
+            _testinternalcapi.set_optimizer(self.old_opt)
 
     def test_for_loop(self):
         def test_func(x):
diff --git a/Lib/test/test_opcache.py b/Lib/test/test_opcache.py
index f4e954fd02148d..92a34113bc0383 100644
--- a/Lib/test/test_opcache.py
+++ b/Lib/test/test_opcache.py
@@ -16,6 +16,8 @@
 
 def disabling_optimizer(func):
     def wrapper(*args, **kwargs):
+        if not hasattr(_testinternalcapi, "get_optimizer"):
+            return func(*args, **kwargs)
         old_opt = _testinternalcapi.get_optimizer()
         _testinternalcapi.set_optimizer(None)
         try:
diff --git a/Lib/test/test_optimizer.py b/Lib/test/test_optimizer.py
index 899a4507317334..fac4d1a4ab44c4 100644
--- a/Lib/test/test_optimizer.py
+++ b/Lib/test/test_optimizer.py
@@ -80,6 +80,8 @@ def func(x=0):
 
 class TestOptimizerSymbols(unittest.TestCase):
 
+    @unittest.skipUnless(hasattr(_testinternalcapi, "uop_symbols_test"),
+                "requires _testinternalcapi.uop_symbols_test")
     def test_optimizer_symbols(self):
         _testinternalcapi.uop_symbols_test()
 
diff --git a/Lib/test/test_weakref.py b/Lib/test/test_weakref.py
index df90647baee31e..16da24d7805b56 100644
--- a/Lib/test/test_weakref.py
+++ b/Lib/test/test_weakref.py
@@ -17,6 +17,7 @@
 from test.support import gc_collect
 from test.support import import_helper
 from test.support import threading_helper
+from test.support import is_wasi, Py_DEBUG
 
 # Used in ReferencesTestCase.test_ref_created_during_del() .
 ref_from_del = None
@@ -960,6 +961,7 @@ def test_hashing(self):
         self.assertEqual(hash(a), hash(42))
         self.assertRaises(TypeError, hash, b)
 
+    @unittest.skipIf(is_wasi and Py_DEBUG, "requires deep stack")
     def test_trashcan_16602(self):
         # Issue #16602: when a weakref's target was part of a long
         # deallocation chain, the trashcan mechanism could delay clearing
diff --git a/Misc/NEWS.d/next/Core and Builtins/2024-04-26-14-06-18.gh-issue-118335.SRFsxO.rst b/Misc/NEWS.d/next/Core and Builtins/2024-04-26-14-06-18.gh-issue-118335.SRFsxO.rst
new file mode 100644
index 00000000000000..e13edbbbe528eb
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2024-04-26-14-06-18.gh-issue-118335.SRFsxO.rst	
@@ -0,0 +1,4 @@
+Change how to use the tier 2 interpreter. Instead of running Python with
+``-X uops`` or setting the environment variable ``PYTHON_UOPS=1``, this
+choice is now made at build time by configuring with
+``--enable-experimental-jit=interpreter``.
diff --git a/Modules/_opcode.c b/Modules/_opcode.c
index 5350adb456b859..85e0ffec900e89 100644
--- a/Modules/_opcode.c
+++ b/Modules/_opcode.c
@@ -367,7 +367,13 @@ _opcode_get_executor_impl(PyObject *module, PyObject *code, int offset)
                      Py_TYPE(code)->tp_name);
         return NULL;
     }
+#ifdef _Py_TIER2
     return (PyObject *)PyUnstable_GetExecutor((PyCodeObject *)code, offset);
+#else
+    PyErr_Format(PyExc_RuntimeError,
+                 "Executors are not available in this build");
+    return NULL;
+#endif
 }
 
 static PyMethodDef
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index 99e80baa223e8f..f7952a119f0bd9 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -985,6 +985,8 @@ get_co_framesize(PyObject *self, PyObject *arg)
     return PyLong_FromLong(code->co_framesize);
 }
 
