Thunderbolt is a full-stack, cross-platform AI chat and agent client built by Mozilla Thunderbird. It ships as a React/Vite web app, a Tauri desktop shell (macOS/Windows/Linux), native iOS/Android apps, and a Bun-based CLI. The system provides chat with multiple LLM providers (OpenAI-compatible, Anthropic, custom Tinfoil enclave), a skills system for user-defined slash-command instructions, voice mode with STT/TTS, MCP tool integration, ACP (Agent Client Protocol) remote agent support, and a widget system for rich chat outputs (weather, maps, citations, document previews, link previews). The backend is an Elysia/Bun server handling OIDC/SAML/OTP auth, an inference usage ledger with signed receipts, a CORS-bypassing proxy with streaming, and PowerSync sync coordination. Data flows from local SQLite (wa-sqlite/bun-sqlite) through an E2E-encrypted PowerSync pipeline to PostgreSQL, with device registration, envelope key storage, and canary proof-of-possession for recovery. The CLI provides a separate agent harness with iroh P2P transport, device-grant auth, and a workspace jail for safe file operations. The repo also contains a substantial AI evaluation framework (scenario-based scoring with Wilson confidence intervals), a Pulumi/K8s/Docker deployment stack, and a marketing/docs site in Astro. While the dominant character is clearly an application, it has a notable data-engineering flavor in its inference usage tracking, eval harness, and telemetry pipelines.