httpx2 2.10 caps a single server-sent event at 1 MiB by default, on both
`client.sse()` and a bare `EventSource(response)`, and raises `SSEError`
past it. A JSON-RPC message is one event and MCP sets no message size
limit, so any tool result or server notification over ~1 MiB delivered
over SSE failed with `SSE stream ended without a response` (the cause
was logged at DEBUG only), and with an event store the client also
replayed the same oversized event on each reconnect attempt.
Pass `max_event_size=None` at all five sites - the POST response stream,
the standalone GET stream, resumption, reconnection, and the legacy SSE
transport - which restores the pre-2.10 behaviour and matches the
unbounded `application/json` response path. The floor moves to
`httpx2>=2.10.0`, the first version that accepts the argument.
Fixes#3332
Max Isbey
dd16fe06c4