+#ifdef _Py_TIER2
+
 static PyObject *
 new_counter_optimizer(PyObject *self, PyObject *arg)
 {
@@ -1012,7 +1014,10 @@ set_optimizer(PyObject *self, PyObject *opt)
 static PyObject *
 get_optimizer(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
-    PyObject *opt = (PyObject *)PyUnstable_GetOptimizer();
+    PyObject *opt = NULL;
+#ifdef _Py_TIER2
+    opt = (PyObject *)PyUnstable_GetOptimizer();
+#endif
     if (opt == NULL) {
         Py_RETURN_NONE;
     }
@@ -1045,6 +1050,8 @@ invalidate_executors(PyObject *self, PyObject *obj)
     Py_RETURN_NONE;
 }
 
+#endif
+
 static int _pending_callback(void *arg)
 {
     /* we assume the argument is callable object to which we own a reference */
@@ -2020,12 +2027,14 @@ static PyMethodDef module_functions[] = {
     {"iframe_getline", iframe_getline, METH_O, NULL},
     {"iframe_getlasti", iframe_getlasti, METH_O, NULL},
     {"get_co_framesize", get_co_framesize, METH_O, NULL},
+#ifdef _Py_TIER2
     {"get_optimizer", get_optimizer,  METH_NOARGS, NULL},
     {"set_optimizer", set_optimizer,  METH_O, NULL},
     {"new_counter_optimizer", new_counter_optimizer, METH_NOARGS, NULL},
     {"new_uop_optimizer", new_uop_optimizer, METH_NOARGS, NULL},
     {"add_executor_dependency", add_executor_dependency, METH_VARARGS, NULL},
     {"invalidate_executors", invalidate_executors, METH_O, NULL},
+#endif
     {"pending_threadfunc", _PyCFunction_CAST(pending_threadfunc),
      METH_VARARGS | METH_KEYWORDS},
     {"pending_identify", pending_identify, METH_VARARGS, NULL},
@@ -2072,7 +2081,9 @@ static PyMethodDef module_functions[] = {
     {"py_thread_id", get_py_thread_id, METH_NOARGS},
 #endif
     {"set_immortalize_deferred", set_immortalize_deferred, METH_VARARGS},
+#ifdef _Py_TIER2
     {"uop_symbols_test", _Py_uop_symbols_test, METH_NOARGS},
+#endif
     {NULL, NULL} /* sentinel */
 };
 
diff --git a/Objects/codeobject.c b/Objects/codeobject.c
index 014632962bfcf3..605167c5c0bd3a 100644
--- a/Objects/codeobject.c
+++ b/Objects/codeobject.c
@@ -1496,6 +1496,8 @@ PyCode_GetFreevars(PyCodeObject *code)
     return _PyCode_GetFreevars(code);
 }
 
+#ifdef _Py_TIER2
+
 static void
 clear_executors(PyCodeObject *co)
 {
@@ -1515,6 +1517,8 @@ _PyCode_Clear_Executors(PyCodeObject *code)
     clear_executors(code);
 }
 
+#endif
+
 static void
 deopt_code(PyCodeObject *code, _Py_CODEUNIT *instructions)
 {
@@ -1739,9 +1743,11 @@ code_dealloc(PyCodeObject *co)
 
         PyMem_Free(co_extra);
     }
+#ifdef _Py_TIER2
     if (co->co_executors != NULL) {
         clear_executors(co);
     }
+#endif
 
