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Python: MCPStreamableHTTPTool cannot set mcp 2's max_sse_event_size, so tool results over 1 MiB fail #10222
Description
Problem
With agent-framework-core 1.24.0 and mcp 2.3.0, a streamable HTTP MCP tool result larger than 1 MiB fails. It looks like a lost connection.
mcp 2 limits one SSE event to 1 MiB by default (mcp/client/streamable_http.py:61, DEFAULT_MAX_SSE_EVENT_SIZE = 1024 * 1024). streamable_http_client accepts max_sse_event_size, and None disables the limit (streamable_http.py:718, :735). mcp 1 had no such limit.
MCPStreamableHTTPTool.get_mcp_client (agent_framework/_mcp.py:4306) calls streamable_http_client(url=..., http_client=..., terminate_on_close=...) (:4417-4421). It passes no max_sse_event_size, and the tool has no parameter for it. So a caller cannot raise or lift the limit without overriding get_mcp_client.
Workaround we use
We subclass the tool and override get_mcp_client. It reads the arguments from the unentered context manager that super().get_mcp_client() returns (args/kwds), then calls streamable_http_client(**kwds, max_sse_event_size=None). This depends on a private detail of contextlib, and it breaks if the call's shape changes.
Ask
Add a max_sse_event_size: int | None parameter to MCPStreamableHTTPTool, passed through to streamable_http_client. Its default should be mcp's own default, so today's behaviour does not change.
Versions
- agent-framework-core 1.24.0
- mcp 2.3.0
- Python 3.13
max_sse_event_size is also absent from _mcp.py in python-1.25.0 and on main (checked 2026-11-10).
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gian-alloy commentedon Nov 10, 2026
One more detail for whoever triages this: the parameter only exists from mcp 2.3.0 (modelcontextprotocol/python-sdk#3600). In mcp 2.2.0,
streamable_http_clienthas nomax_sse_event_size, and the 1 MiB limit can't be changed. Core declaresmcp>=2.2.0,<3, and only in theallextra, so simply passing the new keyword would break existing installs that run mcp 2.2.0.The design I'd propose, keeping the current mcp range:
MCPStreamableHTTPTool(..., max_sse_event_size=...), with a sentinel default (typing_extensions.Sentinel, as_skills.pyalready does).int: passed through.None: passed through, which disables the limit (mcp's own semantics).<= 0raisesValueError, the same waymax_host_payload_size_bytesis validated.streamable_http_clienthas nomax_sse_event_sizeparameter,ValueErrorasks for mcp>=2.3.0.mcp_client=joins the existing "framework-owned HTTP options" conflict error.Noneand a larger limit accept it.MCPStreamableHTTPTooldocstring, plus the core README/AGENTS notes next to the other transport limits.Does this direction look right to you? If it does, I'm happy to open a PR.
gian-alloy commentedon Nov 10, 2026
In case it helps triage, the design above is already implemented on a branch (one commit, no PR yet):
main...gian-alloy:agent-framework:python-mcp-max-sse-event-size
It covers the code, tests and the README/AGENTS notes. The tests pass on mcp 2.3.0 and on the declared floor, mcp 2.2.0. I'll open a PR from it once you're happy with the direction, or change it first if you'd prefer something else.