Eggshell is an open-source token-saving tool built in Lean4 that creates local memory for AI agents, allowing them to reuse work across independent chats without additional LLM calls. It uses semantic search and graph-based evidence tracking to retrieve relevant prior observations, reducing token consumption by up to 81.8% in follow-up conversations. The tool integrates with Codex and experimental adapters for Claude Code, Gemini CLI, Cursor, and OpenCode.