[v2] Expose the SSE max_event_size setting in Streamable HTTP clients #3332

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1. maxisbey

   added

   v2Affects the v2 line (2.x on main)Affects the v2 line (2.x on main)

   on Aug 18, 2026
2. maxisbey

   assigned

   Kludex

   on Aug 19, 2026
3. mguttmann commented on Sep 2, 2026

   

   mguttmann

   on Sep 2, 2026

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   We hit exactly this in production, and it is not limited to tools/call: a tools/list response above 1 MiB is enough to make a whole upstream server disappear behind a Streamable HTTP client.

   Real-world occurrence

   Our MCP gateway (FastMCP 4.0.0, which pulls mcp==2.1.1 and httpx2==2.12.0) proxies a hosted MCP server (Composio, backend.composio.dev) that exposes 1030 tools. That server answers tools/list with HTTP 200 text/event-stream and delivers the whole result as ONE data: event of 2,140,787 bytes. Verified with a raw replay of the identical request bodies the SDK sends (with and without params._meta, protocol version 2025-11-25): the server returns the complete, valid response every time.

   With mcp 1.x (FastMCP 3.4.5, httpx 0.28 + httpx-sse) the client lists all 1030 tools. With mcp 2.1.1 the same request ends in

   MCPError: SSE stream ended without a response
   

   and a proxy built on top of it reports 0 tools for that upstream, without any hint that a size limit was involved. The actual SSEError("Server-sent event exceeded the 1048576 byte limit.") is only visible at DEBUG level.

   Where it happens (mcp 2.1.1)

   • src/mcp/client/streamable_http.py:426: event_source = EventSource(response) - no max_event_size, so httpx2._config.DEFAULT_MAX_EVENT_SIZE_BYTES = 1024 * 1024 applies.
   • src/mcp/client/streamable_http.py:411-460 (_handle_sse_response): the SSEError raised by the httpx2 decoder is caught by except Exception: logger.debug("SSE stream ended", exc_info=True) and the request is resolved via _resolve_abandoned_request(..., "SSE stream ended without a response"), so the caller gets a generic CONNECTION_CLOSED error that points in the wrong direction (network) instead of the real cause (client-side size cap).
   • Neither streamable_http_client() nor StreamableHTTPTransport expose the setting, and FastMCP (checked 4.0.0 and 4.0.1, package fastmcp-slim) has no way to pass it through either.

   Minimal reproduction (no external services)

   A plain Starlette app that answers every JSON-RPC request with text/event-stream containing exactly one data: event, like several hosted servers do (stateless, no mcp-session-id). Run with:

   uv run --no-project --with "mcp==2.1.1" --with uvicorn --with starlette python repro_sse_event_limit.py

   import asyncio, json, socket, threading
   import uvicorn
   from starlette.applications import Starlette
   from starlette.requests import Request
   from starlette.responses import Response, StreamingResponse
   from starlette.routing import Route
   from mcp import ClientSession
   from mcp.client.streamable_http import streamable_http_client
   
   
   def make_app(total_kib: int):
       n = 10
       desc = "x" * (total_kib * 1024 // n)
       tools = [{"name": f"tool_{i}", "description": desc, "inputSchema": {"type": "object", "properties": {}}} for i in range(n)]
   
       async def endpoint(request: Request):
           body = await request.json()
           method = body.get("method")
           if method == "initialize":
               result = {"protocolVersion": "2025-11-25", "capabilities": {"tools": {"listChanged": True}},
                         "serverInfo": {"name": "one-big-sse-event", "version": "0.1.0"}}
           elif method == "tools/list":
               result = {"tools": tools}
           elif "id" not in body:  # notifications
               return Response(status_code=202)
           else:  # e.g. server/discover from 2.x clients
               return Response(json.dumps({"jsonrpc": "2.0", "id": body["id"], "error": {"code": -32601, "message": "Method not found"}}),
                               status_code=400, media_type="application/json")
           payload = json.dumps({"jsonrpc": "2.0", "id": body["id"], "result": result})
   
           async def gen():
               yield f"event: message\ndata: {payload}\n\n".encode()
           return StreamingResponse(gen(), media_type="text/event-stream", headers={"cache-control": "no-cache"})
   
       return Starlette(routes=[Route("/mcp", endpoint, methods=["POST", "GET", "DELETE"])])
   
   
   def free_port():
       s = socket.socket(); s.bind(("127.0.0.1", 0)); p = s.getsockname()[1]; s.close(); return p
   
   
   async def probe(total_kib: int) -> str:
       port = free_port()
       server = uvicorn.Server(uvicorn.Config(make_app(total_kib), host="127.0.0.1", port=port, log_level="error"))
       th = threading.Thread(target=server.run, daemon=True); th.start()
       while not server.started:
           await asyncio.sleep(0.05)
       try:
           async with streamable_http_client(f"http://127.0.0.1:{port}/mcp") as streams:
               async with ClientSession(streams[0], streams[1]) as session:
                   await session.initialize()
                   result = await session.list_tools()
                   out = f"OK, {len(result.tools)} tools"
       except Exception as exc:
           inner = exc
           while isinstance(inner, BaseExceptionGroup) and inner.exceptions:
               inner = inner.exceptions[0]
           out = f"FAIL: {type(inner).__name__}: {inner}"
       server.should_exit = True; th.join(timeout=5)
       return out
   
   
   async def main():
       for kib in (512, 1000, 1023, 1030, 1100, 2200):
           print(f"tools/list as ONE SSE event of ~{kib:>5} KiB -> {await probe(kib)}", flush=True)
   
   asyncio.run(main())

