diff --git a/src/agentex/lib/core/services/adk/streaming.py b/src/agentex/lib/core/services/adk/streaming.py
index 7215f084c..33ca7bc1c 100644
--- a/src/agentex/lib/core/services/adk/streaming.py
+++ b/src/agentex/lib/core/services/adk/streaming.py
@@ -177,6 +177,12 @@ async def add(self, update: StreamTaskMessageDelta) -> None:
         if self._closed:
             return
         async with self._lock:
+            # Re-check under the lock: a concurrent close() (e.g. from a racing
+            # Full) may have drained and shut down the ticker after the check
+            # above but before we acquired the lock. Appending now would strand
+            # the delta in a dead buffer, never published.
+            if self._closed:
+                return
             self._buf.append(update)
             self._buf_chars += _delta_char_len(update.delta)
             if not self._first_flushed or self._buf_chars >= self.MAX_BUFFERED_CHARS:
@@ -415,19 +421,28 @@ async def open(self) -> "StreamingTaskMessageContext":
 
         return self
 
+    async def _reap_buffer(self) -> None:
+        """Drain and stop the coalescing buffer, releasing its background ticker.
+
+        Idempotent: a no-op once the buffer has already been reaped.
+        """
+        if self._buffer is not None:
+            await self._buffer.close()
+            self._buffer = None
+
     async def close(self) -> TaskMessage:
         """Close the streaming context."""
         if not self.task_message:
             raise ValueError("Context not properly initialized - no task message")
 
-        if self._is_closed:
-            return self.task_message  # Already done
+        # Reap the buffer (stopping its ticker) before the _is_closed
+        # short-circuit, so a context already marked done by a Full update can't
+        # leave the ticker orphaned. Draining here also lets consumers see the
+        # full delta sequence in order before DONE.
+        await self._reap_buffer()
 
-        # Drain any buffered deltas before announcing DONE so consumers see the
-        # full sequence in order.
-        if self._buffer is not None:
-            await self._buffer.close()
-            self._buffer = None
+        if self._is_closed:
+            return self.task_message  # Already done (buffer reaped above)
 
         # Send the DONE event
         done_event = StreamTaskMessageDone(
@@ -486,6 +501,14 @@ async def stream_update(self, update: TaskMessageUpdate) -> TaskMessageUpdate |
                 await self._buffer.add(update)
                 return update
 
+        # A Full ends the stream and supersedes buffered deltas. Drain and stop
+        # the buffer BEFORE publishing the Full, so leftover deltas land in order
+        # (deltas -> Full) instead of trailing the terminal Full as a stale
+        # duplicate tail. This also stops the ticker, which would otherwise be
+        # orphaned when __aexit__'s close() short-circuits on _is_closed.
+        if isinstance(update, StreamTaskMessageFull):
+            await self._reap_buffer()
+
         result = await self._streaming_service.stream_update(update)
 
         if isinstance(update, StreamTaskMessageDone):
diff --git a/tests/lib/core/services/adk/test_streaming.py b/tests/lib/core/services/adk/test_streaming.py
index b07c55f74..a8068f307 100644
--- a/tests/lib/core/services/adk/test_streaming.py
+++ b/tests/lib/core/services/adk/test_streaming.py
@@ -22,7 +22,10 @@
     ToolResponseDelta,
     ReasoningSummaryDelta,
 )
-from agentex.types.task_message_update import StreamTaskMessageDelta
+from agentex.types.task_message_update import (
+    StreamTaskMessageFull,
+    StreamTaskMessageDelta,
+)
 from agentex.lib.core.services.adk.streaming import (
     CoalescingBuffer,
     StreamingTaskMessageContext,
@@ -352,6 +355,24 @@ async def on_flush(u: StreamTaskMessageDelta) -> None:
         await buf.add(_text(task_message, "after"))
         assert flushed == []
 
+    @pytest.mark.asyncio
+    async def test_add_racing_close_is_not_stranded(self, task_message: TaskMessage) -> None:
+        """TOCTOU: a delta that passes add()'s pre-lock _closed check but only
+        acquires the lock after close() set _closed must be dropped, not appended
+        to a drained, ticker-less buffer where it would never be published."""
+        buf = CoalescingBuffer(on_flush=AsyncMock())
+        buf.start()
+        # Hold the lock so add() parks *after* its pre-lock _closed check.
+        await buf._lock.acquire()
+        add_task = asyncio.create_task(buf.add(_text(task_message, "racing")))
+        await asyncio.sleep(0)  # add() passes the _closed check, blocks on the lock
+        buf._closed = True  # close() wins the race
+        buf._lock.release()
+        await add_task
+
+        assert buf._buf == [], "racing delta was stranded in the closed buffer"
+        await buf.close()  # cleanup
+
 
 class TestCoalescingBufferCloseDuringFlush:
     @pytest.mark.asyncio
@@ -520,3 +541,59 @@ async def test_open_without_created_at_passes_omit(self) -> None:
 
         kwargs = client.messages.create.call_args.kwargs
         assert kwargs["created_at"] is omit
+
+
+class TestFullMessageClosesBuffer:
+    """A StreamTaskMessageFull must stop the buffer ticker and drain its deltas
+    before the terminal Full. Marking the context done without closing the
+    buffer leaves close()'s _is_closed short-circuit to orphan the ticker, and
+    publishing buffered deltas after the Full reads as a stale duplicate tail."""
+
+    @pytest.mark.asyncio
+    async def test_full_message_stops_ticker(self) -> None:
+        ctx, _svc, tm = await _make_context("coalesced")
+        # A delta makes the buffer and its ticker live.
+        await ctx.stream_update(_text(tm, "hello"))
+        buf = ctx._buffer
+        assert buf is not None
+        task = buf._task
+        assert task is not None and not task.done()
+
+        await ctx.stream_update(
+            StreamTaskMessageFull(
+                parent_task_message=tm,
+                content=TextContent(author="agent", content="final", format="markdown"),
+                type="full",
+            )
+        )
+
+        assert ctx._buffer is None, "Full message left the buffer un-closed"
+        assert task.done(), "coalescing-buffer ticker still running after Full (orphaned)"
+
+    @pytest.mark.asyncio
+    async def test_full_is_terminal_publish_no_trailing_deltas(self) -> None:
+        # Buffered deltas must publish BEFORE the Full, never after (a trailing
+        # delta after the terminal Full reads as a stale duplicate tail).
+        ctx, svc, tm = await _make_context("coalesced")
+        # Two deltas through the buffer. Regardless of how the coalescing window
+        # batches them (1 or 2 publishes), the invariant under test is the same:
+        # every delta publishes before the terminal Full, never after it.
+        await ctx.stream_update(_text(tm, "alpha"))
+        await ctx.stream_update(_text(tm, "beta"))
+
+        full = StreamTaskMessageFull(
+            parent_task_message=tm,
+            content=TextContent(author="agent", content="alphabeta", format="markdown"),
+            type="full",
+        )
+        await ctx.stream_update(full)
+
+        published = [c.args[0] for c in svc.stream_update.await_args_list]
+        assert published, "nothing was published"
+        assert published[-1] is full, (
+            f"Full must be the terminal publish; saw trailing "
+            f"{type(published[-1]).__name__} after it (stale duplicate tail)"
+        )
+        assert any(isinstance(u, StreamTaskMessageDelta) for u in published[:-1]), (
+            "expected the buffered deltas to be published before the Full"
+        )