Keep SSE reconnect tests fast and document stable errors

Author: Kludex

docs/client/transports.md

@@ -56,7 +56,7 @@ Pass `max_sse_event_size` when a server sends a large tool result or notificatio
 
 The default is 1 MiB per event, measured in bytes before the event is parsed. The limit applies to
 POST responses, the GET stream, and resumed streams. An oversized event in a POST response or resumed
-stream fails that request with an error naming the limit. On the background GET stream, the client logs
+stream fails that request with an SSE error. On the background GET stream, the client logs
 the error and retries the stream. Set `max_sse_event_size=None` to disable the cap when you trust the
 server and need larger events. JSON responses are unaffected. If you use `ClientSessionGroup`, set the
 same option on `StreamableHttpParameters`.

tests/client/test_streamable_http.py

@@ -175,10 +175,12 @@ def handler(request: httpx2.Request) -> httpx2.Response:
         if request.method == "GET":
             gets.append(request)
             if len(gets) == 1:
-                content = b'data: {"jsonrpc":"2.0","method":"notifications/message","params":{"data":"'
+                content = b'retry: 0\n\ndata: {"jsonrpc":"2.0","method":"notifications/message","params":{"data":"'
                 content += b"x" * 2048 + b'"}}\n\n'
-            else:
+            elif len(gets) == 2:
                 content = b'data: {"jsonrpc":"2.0","method":"notifications/message","params":{"data":"ok"}}\n\n'
+            else:
+                return httpx2.Response(500)
             return httpx2.Response(200, content=content, headers={"content-type": "text/event-stream"})
         body = json.loads(request.content)
         if body.get("method") == "initialize":