diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index 6b6074be0a5728..32b883213685b2 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -292,6 +292,12 @@ Python/jit.c                  @brandtbucher @savannahostrowski @diegorusso
 Tools/jit/                    @brandtbucher @savannahostrowski @diegorusso
 InternalDocs/jit.md           @brandtbucher @savannahostrowski @diegorusso @AA-Turner
 
+# Lazy imports (PEP 810)
+Objects/lazyimportobject.c                  @twouters @DinoV @pablogsal
+Include/internal/pycore_lazyimportobject.h  @twouters @DinoV @pablogsal
+Lib/test/test_import/test_lazy_imports.py   @twouters @DinoV @pablogsal
+Lib/test/test_import/data/lazy_imports/     @twouters @DinoV @pablogsal
+
 # Micro-op / μop / Tier 2 Optimiser
 Python/optimizer.c            @markshannon @Fidget-Spinner
 Python/optimizer_analysis.c   @markshannon @tomasr8 @Fidget-Spinner @savannahostrowski
diff --git a/Doc/c-api/datetime.rst b/Doc/c-api/datetime.rst
index 127d7c9c91a3d5..d7b4e116c49e35 100644
--- a/Doc/c-api/datetime.rst
+++ b/Doc/c-api/datetime.rst
@@ -30,6 +30,10 @@ macros.
 
       This is not compatible with subinterpreters.
 
+   .. versionchanged:: 3.15
+
+      This macro is now thread safe.
+
 .. c:type:: PyDateTime_CAPI
 
    Structure containing the fields for the datetime C API.
@@ -44,6 +48,11 @@ macros.
 
    This variable is only available once :c:macro:`PyDateTime_IMPORT` succeeds.
 
+   .. versionchanged:: 3.15
+
+      This variable should not be accessed directly as direct access is not thread-safe.
+      Use :c:func:`PyDateTime_IMPORT` instead.
+
 .. c:type:: PyDateTime_Date
 
    This subtype of :c:type:`PyObject` represents a Python date object.
diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 0e440ccfd011f0..cd80947924684d 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -1563,6 +1563,8 @@ New features
   thread state.
   (Contributed by Victor Stinner in :gh:`139653`.)
 
+* Add :c:func:`PyUnstable_SetImmortal` C-API function to mark objects as :term:`immortal`.
+  (Contributed by Kumar Aditya in :gh:`143300`.)
 
 Changed C APIs
 --------------
@@ -1571,6 +1573,9 @@ Changed C APIs
   flag is set then :c:macro:`Py_TPFLAGS_HAVE_GC` must be set too.
   (Contributed by Sergey Miryanov in :gh:`134786`.)
 
+* :c:macro:`PyDateTime_IMPORT` is now thread safe. Code that directly checks ``PyDateTimeAPI``
+  for ``NULL`` should be updated to call :c:macro:`PyDateTime_IMPORT` instead.
+  (Contributed by Kumar Aditya in :gh:`141563`.)
 
 Porting to Python 3.15
 ----------------------
diff --git a/Lib/test/test_repl.py b/Lib/test/test_repl.py
index 6cdb1ca65c6aed..40965835bcec00 100644
--- a/Lib/test/test_repl.py
+++ b/Lib/test/test_repl.py
@@ -143,6 +143,22 @@ def test_multiline_string_parsing(self):
         output = kill_python(p)
         self.assertEqual(p.returncode, 0)
 
+    @cpython_only
+    def test_lexer_buffer_realloc_with_null_start(self):
+        # gh-144759: NULL pointer arithmetic in the lexer when start and
+        # multi_line_start are NULL (uninitialized in tok_mode_stack[0])
+        # and the lexer buffer is reallocated while parsing long input.
+        long_value = "a" * 2000
+        user_input = dedent(f"""\
+        x = f'{{{long_value!r}}}'
+        print(x)
+        """)
+        p = spawn_repl()
+        p.stdin.write(user_input)
+        output = kill_python(p)
+        self.assertEqual(p.returncode, 0)
+        self.assertIn(long_value, output)
+
     def test_close_stdin(self):
         user_input = dedent('''
             import os
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-02-13-12-00-00.gh-issue-144759.d3qYpe.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-02-13-12-00-00.gh-issue-144759.d3qYpe.rst
new file mode 100644
index 00000000000000..46786d0672b0a8
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-02-13-12-00-00.gh-issue-144759.d3qYpe.rst
@@ -0,0 +1,4 @@
+Fix undefined behavior in the lexer when ``start`` and ``multi_line_start``
+pointers are ``NULL`` in ``_PyLexer_remember_fstring_buffers()`` and
+``_PyLexer_restore_fstring_buffers()``. The ``NULL`` pointer arithmetic
+(``NULL - valid_pointer``) is now guarded with explicit ``NULL`` checks.
diff --git a/Parser/lexer/buffer.c b/Parser/lexer/buffer.c
index 63aa1ea2ad4f60..e122fd0d9878ea 100644
--- a/Parser/lexer/buffer.c
+++ b/Parser/lexer/buffer.c
@@ -13,8 +13,8 @@ _PyLexer_remember_fstring_buffers(struct tok_state *tok)
 
     for (index = tok->tok_mode_stack_index; index >= 0; --index) {
         mode = &(tok->tok_mode_stack[index]);
-        mode->start_offset = mode->start - tok->buf;
-        mode->multi_line_start_offset = mode->multi_line_start - tok->buf;
+        mode->start_offset = mode->start == NULL ? -1 : mode->start - tok->buf;
+        mode->multi_line_start_offset = mode->multi_line_start == NULL ? -1 : mode->multi_line_start - tok->buf;
     }
 }
 
@@ -27,8 +27,8 @@ _PyLexer_restore_fstring_buffers(struct tok_state *tok)
 
     for (index = tok->tok_mode_stack_index; index >= 0; --index) {
         mode = &(tok->tok_mode_stack[index]);
-        mode->start = tok->buf + mode->start_offset;
-        mode->multi_line_start = tok->buf + mode->multi_line_start_offset;
+        mode->start = mode->start_offset < 0 ? NULL : tok->buf + mode->start_offset;
+        mode->multi_line_start = mode->multi_line_start_offset < 0 ? NULL : tok->buf + mode->multi_line_start_offset;
     }
 }