   Output (macOS, Python 3.12, mcp 2.1.1, httpx2 2.12.0):

   tools/list as ONE SSE event of ~  512 KiB -> OK, 10 tools
   tools/list as ONE SSE event of ~ 1000 KiB -> OK, 10 tools
   tools/list as ONE SSE event of ~ 1023 KiB -> OK, 10 tools
   tools/list as ONE SSE event of ~ 1030 KiB -> FAIL: MCPError: SSE stream ended without a response
   tools/list as ONE SSE event of ~ 1100 KiB -> FAIL: MCPError: SSE stream ended without a response
   tools/list as ONE SSE event of ~ 2200 KiB -> FAIL: MCPError: SSE stream ended without a response
   

   The cut is exactly at the 1 MiB default. Note that a FastMCP 2.x/4.x test server does NOT reproduce this because it answers with application/json; the trigger is a server that streams the result as a single SSE event, which the spec allows.

   What would help

   1. Expose the limit: a max_event_size parameter on streamable_http_client() and StreamableHTTPTransport (httpx2 terminology), applied consistently to the POST response path (EventSource(response, max_event_size=...)), the GET stream and the reconnection path (AsyncClient.sse(...) currently has no transport-level setting either).
   2. Surface the cause: when the decoder raises SSEError, resolve the pending request with an error that carries the SSEError text (and log at WARNING, not DEBUG). "SSE stream ended without a response" sends people hunting for network problems; it took a wire trace to find the size cap.
   3. Consider a larger default than 1 MiB for the POST response path, since tools/list results of large hosted servers routinely exceed it. The bound itself is valuable (it protects the client from a misbehaving server), it just needs to be configurable.

   Workaround we use in the meantime

   Process-local, at import time, before any client connects:

   import functools, httpx2
   import mcp.client.streamable_http as sh
   sh.EventSource = functools.partial(httpx2.EventSource, max_event_size=32 * 1024 * 1024)

   That keeps a bound in place (memory protection against a broken upstream) while letting the real-world tools/list through. Patching httpx2 itself does not work because the default is bound at function definition time.

   Happy to test a PR against the real server that triggered this.

   👍React with 👍3Reacted by Antoine Balliet, Thomas Delayen and Charles Francis
4. Kludex commented on Sep 8, 2026

   

   Kludex

   on Sep 8, 2026

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   The idea of the limit in HTTPX2 was to prevent an attack, but we can change that limit if proven that a bigger value doesn't bring back a vulnerability.
5. Zhangs-11 commented on Sep 8, 2026

   

   Zhangs-11

   on Sep 8, 2026

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   Thanks — I agree that the safe default should remain unchanged.

   My proposal is not to globally increase or remove the limit, but to expose max_event_size as an explicit client option for applications that legitimately receive larger SSE events. The existing value would remain the default, so current users retain the same protection.

   We can also document that increasing it raises memory/DoS risk and should only be done when the server and payload sizes are trusted or otherwise controlled. Would that address the security concern?
6. Kludex commented on Sep 8, 2026

   

   Kludex

   on Sep 8, 2026

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   @Zhangs-11 are you openclaw or hermes?
7. Zhangs-11 commented on Sep 8, 2026

   

   Zhangs-11

   on Sep 8, 2026

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   Neither — I’m using OpenAI Codex to help investigate and communicate the issue.
8. mguttmann commented on Sep 9, 2026

   

   mguttmann

   on Sep 9, 2026

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   The limit protects a client from a hostile or broken server exhausting its memory, and that protection should stay the default. Exposing max_event_size as an explicit, opt-in parameter on streamable_http_client() and StreamableHTTPTransport does not reintroduce the risk: the default path is untouched, and only an operator who trusts one specific server raises the bound for that one client, the same trust decision they already make with timeout. In our gateway we run exactly that today via a workaround: a 32 MiB bound for one hosted upstream whose tools/list is a single 2.1 MB event (1030 tools), everything else keeps the 1 MiB default.

   Independent of the limit itself: when the decoder raises SSEError, resolving the pending request with that message instead of the generic "SSE stream ended without a response" has no security cost and would have saved us a wire trace to find the cause.
9. aballiet commented on Sep 11, 2026

   

   aballiet

   on Sep 11, 2026

   Last edited by aballiet

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   We hit this exact failure in production through FastMCP 4 / MCP SDK 2 and httpx2 2.12.0. Two successful HTTP 200 tool responses were approximately 1.83 MB and 2.15 MB, so each deterministically exceeded httpx2's 1,048,576-byte per-event limit. The caller only received SSE stream ended without a response.

   Our downstream compatibility fix deliberately keeps the 1 MiB safety cap. It catches SSEError before the SDK's catch-all, logs the exception type/message with exc_info, resolves only the affected request with an actionable message, and raises the caller-facing error from the original SSEError. A separate regression confirms a plain premature EOF still receives the existing generic error.

   The key upstream requirement for us is therefore request-scoped exception propagation, not max_event_size=None: the original parser exception (or an equivalent structured error retaining its details) should reach the call boundary, siblings should remain usable, and the default cap should stay intact.

   I would be happy to contribute the smaller upstream change once maintainers confirm the preferred exception-plumbing shape, especially how the original cause should cross the stream/dispatcher boundary.
10. Kludex

    closed this as completedin #3600on Oct 1, 2026