chore: import upstream snapshot with attribution

examples/stories/sse_polling/server_lowlevel.py

@@ -0,0 +1,45 @@
+"""SEP-1699 polling on the lowlevel `Server`: close the request's SSE stream mid-handler."""
+
+from typing import Any
+
+import mcp_types as types
+from starlette.applications import Starlette
+
+from mcp.server.context import ServerRequestContext
+from mcp.server.lowlevel import Server
+from stories._hosting import NO_DNS_REBIND, run_app_from_args
+from stories.sse_polling.event_store import InMemoryEventStore
+
+_TOOL = types.Tool(
+    name="long_operation",
+    description="Emit progress, close the SSE stream, emit more, return.",
+    input_schema={"type": "object", "properties": {}},
+)
+
+
+def build_app() -> Starlette:
+    async def list_tools(
+        ctx: ServerRequestContext[Any], params: types.PaginatedRequestParams | None
+    ) -> types.ListToolsResult:
+        return types.ListToolsResult(tools=[_TOOL])
+
+    async def call_tool(ctx: ServerRequestContext[Any], params: types.CallToolRequestParams) -> types.CallToolResult:
+        assert params.name == "long_operation"
+        await ctx.session.report_progress(0.5, total=1.0, message="before-close")
+        # The transport only wires this callback when an event_store is configured and the
+        # negotiated version is in the 2025 era; it is None otherwise.
+        if ctx.close_sse_stream is not None:
+            await ctx.close_sse_stream()
+        await ctx.session.report_progress(1.0, total=1.0, message="after-close")
+        return types.CallToolResult(content=[types.TextContent(text="resumed")])
+
+    server = Server("sse-polling-example", on_list_tools=list_tools, on_call_tool=call_tool)
+    return server.streamable_http_app(
+        event_store=InMemoryEventStore(),
+        retry_interval=0,
+        transport_security=NO_DNS_REBIND,
+    )
+
+
+if __name__ == "__main__":
+    run_app_from_args(build_app)