Zenith is an open-source columnar database in Rust, purpose-built for AI agent traces — long, sparse, high-cardinality JSON spans with rich text, late annotations, and bursty ingest. It runs on S3 + Postgres, exposes HTTP / gRPC / OTLP, speaks SQL and ZenithQL, and bakes full-text + vector + JSON-path search directly into segments (no sidecar indexes). Apache 2.0.