     Py_XDECREF(co->co_consts);
     Py_XDECREF(co->co_names);
diff --git a/Objects/object.c b/Objects/object.c
index 8d856939254080..45310a6c22d677 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -2281,9 +2281,11 @@ static PyTypeObject* static_types[] = {
     &_PyBufferWrapper_Type,
     &_PyContextTokenMissing_Type,
     &_PyCoroWrapper_Type,
+#ifdef _Py_TIER2
     &_PyCounterExecutor_Type,
     &_PyCounterOptimizer_Type,
     &_PyDefaultOptimizer_Type,
+#endif
     &_Py_GenericAliasIterType,
     &_PyHamtItems_Type,
     &_PyHamtKeys_Type,
@@ -2304,8 +2306,10 @@ static PyTypeObject* static_types[] = {
     &_PyPositionsIterator,
     &_PyUnicodeASCIIIter_Type,
     &_PyUnion_Type,
+#ifdef _Py_TIER2
     &_PyUOpExecutor_Type,
     &_PyUOpOptimizer_Type,
+#endif
     &_PyWeakref_CallableProxyType,
     &_PyWeakref_ProxyType,
     &_PyWeakref_RefType,
diff --git a/PCbuild/_testinternalcapi.vcxproj b/PCbuild/_testinternalcapi.vcxproj
index a825cac9138674..d4cd8ad1a46f24 100644
--- a/PCbuild/_testinternalcapi.vcxproj
+++ b/PCbuild/_testinternalcapi.vcxproj
@@ -108,6 +108,12 @@
       <ReferenceOutputAssembly>false</ReferenceOutputAssembly>
     </ProjectReference>
   </ItemGroup>
+  <ItemDefinitionGroup>
+    <ClCompile>
+      <PreprocessorDefinitions Condition="'$(UseJIT)' == 'true'">_Py_JIT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions Condition="'$(UseTIER2)' != '0'">_Py_TIER2=$(UseTIER2);%(PreprocessorDefinitions)</PreprocessorDefinitions>
+    </ClCompile>
+  </ItemDefinitionGroup>
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>
diff --git a/PCbuild/build.bat b/PCbuild/build.bat
index 83b50db4467033..43a99aa684483f 100644
--- a/PCbuild/build.bat
+++ b/PCbuild/build.bat
@@ -36,7 +36,9 @@ echo.                 overrides -c and -d
 echo.  --disable-gil  Enable experimental support for running without the GIL.
 echo.  --test-marker  Enable the test marker within the build.
 echo.  --regen        Regenerate all opcodes, grammar and tokens.
-echo.  --experimental-jit  Enable the experimental just-in-time compiler.
+echo.  --experimental-jit          Enable the experimental just-in-time compiler.
+echo.  --experimental-jit-off      Ditto but off by default (PYTHON_JIT=1 enables).
+echo.  --experimental-interpreter  Enable the experimental Tier 2 interpreter.
 echo.
 echo.Available flags to avoid building certain modules.
 echo.These flags have no effect if '-e' is not given:
@@ -66,6 +68,7 @@ set verbose=/nologo /v:m /clp:summary
 set kill=
 set do_pgo=
 set pgo_job=-m test --pgo
+set UseTIER2=0
 
 :CheckOpts
 if "%~1"=="-h" goto Usage
@@ -86,7 +89,10 @@ if "%~1"=="--disable-gil" (set UseDisableGil=true) & shift & goto CheckOpts
 if "%~1"=="--test-marker" (set UseTestMarker=true) & shift & goto CheckOpts
 if "%~1"=="-V" shift & goto Version
 if "%~1"=="--regen" (set Regen=true) & shift & goto CheckOpts
-if "%~1"=="--experimental-jit" (set UseJIT=true) & shift & goto CheckOpts
+if "%~1"=="--experimental-jit" (set UseJIT=true) & (set UseTIER2=1) & shift & goto CheckOpts
+if "%~1"=="--experimental-jit-off" (set UseJIT=true) & (set UseTIER2=3) & shift & goto CheckOpts
+if "%~1"=="--experimental-interpreter" (set UseTIER2=4) & shift & goto CheckOpts
+if "%~1"=="--experimental-interpreter-off" (set UseTIER2=6) & shift & goto CheckOpts
 rem These use the actual property names used by MSBuild.  We could just let
 rem them in through the environment, but we specify them on the command line
 rem anyway for visibility so set defaults after this
@@ -179,6 +185,7 @@ echo on
  /p:DisableGil=%UseDisableGil%^
  /p:UseTestMarker=%UseTestMarker% %GITProperty%^
  /p:UseJIT=%UseJIT%^
+ /p:UseTIER2=%UseTIER2%^
  %1 %2 %3 %4 %5 %6 %7 %8 %9
 
