Claude Desktop: HTTPS MCP tools/call sends GET instead of POST for large payloads

Bug

When using Claude Desktop with a direct HTTPS MCP connection (not stdio proxy), large tools/call payloads fail with:

Request with GET/HEAD method cannot have body

Reproduction

1. Connect Claude Desktop to an HTTPS MCP server (e.g. https://www.lava.so/mcp)
2. Call a tool with a short input — works fine
3. Call the same tool with a longer input (e.g. a multi-paragraph question string) — fails with the GET/body error

Observed behavior

• Short payloads: Request goes through as POST, tool works
• Large payloads: Request is sent as GET with a body, which HTTP spec rejects
• The threshold appears to be payload size — the same tool works or fails depending on input length

Expected behavior

All tools/call requests should be sent as POST regardless of payload size. The MCP Streamable HTTP spec requires POST for all JSON-RPC messages.

Workaround

Using a stdio proxy that always forwards as POST works. Only the native HTTPS MCP client in Claude Desktop is affected.

Environment

• Claude Desktop (macOS)
• MCP server: Vercel-hosted Next.js with WebStandardStreamableHTTPServerTransport
• Connection type: Direct HTTPS (not stdio proxy)