chore: import upstream snapshot with attribution

examples/stories/sse_polling/server.py

@@ -0,0 +1,35 @@
+"""SEP-1699: a tool closes its own SSE stream mid-call; the event store buffers the rest. Exports `build_app()`."""
+
+from starlette.applications import Starlette
+
+from mcp.server.mcpserver import Context, MCPServer
+from stories._hosting import NO_DNS_REBIND, run_app_from_args
+from stories.sse_polling.event_store import InMemoryEventStore
+
+
+def build_app() -> Starlette:
+    mcp = MCPServer("sse-polling-example")
+
+    @mcp.tool()
+    async def long_operation(ctx: Context) -> str:
+        """Emit progress, close this call's SSE stream, emit more progress, then return.
+
+        Everything sent after `close_sse_stream()` lands in the event store and is
+        replayed when the client reconnects with `Last-Event-ID`.
+        """
+        await ctx.report_progress(0.5, total=1.0, message="before-close")
+        await ctx.close_sse_stream()
+        await ctx.report_progress(1.0, total=1.0, message="after-close")
+        return "resumed"
+
+    # event_store enables Last-Event-ID replay; retry_interval=0 makes the client's
+    # reconnect wait a no-op so the example is deterministic without real time.
+    return mcp.streamable_http_app(
+        event_store=InMemoryEventStore(),
+        retry_interval=0,
+        transport_security=NO_DNS_REBIND,
+    )
+
+
+if __name__ == "__main__":
+    run_app_from_args(build_app)