[Bug] MCP client hangs indefinitely when sending large tool arguments

Bug Description
The Claude Code MCP client hangs indefinitely when sending large tool arguments (~27KB) to an MCP server. Small payloads (e.g. 11 bytes) work instantly. Large responses from the server also work fine — the issue is specifically in outbound request serialization/dispatch. Reproducible with a local MCP server using streamable HTTP transport and stateless_http=True. The HTTP request never reaches the server (confirmed via server logs). Waited 2+ minutes with no timeout or error.

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.119
• Feedback ID: e3caa228-b389-4a44-8364-6b4120c46dad

Note: I didn't include the errors because they were unrelated to the actual issue I'm seeing.