diff --git a/packages/reflex-base/news/6801.bugfix.md b/packages/reflex-base/news/6801.bugfix.md
new file mode 100644
index 00000000000..41bc88f9f0b
--- /dev/null
+++ b/packages/reflex-base/news/6801.bugfix.md
@@ -0,0 +1 @@
+Enqueuing an event chained from a parent event that has already finished no longer raises `RuntimeError: Cannot add a child to an EventFuture that is already done.`. Such a late-chained event skips registration under the completed parent and is processed normally.
diff --git a/packages/reflex-base/src/reflex_base/event/processor/event_processor.py b/packages/reflex-base/src/reflex_base/event/processor/event_processor.py
index fcdc90eade5..8afc7b2fb54 100644
--- a/packages/reflex-base/src/reflex_base/event/processor/event_processor.py
+++ b/packages/reflex-base/src/reflex_base/event/processor/event_processor.py
@@ -423,7 +423,10 @@ async def enqueue(
                 # tracker since this event will never enter the queue.
                 tracked.cancel()
                 return tracked
-            parent_future.add_child(tracked)
+            # Skip registration if the parent is already done (late-chained
+            # event) so the child runs instead of crashing.
+            if not parent_future.done():
+                parent_future.add_child(tracked)
         if parent_future is None:
             self._supersede_previous(token=token, event=event, tracked=tracked)
         await queue.put(EventQueueEntry(event=event, ctx=ev_ctx))
diff --git a/tests/units/reflex_base/event/processor/test_event_processor.py b/tests/units/reflex_base/event/processor/test_event_processor.py
index a66de81086e..417bbdc6008 100644
--- a/tests/units/reflex_base/event/processor/test_event_processor.py
+++ b/tests/units/reflex_base/event/processor/test_event_processor.py
@@ -2,6 +2,7 @@
 
 import asyncio
 import contextlib
+import dataclasses
 import logging
 from typing import Any
 
@@ -13,6 +14,7 @@
     QueueShutDown,
     _stream_queue_until_done,
 )
+from reflex_base.event.processor.future import EventFuture
 from reflex_base.registry import RegistrationContext
 
 from reflex.event import Event, EventHandler
@@ -622,6 +624,38 @@ async def test_chained_event_processed(token: str):
     assert _CALL_LOG == [{"value": "chained"}]
 
 
+async def test_enqueue_child_of_done_parent_does_not_crash(
+    mock_event_processor: EventProcessor,
+    token: str,
+):
+    """Regression: a late-chained event whose parent future already completed
+    still runs instead of crashing when registered as the parent's child.
+
+    Args:
+        mock_event_processor: The event processor with mock root context.
+        token: The client token.
+    """
+    async with mock_event_processor as ep:
+        done_parent = EventFuture(txid="parent-txid")
+        done_parent.set_result(None)
+        ep._futures["parent-txid"] = done_parent
+
+        assert ep._root_context is not None
+        child_ctx = dataclasses.replace(
+            ep._root_context.fork(token=token), parent_txid="parent-txid"
+        )
+        future = await ep.enqueue(
+            token,
+            Event.from_event_type(logging_event("late-child"))[0],
+            ev_ctx=child_ctx,
+        )
+        await future
+
+    assert _CALL_LOG == [{"value": "late-child"}]
+    # The child is not registered under the already-done parent.
+    assert done_parent.children == []
+
+
 async def test_join_when_not_started(processor: EventProcessor):
     """join() when not started is a no-op (queue is None).