 @echo off
diff --git a/PCbuild/pythoncore.vcxproj b/PCbuild/pythoncore.vcxproj
index 4cb3e0d3227092..a24667dc74064a 100644
--- a/PCbuild/pythoncore.vcxproj
+++ b/PCbuild/pythoncore.vcxproj
@@ -105,6 +105,7 @@
       <PreprocessorDefinitions>_USRDLL;Py_BUILD_CORE;Py_BUILD_CORE_BUILTIN;Py_ENABLE_SHARED;MS_DLL_ID="$(SysWinVer)";%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <PreprocessorDefinitions Condition="$(IncludeExternals)">_Py_HAVE_ZLIB;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <PreprocessorDefinitions Condition="'$(UseJIT)' == 'true'">_Py_JIT;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions Condition="'$(UseTIER2)' != '0'">_Py_TIER2=$(UseTIER2);%(PreprocessorDefinitions)</PreprocessorDefinitions>
     </ClCompile>
     <Link>
       <AdditionalDependencies>version.lib;ws2_32.lib;pathcch.lib;bcrypt.lib;%(AdditionalDependencies)</AdditionalDependencies>
diff --git a/Python/bytecodes.c b/Python/bytecodes.c
index 18837aef74d78e..28766d6d97cc5f 100644
--- a/Python/bytecodes.c
+++ b/Python/bytecodes.c
@@ -2363,6 +2363,7 @@ dummy_func(
             CHECK_EVAL_BREAKER();
             assert(oparg <= INSTR_OFFSET());
             JUMPBY(-oparg);
+            #ifdef _Py_TIER2
             #if ENABLE_SPECIALIZATION
             _Py_BackoffCounter counter = this_instr[1].counter;
             if (backoff_counter_triggers(counter) && this_instr->op.code == JUMP_BACKWARD) {
@@ -2388,6 +2389,7 @@ dummy_func(
                 ADVANCE_ADAPTIVE_COUNTER(this_instr[1].counter);
             }
             #endif  /* ENABLE_SPECIALIZATION */
+            #endif /* _Py_TIER2 */
         }
 
         pseudo(JUMP) = {
@@ -2401,6 +2403,7 @@ dummy_func(
         };
 
         tier1 inst(ENTER_EXECUTOR, (--)) {
+            #ifdef _Py_TIER2
             int prevoparg = oparg;
             CHECK_EVAL_BREAKER();
             if (this_instr->op.code != ENTER_EXECUTOR ||
@@ -2418,6 +2421,9 @@ dummy_func(
             tstate->previous_executor = Py_None;
             Py_INCREF(executor);
             GOTO_TIER_TWO(executor);
+            #else
+            Py_FatalError("ENTER_EXECUTOR is not supported in this build");
+            #endif /* _Py_TIER2 */
         }
 
         replaced op(_POP_JUMP_IF_FALSE, (cond -- )) {
diff --git a/Python/ceval.c b/Python/ceval.c
index 8b23bc6bcbeb39..59498bc826e941 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -755,7 +755,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
     _Py_CODEUNIT *next_instr;
     PyObject **stack_pointer;
 
-#ifndef _Py_JIT
+#if defined(_Py_TIER2) && !defined(_Py_JIT)
     /* Tier 2 interpreter state */
     _PyExecutorObject *current_executor = NULL;
     const _PyUOpInstruction *next_uop = NULL;
@@ -959,6 +959,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
     goto error;
 
 
+#ifdef _Py_TIER2
 
 // Tier 2 is also here!
 enter_tier_two:
@@ -1113,6 +1114,8 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
 
