Epicenter is a monorepo platform for building local-first, privacy-focused productivity applications with optional cloud sync. It encompasses a suite of apps including Honeycrisp (notes with Yjs CRDT sync), Whispering (voice transcription/dictation with local and cloud providers), Matter (typed markdown vaults with SQLite projection), Local Mail (Gmail mirror with offline triage), Local Books (QuickBooks mirror with CLI tools), Vocab (AI language tutor), Skills (agent instruction management), and the Epicenter desktop host (Tauri + Bun sidecar). The shared platform provides a CRDT-based data layer (Yjs with Durable Object authority), portable blob storage (IndexedDB, OPFS, S3), an OAuth/infrastructure-agnostic auth system, an agent loop framework with tool catalogs, and a server framework built on Hono for Cloudflare Workers or Bun. The architecture emphasizes offline-first, typed error handling (Result/defineErrors pattern), compile-time platform seams (browser/Tauri/desktop-host), and rigorous ADR-driven design with evidence benchmarks.