gh-159303: Document the exception handler context change

Author: drakeo338

Doc/library/asyncio-eventloop.rst

@@ -1493,14 +1493,20 @@ Allows customizing how exceptions are handled in the event loop.
    about context).
 
    If the handler is called on behalf of a :class:`~asyncio.Task` or
-   :class:`~asyncio.Handle`, it is run in the
+   :class:`~asyncio.Handle`, it is run in a copy of the
    :class:`contextvars.Context` of that task or callback handle.
 
    .. versionchanged:: 3.12
 
       The handler may be called in the :class:`~contextvars.Context`
       of the task or handle where the exception originated.
 
+   .. versionchanged:: next
+
+      The handler now runs in a copy of that context. Previously, reporting
+      an exception from inside the task's own context raised
+      :exc:`RuntimeError` and the handler was skipped.
+
 .. method:: loop.get_exception_handler()
 
    Return the current exception handler, or ``None`` if no custom
@@ -1519,6 +1525,13 @@ Allows customizing how exceptions are handled in the event loop.
    *context* parameter has the same meaning as in
    :meth:`call_exception_handler`.
 
+   .. versionchanged:: next
+
+      When no exception handler is set, :meth:`call_exception_handler`
+      runs this method in a copy of the :class:`contextvars.Context` of
+      the task or handle where the exception originated, as for a custom
+      handler.
+
 .. method:: loop.call_exception_handler(context)
 
    Call the current event loop exception handler.

Misc/NEWS.d/next/Library/2026-10-13-12-00-00.gh-issue-159303.Ctx3Dh.rst

@@ -1,5 +1,5 @@
 :meth:`asyncio.loop.call_exception_handler` now runs the default exception
-handler in the context of the task, future or handle that caused the error,
+handler in the context of the task or handle that caused the error,
 as it already did for a custom exception handler. Both handlers now run in a
 copy of that context, so reporting an error from inside the task itself no
 longer fails with :exc:`RuntimeError`.