 #endif  // _Py_JIT
 
+#endif // _Py_TIER2
+
 }
 
 #if defined(__GNUC__)
diff --git a/Python/generated_cases.c.h b/Python/generated_cases.c.h
index 1444f5cdebba4b..602cce4beb1e28 100644
--- a/Python/generated_cases.c.h
+++ b/Python/generated_cases.c.h
@@ -2492,6 +2492,7 @@
             (void)this_instr;
             next_instr += 1;
             INSTRUCTION_STATS(ENTER_EXECUTOR);
+            #ifdef _Py_TIER2
             int prevoparg = oparg;
             CHECK_EVAL_BREAKER();
             if (this_instr->op.code != ENTER_EXECUTOR ||
@@ -2508,6 +2509,9 @@
             tstate->previous_executor = Py_None;
             Py_INCREF(executor);
             GOTO_TIER_TWO(executor);
+            #else
+            Py_FatalError("ENTER_EXECUTOR is not supported in this build");
+            #endif /* _Py_TIER2 */
             DISPATCH();
         }
 
@@ -3432,6 +3436,7 @@
             CHECK_EVAL_BREAKER();
             assert(oparg <= INSTR_OFFSET());
             JUMPBY(-oparg);
+            #ifdef _Py_TIER2
             #if ENABLE_SPECIALIZATION
             _Py_BackoffCounter counter = this_instr[1].counter;
             if (backoff_counter_triggers(counter) && this_instr->op.code == JUMP_BACKWARD) {
@@ -3457,6 +3462,7 @@
                 ADVANCE_ADAPTIVE_COUNTER(this_instr[1].counter);
             }
             #endif  /* ENABLE_SPECIALIZATION */
+            #endif /* _Py_TIER2 */
             DISPATCH();
         }
 
diff --git a/Python/instrumentation.c b/Python/instrumentation.c
index ce97c3add7d0ea..5614f4867a390d 100644
--- a/Python/instrumentation.c
+++ b/Python/instrumentation.c
@@ -1705,10 +1705,12 @@ instrument_lock_held(PyCodeObject *code, PyInterpreterState *interp)
         );
         return 0;
     }
+#ifdef _Py_TIER2
     if (code->co_executors != NULL) {
         _PyCode_Clear_Executors(code);
     }
     _Py_Executors_InvalidateDependency(interp, code, 1);
+#endif
     int code_len = (int)Py_SIZE(code);
     /* Exit early to avoid creating instrumentation
      * data for potential statically allocated code
@@ -1946,7 +1948,9 @@ _PyMonitoring_SetEvents(int tool_id, _PyMonitoringEventSet events)
         goto done;
     }
     set_global_version(tstate, new_version);
+#ifdef _Py_TIER2
     _Py_Executors_InvalidateAll(interp, 1);
+#endif
     res = instrument_all_executing_code_objects(interp);
 done:
     _PyEval_StartTheWorld(interp);
@@ -1986,7 +1990,9 @@ _PyMonitoring_SetLocalEvents(PyCodeObject *code, int tool_id, _PyMonitoringEvent
         code->_co_instrumentation_version -= MONITORING_VERSION_INCREMENT;
     }
 
+#ifdef _Py_TIER2
     _Py_Executors_InvalidateDependency(interp, code, 1);
+#endif
 
     res = instrument_lock_held(code, interp);
 
diff --git a/Python/optimizer.c b/Python/optimizer.c
index a9a35fc902018a..a5e7430c46450e 100644
--- a/Python/optimizer.c
+++ b/Python/optimizer.c
@@ -1,3 +1,5 @@
+#ifdef _Py_TIER2
+
 #include "Python.h"
 #include "opcode.h"
 #include "pycore_interp.h"
@@ -1622,3 +1624,5 @@ _Py_Executors_InvalidateAll(PyInterpreterState *interp, int is_invalidation)
         }
     }
 }
+
+#endif /* _Py_TIER2 */
diff --git a/Python/optimizer_analysis.c b/Python/optimizer_analysis.c
index 9315d7228b5732..842b2e489239af 100644
--- a/Python/optimizer_analysis.c
+++ b/Python/optimizer_analysis.c
@@ -1,3 +1,5 @@
+#ifdef _Py_TIER2
+
 /*
  * This file contains the support code for CPython's uops optimizer.
  * It also performs some simple optimizations.
@@ -603,3 +605,5 @@ _Py_uop_analyze_and_optimize(
     OPT_STAT_INC(optimizer_successes);
     return length;
 }
+
+#endif /* _Py_TIER2 */
diff --git a/Python/optimizer_symbols.c b/Python/optimizer_symbols.c
index 204599b08766c3..d52f490853c006 100644
--- a/Python/optimizer_symbols.c
+++ b/Python/optimizer_symbols.c
@@ -1,3 +1,4 @@
+#ifdef _Py_TIER2
 
