diff --git a/src/mcp/server/streamable_http.py b/src/mcp/server/streamable_http.py
index f8aec6c9e..05c5b9e1f 100644
--- a/src/mcp/server/streamable_http.py
+++ b/src/mcp/server/streamable_http.py
@@ -774,10 +774,17 @@ async def terminate(self) -> None:
 
         Once terminated, all requests with this session ID will receive 404 Not Found.
         """
+        if self._terminated:
+            return
 
         self._terminated = True
         logger.info(f"Terminating session: {self.mcp_session_id}")
 
+        sse_stream_writers = list(self._sse_stream_writers.values())
+        for writer in sse_stream_writers:
+            writer.close()
+        self._sse_stream_writers.clear()
+
         # We need a copy of the keys to avoid modification during iteration
         request_stream_keys = list(self._request_streams.keys())
 
diff --git a/src/mcp/server/streamable_http_manager.py b/src/mcp/server/streamable_http_manager.py
index 648dcc827..e9fbbc9d0 100644
--- a/src/mcp/server/streamable_http_manager.py
+++ b/src/mcp/server/streamable_http_manager.py
@@ -145,6 +145,11 @@ async def lifespan(app: Starlette) -> AsyncIterator[None]:
                 yield  # Let the application run
             finally:
                 logger.info("StreamableHTTP session manager shutting down")
+                for transport in list(self._server_instances.values()):
+                    try:
+                        await transport.terminate()
+                    except Exception:  # pragma: no cover
+                        logger.debug("Error terminating StreamableHTTP transport during shutdown", exc_info=True)
                 # Cancel task group to stop all spawned tasks
                 tg.cancel_scope.cancel()
                 self._task_group = None
diff --git a/tests/server/test_streamable_http_manager.py b/tests/server/test_streamable_http_manager.py
index 0e8afed50..68662fa07 100644
--- a/tests/server/test_streamable_http_manager.py
+++ b/tests/server/test_streamable_http_manager.py
@@ -64,6 +64,19 @@ async def try_run():
     assert "StreamableHTTPSessionManager .run() can only be called once per instance" in str(errors[0])
 
 
+@pytest.mark.anyio
+async def test_run_terminates_active_transports_before_shutdown():
+    app = Server("test-server")
+    manager = StreamableHTTPSessionManager(app=app)
+    transport = AsyncMock()
+
+    async with manager.run():
+        manager._server_instances["session-id"] = transport
+
+    transport.terminate.assert_awaited_once_with()
+    assert not manager._server_instances
+
+
 @pytest.mark.anyio
 async def test_handle_request_without_run_raises_error():
     """Test that handle_request raises error if run() hasn't been called."""
@@ -269,6 +282,22 @@ async def mock_receive():
             assert len(transport._request_streams) == 0, "Transport should have no active request streams"
 
 
+@pytest.mark.anyio
+async def test_transport_terminate_closes_active_sse_writers():
+    transport = StreamableHTTPServerTransport(mcp_session_id="session-id")
+    writer, reader = anyio.create_memory_object_stream[dict[str, str]](1)
+    transport._sse_stream_writers["request-id"] = writer
+
+    await transport.terminate()
+
+    assert transport.is_terminated
+    assert not transport._sse_stream_writers
+    with pytest.raises(anyio.ClosedResourceError):
+        writer.send_nowait({"event": "message", "data": "{}"})
+
+    await reader.aclose()
+
+
 @pytest.mark.anyio
 async def test_unknown_session_id_returns_404(caplog: pytest.LogCaptureFixture):
     """Test that requests with unknown session IDs return HTTP 404 per MCP spec."""