MCP sessions don't mean what you think they mean. We built a probe server that logs every header, _meta field, session ID, TCP socket, and timing pattern, then connected seven MCP clients and tested whether different chat conversations produce different signals. The results: Claude Code creates new sessions per conversation, Codex sends session_id and thread_id in _meta, ChatGPT sends x-openai-session headers, Claude.ai sends changing sentry-trace_ids, and Cursor sends nothing — sharing a pool of 6-8 sessions across all conversations with zero differentiation. We then dug into Cursor's source code and found that composerId flows through the entire tool call chain but is dropped one function call before the HTTP request. This talk presents the full methodology, raw findings, and what it means for MCP server authors who need per-conversation state.