 #include "Python.h"
 
@@ -506,3 +507,5 @@ _Py_uop_symbols_test(PyObject *Py_UNUSED(self), PyObject *Py_UNUSED(ignored))
     Py_XDECREF(val_43);
     return NULL;
 }
+
+#endif /* _Py_TIER2 */
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 790398e944f58e..7726ccc979dbcc 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -624,9 +624,11 @@ static int
 builtins_dict_watcher(PyDict_WatchEvent event, PyObject *dict, PyObject *key, PyObject *new_value)
 {
     PyInterpreterState *interp = _PyInterpreterState_GET();
+#ifdef _Py_TIER2
     if (interp->rare_events.builtin_dict < _Py_MAX_ALLOWED_BUILTINS_MODIFICATIONS) {
         _Py_Executors_InvalidateAll(interp, 1);
     }
+#endif
     RARE_EVENT_INTERP_INC(interp, builtin_dict);
     return 0;
 }
@@ -1272,30 +1274,30 @@ init_interp_main(PyThreadState *tstate)
     }
 
     // Turn on experimental tier 2 (uops-based) optimizer
+    // This is also needed when the JIT is enabled
+#ifdef _Py_TIER2
     if (is_main_interp) {
-#ifndef _Py_JIT
-        // No JIT, maybe use the tier two interpreter:
-        char *envvar = Py_GETENV("PYTHON_UOPS");
-        int enabled = envvar != NULL && *envvar > '0';
-        if (_Py_get_xoption(&config->xoptions, L"uops") != NULL) {
-            enabled = 1;
+        int enabled = 1;
+#if _Py_TIER2 & 2
+        enabled = 0;
+#endif
+        char *env = Py_GETENV("PYTHON_JIT");
+        if (env && *env != '\0') {
+            // PYTHON_JIT=0|1 overrides the default
+            enabled = *env != '0';
         }
         if (enabled) {
-#else
-        // Always enable tier two for JIT builds (ignoring the environment
-        // variable and command-line option above):
-        if (true) {
-#endif
             PyObject *opt = PyUnstable_Optimizer_NewUOpOptimizer();
             if (opt == NULL) {
                 return _PyStatus_ERR("can't initialize optimizer");
             }
             if (PyUnstable_SetOptimizer((_PyOptimizerObject *)opt)) {
-                return _PyStatus_ERR("can't initialize optimizer");
+                return _PyStatus_ERR("can't install optimizer");
             }
             Py_DECREF(opt);
         }
     }
+#endif
 
     if (!is_main_interp) {
         // The main interpreter is handled in Py_Main(), for now.
@@ -1655,10 +1657,12 @@ finalize_modules(PyThreadState *tstate)
 {
     PyInterpreterState *interp = tstate->interp;
 
+#ifdef _Py_TIER2
     // Invalidate all executors and turn off tier 2 optimizer
     _Py_Executors_InvalidateAll(interp, 0);
     _PyOptimizerObject *old = _Py_SetOptimizer(interp, NULL);
     Py_XDECREF(old);
+#endif
 
