@@ -1,10 +1,11 @@
+using static System.FormattableString;
+using System;
+using System.Linq;
+using System.Reflection;
+
+
 namespace Python.Runtime.Native
 {
-    using System;
-    using System.Globalization;
-    using System.Linq;
-    using System.Reflection;
-
     static class ABI
     {
         // GIL builds only. FT splits the refcount; Refcount uses Py_REFCNT.
@@ -19,22 +20,28 @@ static class ABI
         internal static void Initialize(Version version)
         {
             IsFreeThreaded = DetectFreeThreaded();
+            if (IsFreeThreaded && version < new Version(3, 14))
+            {
+                // We need Py_REFCNT to be available as a symbol, which is only
+                // present in Python 3.14 and later.
+                throw new NotSupportedException($"Python ABI v{version} is not supported: Free-threaded builds are only supported on Python 3.14 and later.");
+            }
+
             RefCountOffset = IsFreeThreaded ? -1 : ProbeRefCountOffset();
             ObjectHeadOffset = IsFreeThreaded
                 ? (IntPtr.Size == 4 ? 12 : 16)
                 : RefCountOffset;
 
-            string offsetsClassSuffix = string.Format(CultureInfo.InvariantCulture,
-                                                      "{0}{1}", version.Major, version.Minor);
-
             var thisAssembly = Assembly.GetExecutingAssembly();
 
             const string nativeTypeOffsetClassName = "Python.Runtime.NativeTypeOffset";
-            string className = "Python.Runtime.TypeOffset" + offsetsClassSuffix;
+            string className = Invariant($"Python.Runtime.TypeOffset{version.Major}{version.Minor}");
+
             Type nativeOffsetsClass = thisAssembly.GetType(nativeTypeOffsetClassName, throwOnError: false);
             Type typeOffsetsClass =
                 // Try platform native offsets first. It is only present when generated by setup.py
                 nativeOffsetsClass ?? thisAssembly.GetType(className, throwOnError: false);
+
             if (typeOffsetsClass is null)
             {
                 var types = thisAssembly.GetTypes().Select(type => type.Name).Where(name => name.StartsWith("TypeOffset"));
@@ -53,12 +60,30 @@ static bool DetectFreeThreaded()
         {
             // sys._is_gil_enabled() was added in Python 3.13; absent means GIL build.
             using var sys = Runtime.PyImport_ImportModule("sys");
-            if (sys.IsNull()) { Runtime.PyErr_Clear(); return false; }
+
+            if (sys.IsNull())
+            {
+                Runtime.PyErr_Clear();
+                return false;
+            }
+
             using var func = Runtime.PyObject_GetAttrString(sys.Borrow(), "_is_gil_enabled");
-            if (func.IsNull()) { Runtime.PyErr_Clear(); return false; }
+
+            if (func.IsNull())
+            {
+                Runtime.PyErr_Clear();
+                return false;
+            }
+
             using var args = Runtime.PyTuple_New(0);
             using var result = Runtime.PyObject_Call(func.Borrow(), args.Borrow(), default);
-            if (result.IsNull()) { Runtime.PyErr_Clear(); return false; }
+
+            if (result.IsNull())
+            {
+                Runtime.PyErr_Clear();
+                return false;
+            }
+
             return Runtime.PyObject_IsTrue(result.Borrow()) == 0;
         }
 
@@ -71,7 +96,7 @@ static unsafe int ProbeRefCountOffset()
             using var tempObject = Runtime.PyList_New(0);
             IntPtr* tempPtr = (IntPtr*)tempObject.DangerousGetAddress();
             int offset = 0;
-            while(tempPtr[offset] != (IntPtr)1)
+            while (tempPtr[offset] != (IntPtr)1)
             {
                 offset++;
                 if (offset > 100)
 
@@ -5,10 +5,8 @@
 using System.Text;
 using System.Threading;
 using System.Collections.Generic;
-using System.IO;
 using Python.Runtime.Native;
 using System.Linq;
-using static System.FormattableString;
 
 namespace Python.Runtime
 {