The rmcp Streamable HTTP server transport versions 3.0.0 through 3.5.x have an unbounded cache of tool schema results keyed by tool name. This cache grows with each incoming tools/call request, including those rejected with HTTP 400, potentially leading to memory exhaustion. The vulnerability affects servers using protocol version 2026-07-28 or later and lacking authentication in front of the endpoint. The issue was fixed in version 3.6.0 by removing the cache and calling ServerHandler::get_tool on each request. Mitigation includes upgrading to 3.6.0 or implementing authentication or request filtering to block unknown tool names.