     // Stop watching __builtin__ modifications
     PyDict_Unwatch(0, interp->builtins);
diff --git a/Python/pystate.c b/Python/pystate.c
index 9d7b73b3a071e1..3d6e76e88bd731 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -653,8 +653,10 @@ init_interpreter(PyInterpreterState *interp,
     }
     interp->sys_profile_initialized = false;
     interp->sys_trace_initialized = false;
+#ifdef _Py_TIER2
     (void)_Py_SetOptimizer(interp, NULL);
     interp->executor_list_head = NULL;
+#endif
     if (interp != &runtime->_main_interpreter) {
         /* Fix the self-referential, statically initialized fields. */
         interp->dtoa = (struct _dtoa_state)_dtoa_state_INIT(interp);
@@ -806,9 +808,11 @@ interpreter_clear(PyInterpreterState *interp, PyThreadState *tstate)
         tstate->_status.cleared = 0;
     }
 
+#ifdef _Py_TIER2
     _PyOptimizerObject *old = _Py_SetOptimizer(interp, NULL);
     assert(old != NULL);
     Py_DECREF(old);
+#endif
 
     /* It is possible that any of the objects below have a finalizer
        that runs Python code or otherwise relies on a thread state
@@ -2821,9 +2825,11 @@ _PyInterpreterState_SetEvalFrameFunc(PyInterpreterState *interp,
     if (eval_frame == interp->eval_frame) {
         return;
     }
+#ifdef _Py_TIER2
     if (eval_frame != NULL) {
         _Py_Executors_InvalidateAll(interp, 1);
     }
+#endif
     RARE_EVENT_INC(set_eval_frame_func);
     interp->eval_frame = eval_frame;
 }
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
index 7af363678e8e86..726051521cf574 100644
--- a/Python/sysmodule.c
+++ b/Python/sysmodule.c
@@ -2165,8 +2165,10 @@ static PyObject *
 sys__clear_internal_caches_impl(PyObject *module)
 /*[clinic end generated code: output=0ee128670a4966d6 input=253e741ca744f6e8]*/
 {
+#ifdef _Py_TIER2
     PyInterpreterState *interp = _PyInterpreterState_GET();
     _Py_Executors_InvalidateAll(interp, 0);
+#endif
     PyType_ClearCache();
     Py_RETURN_NONE;
 }
diff --git a/Tools/cases_generator/analyzer.py b/Tools/cases_generator/analyzer.py
index 18cefa08328804..fdb635486b9531 100644
--- a/Tools/cases_generator/analyzer.py
+++ b/Tools/cases_generator/analyzer.py
@@ -411,6 +411,7 @@ def has_error_without_pop(op: parser.InstDef) -> bool:
     "PyCell_New",
     "PyFloat_AS_DOUBLE",
     "_PyFrame_PushUnchecked",
+    "Py_FatalError",
 )
 
 ESCAPING_FUNCTIONS = (
diff --git a/Tools/jit/README.md b/Tools/jit/README.md
index 7b33f99d23f75d..0f5aa9ce656bc9 100644
--- a/Tools/jit/README.md
+++ b/Tools/jit/README.md
@@ -7,18 +7,18 @@ This version of CPython can be built with an experimental just-in-time compiler.
 
 The JIT compiler does not require end users to install any third-party dependencies, but part of it must be *built* using LLVM[^why-llvm]. You are *not* required to build the rest of CPython using LLVM, or even the same version of LLVM (in fact, this is uncommon).
 
-LLVM version 16 is required. Both `clang` and `llvm-readobj` need to be installed and discoverable (version suffixes, like `clang-16`, are okay). It's highly recommended that you also have `llvm-objdump` available, since this allows the build script to dump human-readable assembly for the generated code.
+LLVM version 18 is required. Both `clang` and `llvm-readobj` need to be installed and discoverable (version suffixes, like `clang-18`, are okay). It's highly recommended that you also have `llvm-objdump` available, since this allows the build script to dump human-readable assembly for the generated code.
 
