MCP Python SDK 2.3 max_sse_event_size vs Sume's 256 KiB result cap

If you set max_sse_event_size in the MCP Python SDK 2.3.0 (2026-10-02), keep it above 256 KiB for Sume. Sume's hosted server caps a tool result at 256 KiB and returns an mcp_output_too_large error beyond that, so a limit under that figure could reject a legitimate large result on the client side first.

The two limits protect different ends of the same connection.

What changed in 2.3.0

The release notes add max_sse_event_size, omit empty _meta and params, fail invalid x-mcp-header annotations at registration, add MCPServer(subscriptions=False), and retry tool calls automatically after header mismatches. It also requires httpx2 at 2.10.0 or later.

At a glance

Sume's result cap

Sume's output budget is 256 KiB per tool result. Past it, the result has isError true and the code mcp_output_too_large. The message says it is an output limit and not a job failure, and tells you never to resubmit a paid create.

The fix is to ask for less: lower a list limit, or fetch assets by id one at a time.

• Set max_sse_event_size above 256 KiB plus overhead.
• On mcp_output_too_large, narrow the query.
• Do not retry the create call.

Limits and what is not verified

I did not find the default value of max_sse_event_size in the release notes, so I do not quote one. I did not trigger an oversize result against the live server.