 It's easy to install all of the required tools:
 
 ### Linux
 
-Install LLVM 16 on Ubuntu/Debian:
+Install LLVM 18 on Ubuntu/Debian:
 
 ```sh
 wget https://apt.llvm.org/llvm.sh
 chmod +x llvm.sh
-sudo ./llvm.sh 16
+sudo ./llvm.sh 18
 ```
 
 ### macOS
diff --git a/configure b/configure
index e1d0f36463c44b..01c00d2198548f 100755
--- a/configure
+++ b/configure
@@ -1818,7 +1818,7 @@ Optional Features:
   --disable-gil           enable experimental support for running without the
                           GIL (default is no)
   --enable-pystats        enable internal statistics gathering (default is no)
-  --enable-experimental-jit
+  --enable-experimental-jit[=no|yes|yes-off|interpreter]
                           build the experimental just-in-time compiler
                           (default is no)
   --enable-optimizations  enable expensive, stable optimizations (PGO, etc.)
@@ -8229,11 +8229,19 @@ else $as_nop
   enable_experimental_jit=no
 fi
 
+case $enable_experimental_jit in
+  no) enable_experimental_jit=no ;;
+  yes) enable_experimental_jit="-D_Py_JIT -D_Py_TIER2=1" ;;
+  yes-off) enable_experimental_jit="-D_Py_JIT -D_Py_TIER2=3" ;;
+  interpreter) enable_experimental_jit="-D_Py_TIER2=4" ;;
+  interpreter-off) enable_experimental_jit="-D_Py_TIER2=6" ;;  # Secret option
+  *) as_fn_error $? "invalid argument: --enable-experimental-jit=$enable_experimental_jit; expected no|yes|yes-off|interpreter" "$LINENO" 5 ;;
+esac
 if test "x$enable_experimental_jit" = xno
 then :
 
 else $as_nop
-  as_fn_append CFLAGS_NODIST " -D_Py_JIT"
+  as_fn_append CFLAGS_NODIST " $enable_experimental_jit"
            REGEN_JIT_COMMAND="\$(PYTHON_FOR_REGEN) \$(srcdir)/Tools/jit/build.py $host"
            JIT_STENCILS_H="jit_stencils.h"
            if test "x$Py_DEBUG" = xtrue
diff --git a/configure.ac b/configure.ac
index 7681ea3d3fe3c6..ae65efeac55366 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1768,14 +1768,23 @@ fi
 # Check for --enable-experimental-jit:
 AC_MSG_CHECKING([for --enable-experimental-jit])
 AC_ARG_ENABLE([experimental-jit],
-              [AS_HELP_STRING([--enable-experimental-jit],
+              [AS_HELP_STRING([--enable-experimental-jit@<:@=no|yes|yes-off|interpreter@:>@],
                               [build the experimental just-in-time compiler (default is no)])],
               [],
               [enable_experimental_jit=no])
+case $enable_experimental_jit in
+  no) enable_experimental_jit=no ;;
+  yes) enable_experimental_jit="-D_Py_JIT -D_Py_TIER2=1" ;;
+  yes-off) enable_experimental_jit="-D_Py_JIT -D_Py_TIER2=3" ;;
+  interpreter) enable_experimental_jit="-D_Py_TIER2=4" ;;
+  interpreter-off) enable_experimental_jit="-D_Py_TIER2=6" ;;  # Secret option
+  *) AC_MSG_ERROR(
+      [invalid argument: --enable-experimental-jit=$enable_experimental_jit; expected no|yes|yes-off|interpreter]) ;;
+esac
 AS_VAR_IF([enable_experimental_jit],
           [no],
           [],
-          [AS_VAR_APPEND([CFLAGS_NODIST], [" -D_Py_JIT"])
+          [AS_VAR_APPEND([CFLAGS_NODIST], [" $enable_experimental_jit"])
            AS_VAR_SET([REGEN_JIT_COMMAND],
                       ["\$(PYTHON_FOR_REGEN) \$(srcdir)/Tools/jit/build.py $host"])
            AS_VAR_SET([JIT_STENCILS_H], ["jit_stencils.h"])